i'IHI'llii)''liliH|i i>i 1 'in I -a fiwnilt!!' m l|l|||,|l'l Kil''"' I'lii ■I'nuui'iiiuM'iiuit 1 [i riijii Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/cletails/cu31924073905592 LIBRARY ILLUSTRATED STANDARD SCIEMIFIC WOEKS. VOL. VI. QUEKETT'S , PRACTICAL TREATISE ON TUE USE OF THE MICROSCOPE, inml iMto. LONDON; H. BAILLIERE, PUBLISHER, 219, REGENT STREET AND 290, BBOADWAr, NEW YORK, U.S. PARIS: J. B. BAILLIEBE, RUE HAUTE FE UI LLE. MADRID: BAILLY BAILLIEBE, CALLE DEL FKINCIPE. 18 52. -^ '"j-j't -; ■ s \%^^^^ I r ' ~^ ■, v^ ,v/Av.y, >Yy fV.V '.Vi.vr/,v* ^'/.Vi%'*,*= "^ ' ",' ",'.V, » *T -. T .^. . . //«V/,'.'.'.I :v/.v/"' ' %'» V-s'i't',' ^■''VV '"VvV' f;-.v.v.v.V-/V/ i-r^ '" w ^ S 1 , '^* J J J ■**|4 ry 7. /'an of Fig S 1..7 i;h^m> Aoa instej.J. .t" .'.-■//, -.r 9 D' -if n i.ij\,! ■:.-,!. 20 /'art of a. i!!'-.!-'Mi'-/.r /^j^ri,:i,'.',s ,::t I'l..;. eOO ,ii,u'' ' . ' i:eci --j.,.i.m' PRACTICAL TREATISE ON THE USE OF THE MICROSCOPE, INCLUDING THE DIFFERENT METHODS OF PREPARING AND EXAMINING ANIMAL, VEGETABLE, AND MINERAL STRUCTURES. JOHN QUEKETT, ASSISTANT CONSERVATOR OF THE MUSEDM AND DEMONSTRATOR OF MINUTE ANATOMY AT THE ROYAL COLLEGE OF SCBOEONS OP ENGLAND, kun^i (IFMtinn, raitfj iMinns. ILLUSTRATED WITH TWELVE PLATES AND TWO HUNDRED AM) SEVENTY WOOD ENGRAVINGS. LONDON: H. BAILLIERE, PUBLISHER, 219, REGENT STREET ; AND 290, BROADWAY, NEW YORK, U.S. PARIS: J. B. BAILLIERE, RUE HAUTE FE UILLE. MADRID : BAILLT BAILLIERE, CALLE DEL PRINCIPE, 1852. TO JOSEPH JACKSON LISTER, ESQ., F.R.S., &c., TO WHOSE LABOURS IN PERFECTING THE Irjirnraatic (0nni|rattEi 3liirrnED]iB, SCIENCE IN ENGLAND IS SO DEEPLY INDEBTED, THIS TREATISE IS DEDICATED, ■WITH FEELINGS OF RESPECT, BY HIS FEIEND, THE AUTHOR PREFACE TO THE SECOND EDITION. In presenting the second edition of this work to the public, the Author begs to oiFer his thanks for the favourable reception the former edition has met with. He trusts that the present, from the numerous additions made to it, will be found more worthy of notice. It having been objected, that no mention was made of foreign microscopes in the first edition, this seeming omission is reme- died by the description of all the best instruments now manu- factiir'ed by our continental neighbours. The Author begs to offer his sincere thanks to those friends who have kindly aided him during the progress of the work, and especially to Mr. Lister and Mr. Jackson, for their revision of the whole of the former edition. 32, Blandpoed Sqdaee, December, 1851. PEEFACE TO THE FIRST EDITION. The rapid advances which have been made in modern times, towards a correct knowledge of the intimate structure of animate and inanimate beings, by the employment of the Microscope, have given to this instrument an importance second only to that of the Telescope. By its agency alone have crude notions and theories been swept away, and science in civilized countries made to stand on a firmer basis. In this land of machinery and manufactures, artists have not been found wanting to devote their time and talents to the conver- sion of what might once have been an amusing instrument or a toy, into one of the most powerful auxiliaries that can be employed in scientific research. In proportion to its use, so has been the demand for improvement in its construction, and both amateur and optician have laboured together to bring it to its present state of perfection, the former, in many cases, furnishing the means to enable the latter to carry out his designs. In the present day, so urgent has been the call for Achromatic Microscopes in England, that the demand has far exceeded the supply of information on matters connected with their construction and use; since the works of Sir D. Brewster, Ur. Goring, and Mr. Pritchard, no treatises of a practical nature have been published in this country. The writings of Mr. Pritchard, although very excellent, are chiefly confined to the instruments and apparatus of his own manu- facture; consequently, persons who are in possession of microscopes constructed by others (and these by far the most numerous class) are still without a guide to their , PREFACE. IX management; to remedy this deficiency, the present work has been undertaken. The principal aim the Author has had in view, has been to furnish the uninitiated with a concise and practical account, firstly, of the Microscope as known in former years ; secondly, of the different forms of instruments now generally employed; thirdly, of the methods of applying the same to scientific inquiry; and, lastly, of the various plans of preparing, mounting, and examining animal, vegetable, and mineral substances, together with a classification of a few characteristic and interesting specimens that may be selected from the great volume of Nature. It was, at first, the intention of the writer to have included in the present Treatise the methods of dissecting and injecting, as well as many very important matters, purely of an anato- mical nature ; but he' has found it advisable to defer these and all others relating exclusively to physiological science to a separate work, which he hopes at a subsequent period to lay before the medical profession, to whom the Microscope has now become indispensable as an educational instrument. The different modes of preparing and examining Micro- scopic objects are chiefly the result of the Author's own expe- rience; but as it would be next to impossible for one individual to be fully conversant with aU these subjects, he begs to state that he will always be glad to receive from fellow-labourers any hints bearing on matters relating to the Microscope, and ready to acknowledge the source from whence such informa- tion may have been derived. In order to render the matters treated of, clear and intelligible to the general reader, as many technicalities as possible have been avoided, and the simplest language made use of; which will account for the plainness of style and composition. It may be remarked that the name of Mr. Ross occurs more frequently than that of any other optician; this has arisen from the very valuable papers published by him, from which the Author has made copious extracts; he embraces this X PREFACE. opportunity of acknowledging the kind assistance afforded him on all occasions by Messrs. Powell and Lealand, Mr. Koss, and Messrs. Smith and Beck. He would here, also, beg to tender his best thanks to Dr. Pereira, Mr. Bowerbank, Mr. Jackson, and other gentlemen, who have obligingly aided him with much useful information during the progress of the book, as well as to the artists, Messrs. Leonard and Aldous, and the wood engravers, Messrs. Vasey and Joyce, for the able manner in which their part of the work has been executed. In conclusion, the Author trusts that his endeavours may not be unavailing in affording assistance to those who are engaged in Microscopic investigations; and should his efforts be conducive, in the slightest degree, to the promotion of scientific research, the end for which he has laboured will be fuUy accomplished. 15, DOKOHESTEK PlaOE, Blandfokd Square, Nov. nth, 1848. CONTENTS. PART I. MECHANICAL ARRANGEMENTS. Page HiSTOBV or THE MICROSCOPE " - 1 Hooke's microscope - 4 Leeuwenhoek's microscope 6 Newton's microscope 7 Bonnani's microscope - 8 Grindelius' microscope - - - 9 Stephen Gray's water microscope - ■ - ib. reflecting microscope 10 Wilson's pocket microscope - 11 opaque microscope - 12 Marshall's compound microscope - 13 Lieberkuhn's microscope - - 15 microscope for injections - - 16 anatomical microscope 18 Culpeper's compound microscope - - 20 Cuff's microscope - - 22 George Adams' microscope - 23 juir., microscope 24 Withering's botanical microscope 25 Jewel lenses - 26 Invention of doublet - 28 Herschel's doublet - 29 WoUaston's doublet - 30 HoHand's triplet ' - - 31 History of compound achromatic microscope 32 TuUey's microscope - 37 Lister's discoveries 39 Adjustment of achromatic object-glass - 41 Improvers of achromatic microscope 44 b* CONTENTS. CHAPTER T. Paso The Simple Microscope - ■*' Pocket magnifiers '"• Coddington lens *" Author's dissecting microscope - ^^ Watchmaker's eye-glass - - - ib. Powell and Lealand's dissecting microscope 52 Lister's method of mounting pocket magnifier - 54 Ross's portable dissecting microscope - 55 Slack's dissecting microscope - 56 Ross's simple and compound microscope - 59 Valentine's microscope - "^ Smith and Beck's simple microscope 63 Magnifying powers employed 64 WoUaston's doublet - - 65 Holland's triplet ib. CHAPTER II. Compound Microscope - - 67 Mechanical arrangements . - - 68 Messrs. Powell and Lealand's large achromatic microscope 74 smaller microscope - - 77 portable microscope 78 Ross's compound microscope 81 portable compound microscope 84 Messrs. Smith and Beck's large achromatic microscope 87 Mr. Alfred White's lever stage - 89 Messrs. Smith and Beck's smaller compound microscope 90 achromatic microscope for students 91 Varley's microscope - - 93 Dancer's microscope - - 96 PiUischer's microscope 98 King's microscope 100 Foreign microscopes - 101 Sehiek's microscope 102 Pistor's microscopes 103 Chevalier's microscope - 105 Oberhauser's microscope - 106 N"achet's microscope - 107 CONTENTS. xiii CHAPTER ni. Page AccESsoBY Instruments 110 The diaphragm - - - 111 Dark chamber - - 112 WoUaston condenser - it,. Achromatic condenser - . 113 Mr. Wenham's illuminator - -116 Prism of Dujardin - - 118 Achromatic prism - - - Hi) Oblique prism - ib. Polarizing apparatus 120 Condensing lens - - - 122 Erector - - 125 Lieberkuhn - 126 Side reflector . 127 Dark stops or wells 128 Forceps - ib. Animalcule cages - ] 30 Fishing tubes for animalcules ■■ 1 33 Compressorium - - 137 Troughs for chara and polyps - - 140 Frog-plate - - 141 Fish-troughs - - - 142 Phial-holder - 143 Camera lucida - - - 144 Indicator - - - - 145 Bonnet or hood for the compound body - 147 CHAPTER IV. The Lamp - 149 Chimney shade - - 153 Oil 154 Jatropha oil - ib. Cleaning lamps ... ib. Portable candle-lamp _ _ 155 To clean chimneys of lamps 158 CHAPTER V. On the Magniftikg Powebs used with Simple and Achro- matic Compound Microscopes - 158 xiv (X)N'J'ENTS. PART 11. USE OF THE MICROSCOPE. CHAPTER I. Page Peelimtnary Directions 181 Position '"• Adjustment of the light 182 Transparent objects ib. Adjustment of the focus - 183 Opaque objects 185 CHAPTER II. On the Illumination or Objects 186 Transparent objects ib. Achromatic condenser or Eclairage 189 Direct light 193 Oblique light - - - 194 Back-ground illumination - ib. CHAPl'ER III. Opaque Objects - 196 Lieberkuhn 199 Dark stops - 202 Illuminators of recent construction - ■ 203 Nobert's illuminator - ib. Amici's illuminator - 204 Annular condenser - 205 GiUett's condenser - 206 CHAPTER IV. JIlCBOMETEK - 209 Stage micrometer - - - - 211 Eye-piece micrometer - - 212 Cobweb micrometer - - 216 On the Measurement of Microscopic Objects - 217 CONTENTS. XY Pago With the stage micrometer - 217 By the micrometer eye-piece - 218 To find the value of the lines in the negative eye-piece micrometer ib. To find the value of the divisions in the positive eye-piece micro- meter - - 220 To find the value of each revolution of the screw, or parts of a revolution of the same, in the cobweb micrometer 221 Directions for the use of the eye-piece micrometer - 222 To use the cobweb micrometer - - 223 Measurements of an object made by means of a stage micrometer and a camera lucida - - - ib. CHAPTER IV.A. Ok the Methods of Obtadsistq the Magnifying Power of Single and Compound Microscopes - - 225 To convert Paris lines into English measure - 230 To convert millimetres into English measure - - - ib. CHAPTER V. Camera Lucida ..... 230 Method of using the camera lucida with the microscope - 232 Uses to which the camera lucida may be applied - - 234 CHAPTER VI. On the Polarization of Light - - - . 236 Origin of the term - - ib. Method of using the polarizing apparatus - - 239 Cause of the colours of polarized light - - 245 Advantages of polarized light to the microscopist 250 CHAPTER Vn. Goniometer - - - 251 xvi CONTENTS. PAET III. MANIPULATION. CHAPTER I. I'ncc Diamond for cutting glass - 259 Writing diamonds - 261 CHAPTER II. On CwTTiifG Glass 262 Glass - ib. Cutting-board - ib. Process of cutting 263 Edging the slides 264 On cutting thin glass for covers 265 To cut circular and oval covers - - 266 CHAPTER III. Method of Ckmekting Cjells - - 270 Method of cementing cells without heat - ib. Cementing cells by heat - 271 To cement cells with Canada balsam 273 CHAPTER IV. Oir Cements - - . . 274. Japanners' gold-size - - ib. Sealing-wax varnish - ih. Asphaltum 275 Canada balsam - ib. Marine-glue - - ib. Electrical cement 276 Diamond cement - ib. Suggitt's Hquid jet 277 Coachmakers' varnish ib. Black japan - - ib. CONTENTS. xvii CHAPTER V. Page Ok Peeskrvative FXiUiDs - . 278 Spirit and distilled water ib. Acetate of alumina - ib. Goadby's fluids ib. Solution of creosote 279 Thwaites' fluid jb. Glycerine 280 Castor oil jb. Chromic acid - . ib. Salt and water - ib. Naphtha 281 General directions - - ib. CHAPTER VI. Method of Mounting Objects in Fluid - 283 The concave cell - - 28C White lead cell - ib The thin glass cell - - - 291 Drilled cells - - 29£ Built up cells - 294 Method of mounting objects in deep cells ... 29/ CHAPTER Vn. Method of Mounting Objects in Canada Balsam - - 301 Preliminary directions ... ib Necessary apparatus - - - - 30! Canada balsam ... - ib Wooden forceps ... - 30! Metal forceps ..... 30' Needle-point - . - - . ib Spirit and solar oil-lamp - - 30i To mount sections of wood ... 30( Animal structures ----- 30! Fossil infusoria, &c. - - - 30! Foraminifera, &c. - - - - - 31i Cleaning balsam from the slides - - - 31 CHAPTER VIII. Method of Mounting Objects in the Dby Way - 31 xviii CONTENTS. Page First method - - '■ ^^^ Second method - - ^14 Barker's method - - - " 317 CHAPTER IX. Mounting Opaciue Objects - 318 On discs - - ^°- On cylinders "^21 On sUdes - ^^^ In cells 322 In pill boxes 323 CHAPTER X. To Ma-kb Sections or Bone and Teeth 325 Mounting sections of bone - 328 To make sections of teeth - - 329 To mount sections of teeth - 330 CHAPTER XI. To Make Sections of Shell and other Haed Tissues 331 To make sections of hard vegetable tissues 332 To prepare siliceous skeletons of vegetables - 333 CHAPTER Xn. To Make Sections op Wood . - . . 335 Method of making sections - 338 Methods of mounting sections of wood - - 340 Chippings of wood - - - - 341 Sections of horns, hairs, &c. - - - ib. CHAPTER Xni. On the Dissection and Pbepabation op Vegbtable and Animal Steuctukes . - . . . 343 Dissecting forceps - - - ib. Scissors ... . . 344 Cutting forceps ... 345 Spring scissors ... i^,_ Method of sharpening scissors - - 346 Scalpels - • - - ib. CONTENTS. xix Page Valentin's knife 347 Dissecting needles - 348 Non-cutting instruments ib. Troughs - ib. Loaded corks - - 350 Rests - - 351 CHAPTER XIV. Method of Dissecting Vegetable and Animal Tissues 352 Vegetable tissues - ib. Animal tissues - 353 Dissection of particular tissues 359 Nerve ib. Muscle 360 Tracheae - 361 Spiracles - - 362 CHAPTER XV. Methods of Exhibiting Objects of Interest 363 Circulation of the blood - ib. Method of viewing the circulation in the vertebrata 365 Circulation of blood in the frog - 367 Method of viewing the circulation in the tongue of the frog 370 CHAPTER XVI. On the Cibculation in Plants 373 Chara ib. Method of viewing the circulation 375 Tradescantia virginica — spiderwort 377 Penstemon - 378 Groundsel - - ib. Vallisneria spiralis - 379 Best method of viewing the circulation - . - 380 Method of cultivating chara, vallisneria, &c. - - - ib. Habitat - - - - - 882 CHAPTER XVn. Methods op Procuring iNPusoBr and othbr Animalcules - 383 Localities - - - - ib. Apparatus . - - . 334 XX CONTENTS. Pagu Method of obtaining infusoria - 3b7 Method of obtaining and of keeping hydras - 389 Desmidieae - • - 391 Localities for infusoria 393 The locality of the wheel animalcule 394 Method of feeding infusoria with carmine - 395 Fossil infusoria - 396 Method of preparing fossil infusoria - 397 CHAPTER XVIII. Classification of the most IMPORTANT MiCEOSCOPICAI. OBJECTS 399 Vegetable tissues 400 Cuticles ib. Siliceous cuticles ib. Hairs ib. Cellular tissue 401 Fibro-ceUular tissue ib. Spiral vessels 402 Starch ib. Eaphides ib. Ducts of various kinds 403 Woody fibre 404 Fossil woods 405 Hard tissues ib. Algae 406 Mosses ib. Ferns 407 Pollen ib. Spores ib. Seeds 408 Miscellaneous structures of a fibrous character ib. Method of viewing spiral fibres in the testa of the seeds of salvia collomia, &c. , 409 Animal tissues 411 Siliceous skeletons of recent and fossil infusoria ib. Recent infusoria ib. Fossil infusoria - 412 Sponges 413 Alcyonium 414 Gorgonia 415 Corals ib. Zoophytes ib. Insects 416 CONTENTS. xxi Pnge Eyes of insects, arachnida, and cnistacea 417 Feet of insects, &c. ;i,. Hairs of insects, &c. 418 Parts about the mouth of insects, &c. ib. Parasitic insects 41 9 Method of obtaining the acarus or itch insect 421 Scales of insects 423 Spiracles and tracheae of insects ib. Stings 424 Stomachs ib. Preparations from the higher animals 425 Blood ib. Bone - 426 Teeth 427 Fossil teeth 428 Shell ib. Scales of fish 429 Hairs • 432 Skin 433 Eyes 436 Muscular fibre 437 Mucous membrane 438 Epithelium 439 Method of viewing the ciliary movement - 440 The basement membrane - 442 Method of examining the surface of mucous membranes 443 Objects for polarized light 447 Selenite 449 Crystals of salts ib. Currents in fluids observed during their evaporation 451 Minerals 452 Biniodide of mercury 453 Tongue of whelk and limpet 454 CHAPTER XIX. Methods oe Examining Moebid Structures, &c. 456 CHAPTER XX. Test Objects 458 Angle of aperture 461 Methods of measuring the angle of aperture 464 List of test objects 466 XXU CONTENTS, Page Bat's hair 467 Mouse liair 468 Hair of Dermestes 469 Scales of Hipparchia janira >b. Scales of Pontia brassica 470 Scales of Polyommatus Argiolus ib- Scales of Podura ib. Scales of Lepisma saccharina 472 Scales from the gnat's wing - ib. Battledoor scale of Polyommatus Argiolus ib. Scale of Morpho Menelaus 473 Navicula Hippocampus ib. Navicula Angulata 474 Nobert's tests 475 Method of examining test objects 479 Method of using the adjusting object-glass ib. CHAPTER XXI. Miscellaneous Hints on the Management of the Microscope AND MiCKOscopic Preparations. Apartment 483 To clean the optical part of the microscope 484 Glass sKdes, to clean ib. Cabinets and boxes for holding microscopic objects ib. Labelling slides 486 Mr. C. Brooke's method of viewing opaque objects 487 of making thin glass cells - 488 of mounting opaque objects in fluid ib. of cementing cells ib. of erecting the object for drawing, &c. ib. appp:ndix. Mr. Highley's achromatic gas lamp - 489 M. Nachet's microscope for chemical observations 490 Mr. Hett's microscopes for injections, &c. 492 Mr. Ross's improved achromatic microscope 495 Mr. Kingsley's illuminator 499 Messrs. Smith and Beck's improved microscopes 500 Scale of Amathusia Horsfieldii 501 New test objects - 502 PART I. MECHANICAL ARRANGEMENTS. PRACTICAL TREATISE ON THE DSE OF ERRATA. Instead of Table, page 466, read the following, viz. Object Glasses, 1 1 i i k Angular Aperture. 2 inches 12 degrees 15 22 57 75 105 108 150 Magnifying Powers with the various Eye- Glasses. 20 30 40 60 80 100 >} 33 33 100 130 180 220 350 500 ?J jj 33 420 670 900 650 900 1250 1 6C 12C 33 220 620 1200 2000 A.D, 65, writes that small and indistifroroujects DeconrerKO^r- and more distinct in form when seen through a globe of glass filled with water.* Pliny, who died in a.d. 79, mentions * " Literse quamvis minuts et obscurse, per vitream pilam aquS plenam, majores clarioresque cernuntur." — Nat. Qiioest, lib. i., cap. 7. 1 A PRACTICAL TREATISE ON THE USE OF THE MICEOSCOPE HISTORY OF THE MICROSCOPE. The term microscope, derived from the two Greek words fiiKpoQ small, and aKOTEta I view, and said to have been first suggested by Demisianus, is applied to an instrmnent which enables us to see distinctly and to investigate objects placed at a short distance from the eye, or to see such minute objects as, without its aid, would be invisible. The early history of this instrument, like that of many others of a scientific nature, is involved in considerable obscurity, so that not even the time of its discovery, nor the name of the discoverer, can be fixed on with any degree of certainty; but as, in its most simple form, the microscope consisted of little or nothing else than the magnifying power or lens, which must of necessity have been made of glass or some other transparent and highly refracting material, its invention may with safety be referred to a period anterior to the Christian aera. Aristophanes, who lived five centuries before Christ, speaks in his Clouds of a burning sphere. Seneca, who was bom during the first year of the Christian aera, and died A.D. 65, writes that small and indistinct objects become larger and more distinct in form when seen through a globe of glass filled with water.* Pliny, who died in a.d. 79, mentions * " Literse quamvis minutse et obscurse, per vitream pilam aquft plenam, majores clarioresque cernuntur." — Nat. Qucest, lib. i., cap. 7. 1 2 PRACTICAL TREATISE ON the burning property of lenses made of glass. Ptolemy, the celebrated astronomer of Alexandria, who flourished in the latter part of the first century, was evidently cognizant of the existence of magnifying glasses, and makes use of the word refraction in his work on optics. The testimony of these ancient writers, however, is only important as proving the existence of the microscope in its 'most simple and rudi- mentary form, viz., as an instrument composed of a single magnifying glass or sphere, whose chief application appears to have been that of concentrating the heating power of the sun's rays. It is, however, certain that the simple micro- scope, if we apply this term to every instrument used for magnifying objects, first consisted of a sphere of glass or globe, of the same material, filled with water ; these, no doubt, were soon superseded by lenses of a bi-convex figure, for, according to Dr. Francis Kedi, the latter were in use early in the fourth century. To our countryman, Roger Bacon, who was born at the commencement of the thirteenth century, is attributed the invention of the telescope, the camera obscura, the reading glass, and gunpowder, and, by some, the discovery of the microscope, as he speaks, in his Opus Majus, of principles applicable to it; Record, in his work, entitled Chemin de la Science, published in 1551, relates that Bacon, whilst at Oxford, made a glass which exhibited such curious things, that its effect was generally attributed to some diabolical power. Some centuries were suffered to elapse before the microscope was again noticed, and then we read of it in its improved or compound form, as being supplied with two or more magnifying powers. Several authors, especially Huyghens, assign the invention of the compound microscope to Cornelius Drebbel, a Dutchman, in the year 1621, whilst Fontana, a Neapolitan, claims the discovery for himself in 1618. According to Borellus, it was invented by Zacharias Jansen or Zansz, or his father Hans Zansz, spectacle-makers at Middleburg, in Holland, about the year 1590; they are said to have presented the first microscope to Charles Albert, Archduke of Austria. " One of their microscopes," says Sir D. Brewster, in his Treatise THE MICROSCOPE. 3 on Microscopes, page 2, "which they presented to Prince Maurice, was in the year 1617 in the possession of Cornelius Drebbel of Alkmaar, who then resided in London as mathe- matician to King James I., in which place he made micro- scopes, and passed them oif as being of his own invention." These instruments were said to be six feet in length, and consisted of a tube of gilt copper, one inch in diameter, sup- ported by thin brass pillars, in the shape of dolphins, on a base of ebony, which was adapted to hold the object to be examined ; nothing, however, is known of their internal con- struction, they were, probably, nothing more than telescopes converted into compound microscopes, and there is little doubt that they were similar to the one which ^pinus has described in a letter addressed to the Academy of Sciences of St. Petersburg. We are also told by Viviani, an Italian mathematician, in his Life of Galileo, " that this great man was led to the discovery of the microscope from that of the telescope, and that, in 1612, he sent one to Sigismund, King of Poland;" he adds, "that this philosopher worked twenty years at his apparatus in order to perfect it." But, notwith- standing all the above conflicting statements, the credit of the invention of the compound microscope is given (in this country at least) to Zacharias Jansen, in 1590. Leaving then the region of uncertainty, let us now direct our attention to matters of a more tangible nature. With the foundation of the Koyal Society, in 1660, may be said to have commenced a new sera in optical science, for not only do we now find new microscopes described, but the early volumes of the Trajisactions literally teem with improvements in the construction of these instruments, and with discoveries made through their mediima. One of the first contributors appears to have been the celebrated Kobert Hooke, who, as early as the year 1667, published a work " on some physiological de- scriptions of minute bodies made by magnifying glasses," entitled Micrographia, which may be fairly styled one of the wonders of the day; it is illustrated with 38 plates, and was ordered for publication November 23rd, 1664, but did not appear until three years afterwards. 1* PRACTICAL TREATISE ON The microscope used by Hooke was a compound one with three lenses, and is ihown at fig. 1, and also n the sixth figure of the lirst plate of his work, in vhich figure it wiU be perceived that he likewise ■epresents a method of iUu- ninating opaque objects, practised even at the pre- sent day, the plan being to place a globe of glass filled with salt water or brine immediately in front of the lamp, the pencil of rays from the globe is received by a small planoconvex lens, placed with its convex side nearest the globe, by which the pencil is condensed upon the object. Hooke also informs us of an accurate method of finding the magnifying power of a com- pound microscope, than which a better plan has not been suggested in modern times, and as it would be difficult to make his description shorter or more intelligible than it is, his own words will here be transcribed : — " Having rectified the microscope to see the desired object through it very distinctly, at the same time that I look upon the object through the glass with one eye, I look upon other objects at the same distance with my other bare eye; by which means I am able, by the help of a ruler divided into inches and small parts, and laid on the pedestal of the microscope, to cast, as it were, the mag- nified appearance of the object upon the ruler, and thereby exactly to measure the diameter it appears of through the glass, which being compared with the diameter it appears of to the naked eye, will easily afford the quantity of its mag- nifying." To Hooke also belongs the merit of having first made globule lenses of high power, an invention which Hart- soeker has also claimed; but if the dates of the works of these respective authors be consulted, it will be seen that the Micrographia of Hooke was published in the same year that THE MICROSCOPE. 5 Hartsoeker was bom. Hooke describes exceedingly well the process of making globule lenses, which is as follows: — "If you take a clear piece of Venice glass, and, in a lamp, draw it out into fine threads, and then hold the ends of these threads in the flame, until they melt, they will run into a small round globule or drop, which will hang to the end of the thread; having made a number of these, they are all to be stuck upon the end of a stick with a little sealing-wax, with the threads standing uppermost ; these ends are to be ground off first on a whetstone, and then polished on a metal plate with tripoli. The lenses thus finished, if placed against a small hole made in a thin piece of metal, and fixed there with wax, will both magnify and make some objects more distinct than any of the great microscopes can do." The optical part of the microscope of Hooke consisted of a small object-glass, a field-glass, and an eye-glass; when he wished to examine the parts of an object more accurately, he removed the middle or field-glass, and by that means he states he obtained more light and better definition. The compound body was of the shape represented by fig. 1, and when shut up was seven inches in length, and three inches in? diameter, but was capable of being drawn out like a telescope;, being supplied with four tubes or slides ; it was also capable of being inclined at any angle by means of a ball and socket joint, as represented by fig. 1. Coeval with Hooke were Eustachio Divini, of Rome, and S. Campani, of Bologna, the former of whom, in the year 1668, published, in the Philo- sophical Transactions, an account of his microscope, which consisted of an object-glass and field-glass, like that of Hooke, but, instead of a double convex eye-glass, he substituted two planoconvex lenses, which touched each other in the middle of their convex surfaces ; by this arrangement a flat field of view was obtained, at the same time with a considerable amount of magnifying power. It is said,* that the compound body of this instrument, when shut up, was sixteen inches long, and as large in circumference as a man's thigh, and that the eye-glass was equal in size to the palm of the hand ; ita * Chevalier Des Microscopes et de leur usage, p. 15. PEACTICAL TREATISE ON power was increased by draw tubes from 40 to 140 times. The latter, S. Campani of Bologna, was also a maker of telescopes and microscopes, and a successful rival of the former, his instrument was somewhat similar to that made by Divini, being on the principle of an inverted telescope. Campani'a lenses are said to have been worked on a turn-tool, and not moulded. In 1672, we find that S. P. Salvetti made micro- scopes in imitation of those of Divini and Campani, which were found to far exceed those of the above-mentioned artists in their magnifying and defining powers ; but we are not told in what points of construction these instru- ments differed from those of his prede- cessors. In the year 1673, the name of the immortal Leeuwenhoek first appears in the Philosophical Transactions of this country, as a discoverer of nu- merous wonders by aid of the micro- scope; his instruments, which were composed of single lenses, are said to have been greatly superior to all that had been pre- viously made. According to Baker, they were also remarkable for their simpUcity, each one consisting of a single lens set between two plates of silver, perforated with a small hole, with a moveable pin before it, to place the object on and adjust it to the eye of the beholder. "It has been stated by many authors," says Baker ( On Microscopes, vol. ii.), « that the magnifiers used by Leeuwenhoek were globules or spheres of glass, like those invented by Hooke, but such is not the case ; he assures us that in the cabinet of the twenty-six micro- Fig. 2. THE MICROSCOPE. scopes, left by that famous man at his death to the Royal Society as a legacy, each instrument has a double convex lens, and not a sphere or globule." An account of these microscopes was drawn up by Baker, in 1740, and published in the Philosophical Transactions for that year. Fig. 2 is a front view of the instrument, and fig. 3 a back view, both being of the exact size of the original: a, fig. 2, represents a flat plate of silver, which is ri- vetted to fig. 3 by rivets, b b b; between these plates a small double convex lens is let into the socket, and a hole drilled in each plate for the eye to look through the lens at c ; a limb of silver, d, is fasteiied to the plate, a, by a screw, e, this has another piece of silver joined to it at right angles, f, fig. 3, through this a long fine-threaded screw, g, runs, which turns in and raises or lowers the stage, h, whereon is fastened a pin, i, for the object to be attached to ; this pin can be turned about by the little handle, k, and the stage itself is adjusted to or from the lens by the screw, I, which passes through the stage in a horizontal position, and when the screw is turned, the stage is forced from or brought nearer to the lens at c. " All the parts of these microscopes," says Baker, " are of silver, and fashioned by Mr. Leeuwenhoek's own hand, and the glasses, which are excellent, were all ground and set by himself, each instrument being devoted to one or two objects only, and could be applied to nothing else. This method induced him to make a microscope with a glass adapted to almost every object, till he had got some hundreds of them. The highest magnifying power was 160 diameters, and the lowest 40." About this time, the end of the 17th century. Sir Isaac Newton was in the zenith of his glory; having discovered, in 1672, the theory of light and colours, he was led to the im- provement of the telescope, by substituting mirrors for lenses, and he commences his memorable paper in the Philosophical Transactions with these words: — "When I had found that light consists of rays differently refrangible, I left off my glass works, for I saw that the perfection of telescopes was hitherto limited not so much for want of glasses truly figured, 8 PRACTICAL TKEATISE ON as because that light itself is a heterogeneous mixture of dif- ferently refrangible rays." Having constructed a telescope on the reflecting principle, Newton was soon led to apply the same principles to the microscope, and we find that m^ the year 1672 he invented the first compound reflecting micro- scope, since so greatly improved by Amici, Cuthbert, and Dr. Goring. Newton also suggested that the compound refracting microscope would be rendered more perfect "if the object to be viewed were illuminated in a darkened room by light of any convenient colour not too much com- pounded;" in fact, monochromatic hght. In the year 1698, Philip Bonnani, in his work entitled Observationes circa viventia, quae in Rebus non viventibus repe- riunfur, describes a compound microscope in use by him. This microscope, which is represented by fig. 4, was placed Fig. 4. on a stand in the horizontal position, and was provided with a stage for the objects ; and, with a coarse and fine adjustment to the compound body, the former was obtained by means of a rack and pinion, which moved the entire frame-work, supporting the compound body, whilst the latter was efiected by a screw in the end of the body itself near to the object glass; and to steady the opposite end of the THE MICROSCOPE. body, a triangular support was provided, on which the body was readily turned. In order to make the light of a lamp, or even day%ht, more efficient, this instrument was supplied with a short tube, in which were two double convex lenses, as in a magic lanthorn, which served to condense the light upon the object. A work entitled Oculus Artificialis Tehdioptncvis, &c., was published at Nuremberg, in 1702, by Jean Zahn, in which were contained numerous curious aphorisms, and a description of many compound microscopes, and amongst others, two binocular ones, and also a figure of the microscope of Francis Grindelius, represented by fig. 5. It will be seen that this instrument was used for opaque objects, and that its optical part consisted of six planoconvex lenses, but of its size we have no record. About this period, 1696, we find that Mr. Stephen Gray of the Charterhouse {Philosophical Transactions, No. 221, p. 280), suggests that globule lenses should be formed of small pieces of glass melted into a globule on Charcoal by means of a blow-pipe ; but finding that he could not always succeed, and that on the side upon which they rested on the charcoal they were more or less flattened or opaque, he was led to the construction of his water microscope; this was nothing more than a drop of that fluid lifted up with a pin and deposited in a small hole in a piece of brass. The drop retained nearly a spherical form, and showed objects with some degree of distinctness. He subsequently contrived the apparatus represented by fig. 6, to be used as a water microscope: a 5 is called the frame of the microscope, and was made of brass one-sixteenth of an inch thick ; at a is a small hole one-thirtieth of an inch in Fig. 5. 10 PRACTICAL TREATISE ON Fig 6. diameter, to contain the water, which can be dropped into it by a pin or large needle, and there forms a double convex lens of water : c d e is another piece of brass, well hammered, so as to be springy, and called the object supporter; it is attached to the plate a b, by the screw e; it has a point for opaque objects Atf, and a hole for fluids at c, both of which can be brought opposite to the lens a, and can be made to approach or recede from the lens by turning the screw g in the round plate. This screw is attached to the object supporter cde, and passes throught it to the plate a b, against which it works. The supporter, being made springy, obeys readily the turns of the screwy. Mr. Stephen Gray was also the inventor of a simple reflecting microscope, represented by fig, 7. A represents a brass ring, one-thirtieth of an inch thick, whose inner diameter is about two-fifths of an inch. Having dissolved a globule of quicksilver in one part nitric acid and ten parts water, he rubbed with it the inner surface of the ring, which became silvered : having wiped it dry, he put a drop of quicksilver within it, this, when pressed with the finger, adhered to the ring, and formed a convex speculum. When the ring was taken up ^^ I I — ^^^ carefully and laid on the margin of the h- III,,! J, ^ cylinder B, the mercury sank down and 'W^^^^^^Piiii/ formed a concave reflecting speculum. The cylinder B is supported by a pillar, attached to the foot D; CC, F, G, repre- sents a stage, which is capable of being raised or depressed by the screw on the pillar. The object is placed on the ring G, and is adjusted to the focus of the speculum by the abovementioned screw. This ingenious gentleman, also in May, 1697, suggested the Fig. 7. THE MICROSCOPE. 11 plan of making lenses by letting drops of water fall on pieces of plane glass which form themselves into planoconvex lenses, and he found that they magnified greatly; but as the fluidity of the water obliged him to keep the glass horizontal, he was led to try isinglass dissolved in hot water, whereby the drops, when cold, although less transparent than the pure water, nevertheless allowed of these lenses being used in any position, a plan which many years after was followed up and greatly improved by Sir David Brewster, who employed minute drops of varnish or other viscid fluids placed on the thin pieces of flat ' glass. When the lens so formed was required to be very convex, the glass was held so that the drop was downward ; but when less convex, then the drop was allowed to dry with the plate of glass downwards. In the year 1702, we find in the Philosophical Transactions a description of the pocket microscope of Mr. J. Wilson, who, following the opinion of Hooke, that single magnifying glasses, when they can be used, are preferable to microscopes composed of two or more magnifying glasses, was led to the construction of this in- strument, which, from its fre- quent mention by Baker and other authors, appears to have had a far-famed cele- brity, and, indeed, many specimens of it are still to be met with ; one of the ear- liest forms of this instrument is represented by fig. 8. The body, A A A A, which was made either of ivory, brass, or silver, was of a cylindrical figure, and about two inches in length, and one inch in diameter ; to the lower end, B, the magnifiers are adapted, whilst into the upper screws a piece of tube, D, having at 12 PRACTICAL TBEATISE ON the end, C, a convex glass, and on its outside a male screw. Three thin plates of brass, E, are made to slide easily in the inside of the body to form the stage, one of these plates, F, is bent semi-circularly ia the middle, for the reception of a tube of glass, for viewing the circulation of the blood in small fish, whilst the other tw6 are flat, and between these last all the object sliders are introduced ; between the stage and that end of the body into which the magnifier screws is a bent spring of wire, H, this answers the purposes of keeping the objects fixed between the plates of the stage, and of pressing the stage firmly against the screw-tube. The magnifiers suppUed with this microscope were eight in number, and the objects were adjusted to their focus by the screw-tube, D, for which purpose the screw was made of nearly the same length as the body. This instrument was held in the hand in such a position, that the direct hght from a candle or lamp might pass directly into the condensing glass; it was subsequently much improved by the addition of a spiral spring, instead of the curved one, and of a handle which screwed into the body at right an- gles to its length, and served the purpose of keep- ing the body in the hori- zontal position, Mr. "Wilson was also the inventor of a microscope for opaque objects, represented by fig. 9; this consisted of a thin piece of flat brass, B, about six inches long and half-an-inch wide, one end of which served as a handle, and to the other. A, the mag- nifier was screwed; con- nected with the middle of this piece of brass by a hinge was a jointed arm, PP, Fig. 9. THE MICROSCOPE. 13 carrying at its free extremity a sliding wire, G, to one end of which was attached a pair of forceps, I I, and to the other a small disc of ivory, H, blackened on one side and white on the other; the arm was capable of being adjusted to or from the lens by means of a screw, C, having a nut with a milled head, D ; the spring, E, served to keep the lens holder, A B, in contact with the nut; this form of in- strument is in use at the present day, and a modification of it was adopted by the celebrated Lieberkuhn about forty years afterwards. ' The wonderful discoveries of Leeuwenhoek made by the single microscope, gave to this kind of instrument an universal reputation, and we find, accordingly, that the compound form was laid aside for a time, and the pocket microscope of Mr. Wilson was in great demand. Upwards of thirty years, however, were suffered to elapse before any step was taken (in this country, at least) towards the improvement of this instrument ; the compoimd mi- croscope, then in use, was the con- trivance of Mr. John Marshall, and, from its un- wieldy natiure, was very little em- ployed. It was. Fig. 10. however, the first of the compound kind made for sale in England, and is 14 PRACTICAL TREATISE ON represented by fig. 10. It consists of an octagonal base of wood, z, which supports a square pillar of brass, I k, having a ball and socket joint at m. On the pillar, I k, an arm, d, carrying the compound body a^aP', is made to slide up and down, and above it another smaller arm, g, which has a screw for tightening it at h; f is a long screw attached to the arm, d, carrying the compound body, and when the arm g is fixed by the screw h, the nut i will raise or depress the compound body ; p is the stage, which is fixed to the piUar by the arm n n and the nut o, a fish is laid on it for examination ; r is a convex lens, for concentrating" on the stage the rays of light from the candle, s, which was placed on a stool, or on the ground, whilst the microscope stood on the edge of a table; v is termed a leaden coffin, for putting over the fish to keep it from moving. The optical part of this microscope consisted of two convex lenses, forming the eye-piece in the compound body, and of six magnifiers, which could be screwed to the tube c. The pillar, I k, was marked with the numbers 1, 2, 3, &c., to show the respective distances of the magnifiers from the object. There was no mirror to this microscope, but direct light could be used when the body, by means of the ball and socket-joint, was turned horizontally. A drawer, t, in the stand, z, served to contain the magnifiers and other appa- ratus. This instrument was subsequently much improved upon by Mr. Culpeper and Mr. Scarlet, and will be pre- sently described. In 1738, a new aera in microscopic science presented itself, viz. : the invention, by Lieberkuhn,* of the solar microscope, and of a concave silver speculum for viewing opaque objects, which still bears his name, both of these instruments were subsequently greatly improved upon by our countryman, Mr. Cuff. The solar microscope, as invented by Lieberkuhn could not be employed unless the sun's rays fell directly upon a condensing lens, therefore its use was limited to a short portion of the day. Cuff, however, applied a moveable mirror to it, and made it more available for general use. * Dr. Nathaniel Lieberkuhn of Berlin. THE MICROSCOPE. 15 Lieberkuhn himself exhibited his microscopes to some Fellows of the Eoyal Society of London in 1739. The solar microscope, as improved by Mr. Ouff, for a length of time created great wonder and astonishment; it was principally used for the exhibition of animalcules, and the circulation of the blood in the newt, frog or eel; and was also recommended for getting the exact figure of objects on a large scale, the image being received upon a screen of paper, on which the outline was traced either with a pen or pencil ; when the paper was sufficiently thin, the artist, standing behind the screen, was enabled to draw the image much better than when standing in front of it, and with this great advantage, that the shadow of the hand did not interfere with, or obstruct, any portion of the light. By far the most useful of Lieberkuhn's microscopes, how- ever, was the one for viewing opaque objects, by means of which he made so many important discoveries in the minute structure of the mucous mem- brane of the alimentary canal, as to im- mortalize his name. The most simple form of this instrument is represented by fig. 11 ; it is not unlike the pocket micro- scope of Wilson, represented by fig. 9, being also held in the hand by the handle, ^; a is a flat piece of brass attached to the handle, JO, it supports the lens holder, i, and through it passes the screw, b, which is connected to the back-plate, c; a spring, e, keeps the plates a, c, apart, and the nut, d, adjusts the lens to the focus of any object placed either on g or h. But the chief point of merit in its construction consists in a concave speculum of silver, k, highly polished, to the centre of which the magnifying glass, /, is adapted ; this being screwed into the ring, i, and the object being fixed upon the point, ff, or held in Fig. 11, the forceps, h, the instrument is placed 16 PRACTICAL TREATISE ON 1 in such a position, that the light from ^e sun, or bright cloud, being received upon the speculum, the rays are con- centrated upon it, and it becomes brightly illuminated, and is adjusted to the focus of the lens by turning the nut, d; all loss of time in the screw being prevented by the spring, e. The speculum, is that part of the instrument which is the most important, and is in general use even in the present day. Lieberkuhn was also celebrated B for his beautiful injections of the minute tissues and organs of verte- brate animals ; many specimens of which are still extant. ^ In the museum of the Royal College of Surgeons of England, there is a small cabinet of two drawers, con- D taining twelve of these valua- ble relics, each injection being A provided with a separate mi- croscope, of the form shown by fig. 12. A B represents a piece of brass tube, about an inch long, and an inch in diameter, pro- vided with a cap at each extremity, the one at A carries a small double convex lens of half an inch in focal length, whilst the one at B carries a condensing lens three-quarters of an inch in diameter. A vertical section of one of these instruments is seen at fig. 13. A represents the magnifier, which is lodged in a cavity, formed by the Fig. 13. Fig. 12. THE MICROSCOPE. 17 cap A and partly by the silver cup or speculum I. In front of the lens is the speculum I, a quarter of an inch thick at its edge, having a focus of half an inch, and in front of this there is a disc of metal c, three-eighths in diameter, connected by a wire with the small knob D ; upon this disc the injected portion is fastened, and is covered over with some kind of varnish which has dried of a hemi- spherical figure. Between this knob and the inside and out- side of the tube there are two slips of thin brass, which act as springs to keep the wire and disc steady. When the knob is moved, the injected object is carried to or from the lens, so as to be in its focus, and to be seen distinctly, whilst the condensing lens B serves to concentrate the light on the speculum. To the lower part of the tube a handle of ebony, about three inches in length, is attached by a brass ferrule and two screws. The use of this instrument is obvious; it is held in the hand in such a position, that the rays of light, from a lamp or white cloud, may fall on the condenser B, and by it be concentrated on the speculum I, which again further condenses them upon the object on the disc C; the object, so illuminated, can readily be adjusted by the little knob D, so as to be in the focus of the small magnifier at A. The injected preparations in these twelve microscopes, now nearly a century old, are remarkably beautiful, and the only injury which they have sustained, is that of the cracking of the varnish. Lieberkuhn's principal researches were confined to the minute structure of the mucous membrane of the alimentary canal ; and for the investigation of these opaque parts, he is said to have invented the silver speculum bearing his name, although, from a description and figure in the works of Leeuwenhoek,* one would be inclined to suppose that that illustrious man was cognizant of its prin- ciples and use. The other microscope which Lieberkuhn used for the ex- amination of the mucous membranes, and the circulation of the blood and chyle in the mesentery of small animals, is repre- * Vol. ii. p. 280, Works by Hoole. 2 18 PRACTICAL TREATISE ON sented by figs. 14 and 15, and will be found to be accurately described in a work entitled DUsertationes quatuor Johannis N. Lieberhihn, col- lected and revised by John Sheldon, sur- geon, 1782. It con- sists of a plate of copper or brass, about one-eighth of an inch thick, twelve inches long by eight broad, and fashioned into the shape represented by the figures. It is sup- ported, in a vertical position, on a tripod stand, the back of the instrument is repre- sented by fig. 14, and the front by 15. At each corner there is a small sliding wire, H H, with a hook at one end, and opposite to the three holes in the plate marked A B and C are four smaller hooks, h h, the former are for the purpose of fixing into the legs of any small animal, the circulation in whose mesentery, either of the blood or of the chyle, is about to be examined; and the latter, or the small hooks, are used for bringing successive portions of the mesentery opposite the holes. The part of the microscope carrying the magnifying powers is attached to the plate by pegs ; it consists of a thin plate of brass, 1, fig. 15, to which plate is attached another, 2, by a rivet, 3, this last plate is a little curved, and is also made elastic; in its centre is a screw, 4, and at its free end is a hole, 5, into which the magnifier screws; a section of this part of the microscope is seen in fig. 16, where A re- Fig. 14. THE MICKQSCOPE. 19 presents the connection of the Wo plates by a rivet, and B the bend in the top or lens holder, C the screw for adjust- ment, and D the hole into which the lens E screws. The Fig. 15 Fig. 16. animal being properly secured by the large hooks, and the portion of it to be examined being brought before the hole B by the small hooks, the microscope is so placed, that the light from a window or lamp may pass through the hole, the arm provided with the lens being brought opposite the hole, and over the piece of brass, 1, the lens can be adjusted to or from the object by the screw, 4, and the plate being curved and elastic, will always obey the turns of the screw. This form of instrument appears not to have been constructed for sale in this country, and was never improved upon like the solar microscope, or that for opaque objects. It, no doubt, was entirely superseded by others more generally useful. 20 PRACTICAL TREATISE ON At this time, 1740, we find many makers of eminence residing in the metropolis, amongst whom, the names of Cuff, Benjamin Martin, and Adams, require especial notice. Mr. Cuff has already been mentioned as the improver of the solar microscope and of that for opaque objects, both of which were of Lieberkuhn's invention, and we find that in the year 1747 he improved for Martin Folkes the pocket microscope of Wilson, by fixing it to a stand, and by adding a mirror to it; he subsequently improved the stand by mounting the lens on a moveable arm, and making the stage to slide up and down on a square stem ; the instrument in this im- proved form was used by EUis in his examinations of coral lines, and a figure and description of the same is given in his work on Zoophytes, published in 1756. The cumbrous compound instru- ment of Mr. Marshall, before described in page 13, was, in 1750, improved by Mr. Culpeper, and Mr. Scarlet; they first em- ployed a concave mirror for re- flecting the light through the object and the compound body. Their instrument is represented by fig. 17, it was composed of two tubes, a h, either of wood or paper, sliding one within the other; to the tube a were attached the pillars c d, c d, which rose from the base, e, and supported the round stage, g, in which was a large circular hole for a spring object holder to be fixed, and some smaller holes for the re- ception of the forceps, small condensing lens and fish-pan. Fig. 17. THE MICROSCOPE, 21 To the inner tube, b, all the optical apparatus was adapted, the magnifiers, from four to six in number, being screwed to the end of the small tube, i, and the eye-piece, which consisted of two convex lenses, being fitted into the wooden top of the com- pound body, b. A concave mirror, k, was used for reflecting the light, and a drawer, f, in the base, e, served to contain all the magnifiers and other parts of the apparatus. The only adjustment for focus with which this microscope was pro-' vided, was that accomplished by sliding the tube b up and down in the outer tube a, the tube b being marked with lines at h, to denote the distances through which it should be moved for the different magnifying powers. This instru- ment was subsequently much improved in shape, and was made either of brass or silver, and a rack and pinion were used for the adjustment. It was in great demand at one time, and, with its pyramidal case and drawer with apparatus, may even now be frequently seen exposed for sale. This microscope was styled the double reflecting one, and was the first instrument to which the concave mirror was apphed for illuminating transparent objects, the mode of mounting it being similar to that now adopted. In the year 1744, we are told by Baker,* — "That the microscopes of Hooke and Marshall having been reduced to a manageable size, improved in their structure, and supplied with an easy way of enlightening objects by a speculum under- neath, and, in many other respects, rendered agreeable to the curious, by Mr. Culpeper and Mr. Scarlet. Some further alterations were, however, wanted to make this instrument of more general use, as I fully experienced in 1743, when examining daily the configurations of saline substances, the legs were continual impediments to my turning about the slips of glass, besides pulling the body of the instrument up and down, was likewise subject to jerks, wliich caused a difficulty in fixing it exactly at the focus : there was also no good contrivance for viewing opaque objects. Complaining of these inconveniences, Mr. Cuff, the optician, applied his thoughts to fashion a microscope in another manner, leaving * Vol. ii., Employment for the Microscope, p. 422. 22 PRACTICAL TREATISE ON the stage entirely free and open by taking away the legs, applying a fine threaded screw to regulate and adjust its motions, and adding a concave speculum for objects that are opaque." This microscope was made entirely of brass, and was fastened to the top of a box, by a scroU or bracket, from which rose two flattened pillars, one having a horizontal arm, was made to sHde up and down against the other, and carried the compound body, the coarse adjustment of which was effected by this movement, but the fine, by a screw two inches in length, fixed to the back of one of the pillars, and when its nut was seciu*ed by a screw, which clamped the sliding piUar, then the body could be moved slowly up and down. The stage, somewhat of the shape of a cross, had several holes in it, for the reception of the condensing lens, forceps, and fish-pan. The lower part of the compound body was cylindrical for the space of two or more inches, and marked with numbers corresponding to those of the lenses, upon this, a Lieberkuhn with a long tube was made to slide, and when set to the figures there marked, an object placed on the stage would be in its focus. In the year 1747, Mr. Cuff invented a micrometer for this instrument; it was made of a lattice of fine silver wires, distant from each other one-fiftieth part of an inch, intersecting at right angles, and so placed in the focus of the eye-glass, as to divide the whole visible area of the microscope into squares, whose sides were each one-fiftieth of an inch. The microscope of Benjamin Martin, described in a work published at Reading in 1746, was of the compound form, and adapted for being carried in the pocket ; it was of a cylindrical shape, like the body of Culpeper's, and, like it, the adjustment was made by sliding one tube within the other, the mirror was placed in the bottom of the tube in an inclined position, and was not capable of being moved. It was also supphed with a screw micrometer of a peculiar construction, which had, on the out- side of the body, a dial-plate and hand resembhng the face of a watch. To this ingenious optician we are indebted for the invention of the hand magnifier, with one or more lenses, which has undergone little or no change since his time. We THE MICROSCOPE. 23 are told that Benjamin Martin greatly improved the micro- scope of Cuff before described, by the addition of a joint, so that the compound body might be inclined to any angle, and also by the setting of all the lenses in a circular disc of brass, which was capable of being revolved in such a manner, that each lens in succession might be brought under the com- pound body ; this did away with the necessity of screwing and unscrewing when the powers were required to be changed. The compound body could be removed from the lenses, and the lenses themselves then constituted it a single microscope, the arm which supported them was capable of being moved backwards and forwards by means of a rack and pinion, a plan now in use. In the year 1746, a philosophical instrument maker of some eminence, named George Adams, published a quarto work, entitled Micrographia Illustrata; or, the Knowledge of the Microscope Explained. In this work were contained a description of the nature, uses, and magnifying powers of microscopes in general, together with full directions how to prepare, apply, and examine, as well as preserve, all sorts of minute objects. This work was the first of the kind published in this country, and contributed not a little to the advancement of microscopic science. The microscopes made by Adams were of two kinds, the single and the compound ; their chief peculiarity consisted in the arrange- ment of the lenses, which were six in number, and were all set in a large plate of brass, capable of being turned upon the central piUar of the instrument, and each lens in suc- cession could be brought underneath a hollowed plate or cup, which served as an eye-piece. For the coarse adjustment, the plate was made to sMe up and down the pillar, whilst for the fine a screw was used, which slowly raised or depressed that portion of the pillar to which the stage was attached. Besides these microscopes of his own invention, we find that he was in the habit of making those of Wilson, Lieberkuhn, and Culpeper, aU of which are fuUy described in the work above-named. We have now entered on a period, fertile both in alterations 24 PEACTICAL TREATISE ON of the microscope, and in discoveries made by its agency; we have amongst the former, the results of the labours of Adams, Martin, Baker, and Dellebarre ; and amongst the latter, the works of Trembley, Ellis, Baker, Adams, HiU, Swammerdam, Lyonet, Needham, and Withering. Every optician, says Adams,* now exercised his talents in improving (as he called it) the microscope ; in other words, in varying its construction, and rendering it different from that sold by his neighbour. The principal object seemed to be only to subdivide it and make it he in as small a compass as possible, by which means they not only rendered it complex and troublesome to manage, but lost sight also of the extensive' field, great light, and other excellent properties of the more ancient instruments. In 1770, Dr. HiQ published a treatise, entitled The Construction of Timber explained by the Micro- scope, in which not only were the nature and oflSce of its several parts pointed out, but the way of judging from the structure the uses to which the different kinds could be best applied. This work created a great sensation at the time, and revived the ardour for microscopic pursuits. Adams at this period invented a machine for cutting transverse sections of wood so thin, that they might readily be examined by the microscope. This instrument was subsequently improved on by Mr. Gumming, and with it very beautiful sections were made by Mr. Custance, some of which stand unrivalled even at the present day. In 1771, a new edition of the Micrographia Illustrata of Adams appeared, in which he described a lucernal microscope of his own invention; this was subsequently im- proved by his son, George Adams, in 1774, and served to exhibit opaque as well as transparent objects. The solar microscope, too, at this time had been greatly improved by Benjamin Martin, and was made capable of showing on a screen a magnified image of the surfaces of opaque objects. In 1787, the Microscopical Essays of the younger Adams were published, in which were described aU the instruments at that time in use. Of the single form, we have Wilson's, shown at fig. 8 ; those of EUis, and Lyonet ; also that of * Microscopical Essays, p. 19. THE MICROSCOPE. 25 Fig. 18. Dr. Withering, represented by fig. 18, which even now is manufactured for sale ; it consists of three brass plates, a b c, parallel with each other, to the upper and lower of which three stout wires, d e f, are rivetted; the middle plate, b, forming the stage, is made to slide up and down on these three wires. The upper plate, a, carries the lens, i, the lower one, c, the mirror. Into the stage a dissecting knife, h, a pointed instrument, f, and a pair of forceps, g, are made to fit, and can be readily taken out for use by sliding the stage down nearly to the mirror; this instrument was recommened by Dr. Withering, and was first described in his Botanical Arrangements, its chief merit being its simplicity. The compound microscopes described by Adams, are merely modifications of that of his father, of Culpeper, of Cuff, and of Benjamin Martin. The first, or that of the elder Adams, was improved by the addition of a rack and pinion movement, and by having aU the lenses set in a brass slider, so that they may be placed one after the other under the compound body. The second, or that of Culpeper, was made of brass, and was improved in its optical part. Cuff's compound instrument was much the same as that described at page 22 ; whilst that of Benjamin Martin was improved by Adams himself, and was made capable of receiving a single lens as well as a com- pound body, and was furnished with a cradle joint, by which the compound body could be inclined at any angle; the mirror was double, both plane and concave; the legs, for convenience of package, were made to slide one within the other. With the work of Adams, in 1787, we may close our history of the single and compound microscopes in their unachromatised state, the discoveries at this time were few and comparatively unimportant, and little or nothing more was exhibited by them than the objects contained in the ivory sliders, with which all the above described microscopes were supplied ; and he who 26 PRACTICAL TREATISE ON could exhibit these objects well, was considered a proficient in the art. These instruments, as described by Adams, without any material alteration in the optical part, continued in use up to the time of the invention of the achromatic form, in 1824, but a new and most important jera in microscopic science commenced in this country with the improvement in the reflecting microscope, constructed by Amici in 1815, and with the manufacture of lenses of the precious stones by Sir David Brewster, Dr. Goring, and Mr. Pritchard. At this period it will be necessary to divide our history into two parts. The first to include the improvements made in the single, and the second those in the compoimd micro- scope. In consequence of the great loss of light, and the presence of the prismatic halo enveloping every object seen through the uncorrected compound microscope, the single microscope was generally used by all scientific investigators ; but when high powers were wanted, the glass of which they were made being of such low refractive power, it became necessary to use lenses of very short foci, these were of very small diameters, and allowed only a slight amount of light to enter the eye; to remedy these incon- veniences. Sir David Brewster first suggested the value of usiag other materials of a more highly refracting nature, for the construction of lenses; and he remarked,* ''that no essential improvement could be expected in the single microscope, unless from the discovery of some transparent substance, which, like the diamond, combines a high re- fractive with a low dispersive power. Having experienced the greatest difficulty in getting a small diamond cut into a prism in London, he did not conceive it practicable to grind and polish a diamond lens ; and, therefore, he did not put his opinion to the test of experiment, but he got two lenses, one made of ruby, the other of garnet, which he found to be greatly superior to any lenses that had previously been used." Dr. Goring, in the summer of 1824, having directed the attention of Mr. Pritchard to certain passages in Sir David Brewster's admirable Treatise on New Philosophical " Treatise on the Microscope, p. 13. THE MICROSCOPE. 27 Instruments respecting the value of the precious stones for single microscopes; and having seen their ftJl force, it was agreed that they should undertake to grind a diamond into a magnifier. The first diamond operated on was a small brilliant, and it was proposed to give it the curves that in glass would produce a lens of a twentieth of an inch focus. " This stone, when nearly finished," says Mr, Pritchard, " fate decreed that I should lose,* but having proved the possibility of working lenses of adamant, I set about another, and selected a rose diamond, in order to form a planoconvex lens." After great labour and expense, this Mr. Pritchard so far accom- plished, that on the 1st of December, 1824, he states, "he had the pleasure of first looking through a diamond microscope." Dr. Goring, who tried its performance on various objects, both as a single microscope and as an objective of a com- pound, was well satisfied with its superiority over other forms of lenses. But here Mr. Pritchard's labours did not end, he subsequently found that this stone had many flaws in it, which led him to abandon' the idea of finishing it. Having been prevented from resuming his operations on this refractory material for about a year, Mr. Pritchard, in his third attempt, met with another unexpected defect; he found that some lenses, unlike the first, gave a double or triple image, instead of a single one, in consequence of some of their parts being either harder or softer than others. These defects were after- wards found to be due to polarisation. Mr. Pritchard having learnt how to decide whether a diamond is fit for a magnifier or not, subsequently succeeded in making two planoconvex lenses of adamant, whose structure was quite perfect for microscopic purposes. " One of these," he tells us, " of one twentieth of an inch in focal length, is now in the possession of his Grace the Duke of Buckingham ; the other, of one-thirtieth of an inch focus, is in his own hands." " In consequence of the high refracting power of a diamond lens over that of glass, a lens of the former material may be at least one-third as thin as that of the latter, and if the focal * Those who would wish to enter more in detail into this matter, are referred to Pritchard's Microscopic Cabinet, p. 108. 28 PEACTICAl, TREATISE ON length of both be equal, say/' says Sir D. Brewster,* « one- eightieth of an inch, the magnifying power of the diamond lens wiU be 2133 diameters, whereas that of glass would be only 800." Mr. Pritchard, in later times, succeeded, with much less difficulty, in making lenses of other precious stones, viz., the sapphire, ruby, and garnet, all these substances, although coloured to a certain extent, nevertheless were not unfitted for magnifying powers; and Sir David Brewster, whose authority is indisputable in these matters, states : f — " That they all exhibit minute objects with admirable accuracy and precision, and that the colour of the garnet, which diminishes with its thickness, disappears almost whoUy in very minute lenses." The durabihty of lenses made of the diamond and other precious stones, is, however, an exceedingly valuable property; but the vast expense incurred in their manufacture, and the great superiority of the compound instrument, as now constructed, wHl ever be a barrier to their introduction into general use. The microscope, with a single lens, having been brought to the greatest state" of perfection by the labours of Sir David Brewster, Dr. Goring, and Mr. Pritchard, we must here leave it, and direct our attention to certain combinations of lenses termed doublets and triplets, by means of which microscopic science has been considerably advanced, and, with the ex- ception of the achromatic compound microscope, no more important improvement in the optical part of the microscope has ever yet been accomphshed. As long ago as the year 1688, a doublet was described in the Philosophical Transactions, as made by Eustachio Divini,| in which a large and flat field was obtained by placing two planoconvex lenses so as to touch each other in the middle of their convex siirface. "This instrument," it is there stated, "hath this peculiar, that it shews the objects flat and not crooked, and although it takes in much, yet nevertheless magnifieth extraordinarily." In the year 1812, a periscopic doublet lens was proposed by Dr. WoUaston, II it was composed of two planoconvex lenses, * Treatise on the Microscope, p. 21. f Op. Cit, p. 24. JNo. 42, p. 842. \\ Philosophical Transactions, 1812, p. 375. THE MICROSCOPE. 29 ground to the same radius, and applied by their plane sur- faces to a flat piece of metal, having an aperture of the same diameter as would be suitable for a lens of equal size, but composed of one piece of glass; and the size of the aperture which, on experiment, was found always to give the best definition, was about one-fifth part of the focal length in diameter. This form of doublet was subsequently improved on by Sir David Brewster, who, instead of using the flat piece of metal, and two planoconvex lenses, employed two hemispherical lenses, cemented to the ends of a tube of brass, and filled all the interspace with a fluid of the same refractive power as the glass. This led Sir David to the idea of the grooved sphere, which is nothing more than a spherical lens having a deep groove cut round it in a plane per- pendicular to the axis of vision; a plan analogous to that of the Coddington lens. Experiments on doublets were now carried on by Sir John Herschell, Sir David Brewster, Mr. Coddington, and others, and we have various forms recommended for use by each of these gentlemen; by the former we have three, viz., the periscopic doublet, consisting of a double convex lens of the best form, but placed in its worst position (radii as 6 to 1) for the lens next the eye, and a planoconcave, whose focal length is to that of the other, as 2, 6 to 1, or as 13 to 5, placed in contact with its flatter surface, and having its concavity towards the object. The second consisted of the planoconvex doublet, which is made with two convex lenses of equal focal lengths, the convex sides being placed in contact, and the eye and object opposite the plane sides; and the third, the doublet of no aberration, consisting of a planoconvex lens, and a meniscus placed in such a manner, that the convex sides of both were in contact. This latter form of doublet Sir John pro- poses as the best for obtaining perfect distinctness in micro- scopical observations, and Mr. Pritchard states:*— « That doublets of this kind answer remarkably well, but their angle of aperture is small as compared with combinations of double achromatics." * Microscopic Cabinet, p. 163. 30 PEACTICAL TREATISE ON By far the most important contribution to microscopical science at this period, was the microscopic doublet, the in- vention of Dr. Wollaston, it is described in the Philosophical Transactions for 1829,* and the mode of illumination therein recommended, gave to the single microscope an importance and degree of usefulness, which it had never yet received in this or any other coimtry. The doublet of Wollaston consisted of two planoconvex lenses, having their focal lengths in the pro- portion of 1 to 3, and placed at such a distance from each other, as was ascertained to be best by experiment. It is said that he was led to this invention by a knowledge of the construction of the achromatic Huyghenian eye-piece, which, if reversed, would make a microscope ; but impaired health caused hiTn to communicate his paper to the Boyal Society earlier than he at first intended, and his premature death deprived him of the satisfaction of ever witnessing the great improvement subse- quently made in his doublet, by the introduction of a stop or diaphragm between the two lenses. The microscope stand, with which the doublet was used, was as simple and as elegant in its construction as the doublet itself; and is shown in section, by figtire 19, where A B re- presents a brass tube, about six inches long and an inch or more in diameter, capable of being screwed into the cover of a box or stand, by the screw D. At C a circular perforation is made for the purpose of admitting the light to the mirror E. Above the mirror at F is a diaphragm or stop, for cutting off the outer rays of light reflected from the mirror. At the upper end of the tube is a planoconvex lens of about three quarters of an inch focal length, set in a metal frame at G, with its plane side uppermost; its use being to bring the rays of light to a focus on an object placed across the top of tube at P, which acts as a stage. At I is fixed a small rack, upon which an arm, H, carrying the doublet, M N O, can be moved up * Philosophical Transactions, 1829, p. 9. Fig. 19. THE MICROSCOPE. 31 or down by the pinion, K, whicli is tximed by the milled head, L, The doublet has been before alluded to, it consists of two planoconvex lenses, set each in a separate cell, M N. The cell carrying the upper lens screws into that which carries the lower lens, so that the distance between the indi- vidual lenses may be regulated for perfect definition ; when in use, the doublet is placed in a hole in the arm H. Since Wollaston's time, the stand has been much improved, it has been fitted up with an adjustable stage, and with fine and coarse adjustments, and otherwise much altered in appearance; but the one we have described is copied from his paper in the Philosophical Transactions. A modification of this form of instrument is at present in use, as an illuminator with many microscopes, both simple and compound, and will be again referred to in the chapter on "Illumination of Transparent Objects." "With this microscope," Dr. WoUaston says "that he was able to see distinctly the finest markings upon the scales of the Lepisma and Podura, and upon those of the gnat's wing." The doublet itself is, at the present time, much employed, and is preferred by many to the compoimd micro- scope for the examination of such objects as are perfectly flat, and by reason of its portability, its value is much enhanced. It is infinitely superior to a single lens, and is capable of transmitting a pencil of an angle of 35° to 50° without any sensible errors, and exhibits most of the test objects in a very beautiful manner. The next great improvement in the single microscope, and the last we shall here notice, was effected by Mr. Holland in 1832, and described by him in the forty-ninth volume of the Transactions of the Society of Arts. It consists, as shewn in fig. 20, of three planoconvex lenses, aic, the first two, a b, being placed close together, and the diaphragm or stop between them and the third lens, c. " The first bending," says Mr. Koss,* " being effected by two lenses instead J^ig- 20- q£ QQg^ jg accompanied by smaller aberrations, which are, therefore, more completely balanced or corrected at * Penny Cychpadia, Art., Microscope. 32 PRACTICAL TREATISE ON the second bending, in the opposite direction, by the third lens." This combination, though called by Mr. Holland a triplet, is essentially a doublet, in which the anterior lens is divided into two, and is capable of transmitting a pencil of 65°. Here we must take our leave of the history of the single microscope, and commence that of the achromatic compound instrument. Notwithstanding the great improvements which had taken place in the compound microscope during a period of nearly two centuries, we find, says Mr. Eoss,* that it was " a com- paratively feeble and inefficient instrument, owing to the increase in the chromatic and spherical aberrations occasioned by the great distance through which the light had to pass. The image formed by the object-glass was not a simple one, but made up of an infinite number of variously coloured and variously sized images. Those nearest the object-glass would be blue, and those nearest the eye-glass would be red. The effect of this being the production of so much confusion, that the instrument was reduced to a mere toy, although these errors were diminished to the utmost possible extent by limiting the aperture of the object-glass, and thus restricting the angle of the pencil of light from each point of the object. But this proceeding made the picture so obscure, that, on the whole, the best compound instnunents were inferior to the simple microscopes having a single lens, with which, indeed, almost aU the more important observations of the preceding century were made." The compound microscope, in its chromatic condition, having been found to be incapable of further advancing in a right way scientific research, many artists of eminence applied themselves to the work of im- provement; we are told that achromatism had been discovered in 1729 by a private gentleman in Essex, named Chester More Hall, who, in 1733, constructed and applied to a teles- cope an achromatic object-glass, having been led to its dis- covery by the study of the human eye, and by finding that two kinds of glasses combined, refracted light without decom- posing it. Two of his achromatic telescopes were for a long * Op. Cit., p. 6. THE MICROSCOPE, 33 time in the hands of persons who were not aware af their full value, and Mr. Hall himself paid the debt of nature without revealing the secret of their construction. In 1747, we are told that Euler suggested the construction of achromatic object glasses, a problem which for a long time agitated the learned in England, Holland, Italy, and France ; and in 1774 he proposed the application of an achromatic combination for the object glasses of micro- scopes. Our countryman, DoUond, " on the faith of Sir Isaac Newton's conclusions, zealously denied the possibility of doing what Euler proposed, but, nevertheless, commenced a series of experiments, beginning with that which had led Sir Isaac Newton to his unfavourable opinions, and which ended in accomplishing all that Euler had declared and Newton had hoped to be possible. These experiments, which included the spherical as well as the chromatic correction, were completed in the year 1757, and the glory of achieving this most valuable result is in no respect lessened by the fact, of which there is now no doubt, that a chromatic correction had been, to some extent, produced in the year 1733, by Mr. Chester More Hall."* Although DoUond constructed many achromatic telescopes, he did not apply the same principle to microscopes ; but those which he sold were only modifications of the compound instrument of Cuif. Chevalier tells usf that there exists a very rare work, published at St. Petersburg in 1774, under the following title: — Detailed instruction for carrying lenses of all different kinds to a greater degree of per- fection, with the description of a microscope which may pass for the most perfect of its kind, taken from the dioptric theory of Leonard Euler, and made cmnprehensible to workmen by Nicholas Fuss. It contains a description of the object-glass of the microscope, of which the following is the substance : — " The object-glass will be composed of three glasses ; the first and third of which will be of crown-glass, and the second of flint The focal distance will be half-an-inch, and the aperture of * A. Ross. Prcwtical Illustrations of the Achromatic Telescope. Parti., p. 11. t Op. at. p. 86. 3 34 PRACTICAL TREATISE ON the lens one-eighth of an inch. The least thickness possible should be given to the glass composing the lens; the two lenses of crown glass will be bi-convex, and the middle one bi-concave, &c." This glass, however, appears never to have been executed. In 1784, -^pinus made many fruitless trials to achromatize the microscope, and, although he was successful to a certain extent in destroying colour, he diminished rather than in- creased the magnifying power of the instrument, and he made it, says Adams, " rather more like a microscopic telescope than a microscope." A blank now occurs in the pages of micro- scopic history, from 1784 until 1800, and the microscopes in use in those days were more remarkable for the improve- ment in the mechanical construction of their stages and adjustments, than for that of the optical part ; " and at this period," says Chevalier, "it is to be remarked, with a sentiment of regret, that England was more laborious than Trance, and appeared to have the monopoly of the manufac- ture of the best instruments."* From the year 1800 to 1810, we are told by Chevalier that experiments were carried on by M. Charles, of the Institute, to achromatize small lenses ; but the numerous imperfections of these lenses were such, as to render their apphcation to the microscope completely impossible, as they were not so ar- ranged as to be cemented or superposed, and their centering and curves, so full of imperfections, rendered them unfit for microscopic purposes. In the year 1812, a very simple method was employed by Sir David Brewster f to render both simple and compound microscopes achromatic, which was as follows : — Starting -with the principle that all objects, however delicate, are best seen when immersed in fluid, he placed an object on a piece of glass, and put above it a drop of some kind of oil, having a greater dispersive power than the single or concave lens, forming the object-glass of the microscope. The lens was then made to touch the fluid, so that the surface of the • Op. at. p. 85. t Treatise on the Microscope, p. 73 et seq. THE MICEOSCOPE. 35 fluid was, as it were, formed into a concave lens, and if the radius of the outward surface were such as to correct the dis- persion, we should have a perfect achromatic microscope, both simple and compound. This method, however ingenious, was attended with considerable inconvenience, and our eminent and time-honoured philosopher was led to the construction of a permanent achromatic object-glass, by placing some butter of antimony between a meniscus and a planoconvex lens of crown-glass ; the antimony was retained between the glasses by capillary attraction, and could be removed as often as its properties were deranged. About this period, 1812, we find that numerous experi- ments were carried on by Professor Amici, of Modena, to improve the achromatic object-glass, and during his investiga- tions he invented a reflecting microscope, far superior to those of Newton, Baker, or Smith, which had been made as early as the year 1738, and had been abandoned for many years; this invention so far excelled any microscope pre- viously made, that Amici was induced, in 1815, to lay aside his experiments on the refracting instrument for a con- siderable period. An account of this microscope having soon reached England, Dr. Goring, in 1824, with the assistance of Mr. Cuthbert, succeeded in greatly improving it, and for a few years it was the most perfect form of microscope manufactured in this country; but, owing to the difficulty in constructing the reflectors, and the great trouble in ma- naging them, this instrument, like the reflecting telescope, fell into disuse, and even Amici himself entirely abandoned it, and returned to his former experiments on the refracting achromatic object-glasses. In the year 1816, Frauenhofer, a celebrated optician of Munich, constructed object-glasses for the microscope of a single achromatic lens, in which the two glasses, although in juxta position, were not cemented together, these glasses were very thick and of long focus ; but although such considerable improvements had been made in the telescopic achromatic object^lass since its first discovery by Euler in 1776, we find that even at so late a period as 1821, M. Biot wrote, "that 3* 36 PRACTICAL TREATISE ON opticians regarded as impossible the construction of a good achromatic microscope." Dr. Wollaston, too, was of the same opinion that the compound would never rival the simple microscope. In the year 1823 experiments were commenced in France by M. Selligues, which were followed up by Frauenhofer, in Munich, by Amici, in Modena, by M. Chevalier, in Paris, and by the late Dr. Goring and Mr. Tulley, in London. To M. Selligues we are indebted for the first plan of making an object-glass composed of four achromatic compound lenses, each consisting of two lenses. The focal length of each object-glass was eighteen lines, its diameter six lines, and its thickness in the centre six lines, the aperture only one line. They could be used combined or separate. A microscope, constructed on this principle by M. Chevalier, was presented by M. Selligues to the Academie des Sciences, on the 5th of April, 1824. In the same year, and without a knowledge of what had been done on the Continent, the late Mr. Tulley, at the instigation of Dr. Goring, constructed an achromatic object-glass for a compoiind microscope of one-third of an inch, focal length, composed of three lenses, and transmitting a pencil of eighteen degrees : this was the first that had been made in England, and it is due to Mr. Tulley to say, that as regards accurate correction throughout the field, that glass has not been excelled by any subsequent combination of three single lenses. Mr. Tulley afterwards made a combination to be placed in front of the first mentioned, which increased the angle of the transmitted pencil to thirty-eight degrees, and bore a power of three hundred diameters. Mr. Lister, who was engaged with Mr. TuUey in perfecting the achro- matic object-glass, finding that all the microscope stands hitherto made were not sufficiently steady for the use of high powers, directed his attention to the improvement of this part of the instrument ; and, in order to carry out his views, he employed Mr. James Smith, now one of our first opticians, to execute a stand on the plan represented by fig. 21. This instrument was finished by Mr. Smith, on the 30th of May, 1826, and was the first of the kind constructed in this country THE MICEOSCOPE. 37 with a double stage movement, a diaphragm, and a disc or dark well for opaque objects, when to be viewed by a Lieberkuhn. It was supported on three flat feet, capable of being shut up Fig. 21. one within the other, for convenience of package ; from these a short but stout piUar rose, having at its upper part a cradle joint, to which was attached the. stage, :;;, and the arm, a, supporting the compound body, h, consisting of three tubes, one within the other. Into the inner tube, i, called the draw tube, the eye-piece, Ji, was screwed; this tube was capable of being drawn out from the middle one for the space of four or five inches, and had engraved on it a scale of inches and parts, and to its lower end an erecting glass could be adapted. To the middle tube was attached a rack, which, with its tube, was moved by a pinion connected with the 38 PRACTICAL TREATISE ON milled head, g, this formed the coarse adjustment, the lower end of this tube, e, was conical, and to it the object-glasses, f, were screwed. The third or outer tube was firmly fixed to the arm, a, by a curved plate of brass and by the screw, c. When the compound body was placed in the inclined position, as represented by the figure, the tubular rods, d d, were used to steady it, the nuts, d d, serving to fix them when the proper inclination had been obtained; these rods were attached to the two hindmost feet. When the draw tube was in use, it could be fixed by the moveable band surrounding the body, and having a clamping screw, j. To the stage, x, was attached the tube, n, for carrying the mirror, o, and the ring, p, for holding the forceps, the condenser, and other things. The stage was moved from side to side by the milled head, m', and up and down by that at m. A condensing lens, q, was attached by a moveable arm to the ring, p. This form of instrument was adopted by the Tulleys (father and son), and by these eminent opticians some of the first microscopists of the day were supplied with it, amongst whom the names of Mr. Lister, the late Mr. Loddiges, and Mr. Bowerbank re- quire especial notice, as these gentlemen are intimately associated with the rise of microscopic science in this metropolis. While these experiments were in progress, Dr. Goring is said* to have discovered that the structure of certain bodies could be readily seen in some microscopes and not in others. These bodies he named test objects, he then examined these tests with the achromatic combination before noticed, and was led to the discovery of the fact that " the penetrating power of the microscope depends upon its angle of aperture." On the 30th of March, 1825, M. Chevalier presented to the Society of Encouragement an achromatic lens of four lines focus, two lines in diameter, and one line in thickness in the centre: this lens was greatly superior to the one before noticed, which had been made by him for M. SeUigues. In 1826, Professor Amici, who, from the year 1815 to 1824, had abandoned his experiments on the achromatic * Microscopic Objects, p. 21. THE MICROSCOPE. 39 object-glass, was induced, after the report of Fresnel to the Academy of Sciences, to resume them, and in 1827 he brought to this country and to Paris a horizontal microscope, in which the object-glass was composed of three lenses superposed, each having a focus of six lines and a large aperture. This micro- scope had also extra eye-pieces, by which the magnifying power could be increased. A microscope constructed on Amici's plan, by Chevalier, during the stay of that philo- sopher in Paris, was exhibited at the Louvre, and a silver medal was awarded to its maker. "Whilst these practical investigations were in progress," says Mr. Koss,* "the subject of achromatism engaged the attention of some of the most profound mathematicians in England." Sir John HerscheU, Professors Airy and Barlow, Mr. Coddington, and others, contributed largely to the theo- retical examination of the subject, and, though the resiolts of their labours were not immediately applicable to the micro- scope, they essentially promoted its improvement. For several years prior to 1829, the subject had occupied the mind of a gentleman who, not entirely practical Hke the first, nor purely mathematical Hke the last-mentioned class of observers, was led to the discovery of certain properties in an achromatic combination, which had been before unobserved. These were afterwards experimentally verified; and in the year 1829, a paper on the subject, by the discoverer, Joseph Jackson Lister, Esq., was read to and published by the E.oyal Society. The principles and results thus obtained enabled Mr. Lister to form a combination of lenses, capable of trans- mitting a pencil of fifty degrees with a large field correct in every part. This paper, which was the ground-work of aU the great improvements that have been efiected in this coimtry in the achromatic object-glasses, has tended to raise the compound microscope from its primitive and almost useless condition to that of being the most important instrument ever yet bestowed by art upon the investigator of natm:e, and has gained for the discoverer a lasting reputation. As the results arrived at by Mr. Lister are indispensable to all who would * Art. , Microscope, Penny Cyclopcedia. 40 PEACTICAi TREATISE ON make or understand the instrument, I would refer them to the paper itself, which is contained in the 121st volume of the Philosophical Transactions. From this discovery of Mr. Lister's, in 1829, we may fairly date the rise and continued progress towards perfection of the achromatic compound microscope in England, and all cultivators of natural science, as weU as the makers of the instruments themselves, are largely indebted to Mr. Lister for publishing to the world the valuable results of those labours, which certainly have formed the groundwork of the plan on which all our first-rate opti- cians now work, for whose success he has always most zealously exerted himself, even to the examination, from time to time, of their wonderful productions ; and it is but common justice here to state, that we have now in this metropolis three most eminent manufacturers of the compound achromatic microscope, viz., Messrs. PoweU, Koss, and Smith, whose instruments are without equal in this or any other country. On consulting the dates at which these opticians respectively commenced the manufacture of achromatic object-glasses, we find that as early as March, 1831, Mr. Andrew Ross had completed for Mr. Wm. Valentine a dissecting microscope on an entirely new plan, being provided with coarse and fine adjustments, stage movements, and a WoUaston con- denser. This instrument, first described in the forty-eighth volume of the Transactions of the Society of Arts, will be more fully mentioned in the chapter devoted to the simple microscope; although generally employed for dissecting, it was nevertheless made capable of receiving a compound body. The first microscope of this kind made by Mr. Ross is now in the possession of R. H. SoUy, Esq., for which, in 1832, Mr. Ross was also employed to construct a triple object-glass, he, previous to the year 1831, having made lenses of the precious stones, and acquired a knowledge of achromatism by being connected with Professor Barlow, during the con- struction of his fluid object-glass, and also in the arrangement of his formula for computing the radii of curvature of an achromatic one. Since the period above mentioned, Mr. Ross has been constantly and actively employed in bringing these THE MICROSCOPE. 41 Instruments to perfection, and during the manufacture of the object-glasses, he effected a most important improvement in their construction, which he thus describes:* — "Having applied Mr. Lister's principles with a degree of success never anticipated, so perfect were the corrections given to the achromatic object-glass, so completely were the errors of sphericity and dispersion balanced or destroyed, that the cir- cumstance of covering the object with a plate of the thinnest glass or talc disturbed the corrections, if they had been adapted to an uncovered object, and rendered an object-glass which was perfect under one condition sensibly defective under the other." This defect, if that be called a defect which arose out of an improvement, he (Mr. Ross) first detected, and immediately suggested the means of correcting, and in 1837 communicated his discovery to the Society of Arts, in a paper published in the fifty-first volume of their Transactions, to which paper the author would refer those of his readers who would wish to enter more fully into the subject ; the desired object being effected by separating the anterior lens in the combination from the other two; and figure 22, which is a section of an achromatic object- glass, will explain how the principles established by Mr. Ross were put into practice. A represents a tube, in the end of which the anterior lens is set ; this shdes on the cylinder, B, containing the remainder of the combination; the tube, A, holding the lens nearest the object, may then be moved upon the cylinder, B, for the purpose of varying the distance, according to the thickness of the glass covering the object, by turning the » Op. at. p. 8. Fig. 22. 42 PRACTICAL TREATISE ON screwed ring, C, or more simply by sliding the one on the other, and clamping them together. When adjusted, an aperture is made in the tube. A, within which is seen a mark engraved on the cylinder, and on the edge of which are two marks, a longer and a shorter, engraved upon the tube ; when the mark on the cylinder coincides with the longer mark on the tube, the adjustment is perfect for an uncovered object, and when the coincidence is with the short mark, the proper distance is obtained to balance the aberrations produced by glass one-hundredth of an inch thick, and such glass can readily be obtained. When Mr. Ross first effected this im- provement, he made the adjustment by sHding the outer tube, A, upon the cylinder, B ; but Mr. Powell, we are told, was the first to apply the screw coUar, C, by which the correction can be performed with greater nicety, and Mr. Smith after- wards, as a refinement, added a graduation to it. Mr. Ross, however, has found that for the adjustment to be perfectly correct, it must be tested experimentally. The method of using this improved achromatic object-glass wiU be again alluded to in the chapter devoted to the com- pound microscope. From the peculiar construction of Mr. Ross's higher powers, he is enabled to transmit extraordinarily large angular pencils of light: on several occasions he has obtained the enormous aperture of 135. Mr. PoweU, in early life, was engaged in the manufacture of philosophical instruments, but not of microscopes; and it was only in the year 1834 that he devoted his attention to the last mentioned instruments. In the same year, we find a con- tribution of his to the fiftieth volume of the Transactions of the Society of Arts, entitled, " On a fine adjustment for the Stage of a Microscope." This ingenious contrivance was applicable to any instrument, but Mr. Powell used it with the adjustable stage made by Mr. Turrell, and described by him in the forty- ninth volume of the same transactions. The slow movement was obtained by making the stage stand on three feet, under which three inclined planes were moved simultaneously by one screw, a single turn of which raised or lowered the stage only the three-hundredth part of an inch, THE MICROSCOPE. 43 and twenty divisions marked on the screw-head gave mea- sures of the one six-thousandth part of an inch, and hence its use as a micrometer as well as a fine adjustment. In the year 1841, Mr. Powell made another communication to the Transactions of the same society, "On a new way of mounting the compound body of a microscope," a plan which will be again alluded to under the head Compound Microscope ; and in the year 1840, he succeeded in making an achromatic object- glass of one-sixteenth of an inch in focal length, the first that had been seen in this country ; it is in itself a wonderful production, both for delicacy of workmanship and correctness of definition. About this period, his brother-in-law, Mr. P. H. Lealand, who had for some time assisted him in the manufacture of object-glasses, became a partner with Mr. Powell, and from that time up to the present, these opticians have given their undivided attention to the manufacturing, as well as to the improving and perfecting, the optical and mechanical parts of the achromatic compound microscope. Mr. Smith, who had been for many years engaged in the manufacture of microscopes of all the ordinary kinds, was Ln 1826 employed by Mr. Lister to construct the instrument represented by fig. 21 ; but he did not turn his attention to those of the achromatic form on his own account until 1839, at which time he likewise made object-glasses on Mr. Lister's principles; these, which are of large aperture, were at first constructed on a plan rather different from those of Messrs. Powell and Eoss, the lowest ampUfication was produced by a single achromatic lens, and to increase the magnifying power, another, or, for a stUl higher, a combination of two, was slid over the first. This plan was adopted with the object of furnishing the glasses at a cheaper rate, but more recently Messrs. Smith and Beck make each power a separate com- pound glass, like the others. In the year 1841, Mr. Smith was applied to by the council of the Microscopical Society to furnish them with one of his newly constructed achromatic compound microscopes, and on the 24th of November in the same year, the instrument, of which a figiire is given in the second volume of the Micro- 44 PEACTICAL TREATISE ON scopic Journal, was delivered to the society. Tliis microscope had the compound body mounted so as to slide in the groove of a strong bell-metal arm, the contrivance of Mr. George Jackson, a plan now adopted by Mr. Smith in all his large instruments; the object-glasses were four in number, the highest being the fourth of an inch, which, with the deepest eye-piece, was capable of magnifying 800 diameters. During the last nine years, Mr. Smith has made many and rapid advances in the manufacture of microscopes, and, in conjunction with his partner, Mr. Beck, has successfully endeavoured to reduce the cost of his instruments by simplifying the form of stand, by which they are brought more within the compass of those whose means are limited. Amongst those in this country by whose agency the micro- scope has been much improved, may be mentioned the names of Mr. Varley and Mr. Pritchard, both of whom are well known to the scientific world by their valuable publications. To Mr. Varley, in 1831, we are indebted, first, for a micro- scope with a lever stage movement, for following animalcules, together with capillary cages for containing the same, fishing tubes and other apparatus equally ingenious and useful, and for his lathe for grinding and polishing lenses ; secondly, for his vial microscope, for viewing the circulation in chara; thirdly, for his graphic telescope and microscope ; fourthly, for his valuable instructions and hints concerning the best forms of eye-pieces for telescopes and microscopes ; and, lastly, for his improved lever microscope, all of which in- ventions have been fully described in the Transactions of the Society of Arts. To Mr. Pritchard, we are in- debted for three valuable works on the microscope, viz: — The Microscopic Cabinet, The Microscopic Illustrations, and The Micrographia, in which are admirably explained the con- struction of the instruments made and improved upon by Dr. Goring and himself, together with the history of the doublet, jewel, reflecting, and achromatic microscopes, the methods of testing and using the same, with the descriptions of many interesting objects observed by them. These works, which were the first of the kind published in England, have THE MICROSCOPE. 45 long since obtained a well-deserved reputation. The names of Chevalier, Frauenhofer, Oberhauser, Schick, Nachet, and many- other continental opticians, here deserve honourable mention for their various productions; and the author would be wanting in justice and candour, were he not to acknowledge the valuable information which has been derived in this History of the Microscope from the excellent work of M. Che- valier, entitled, Des Microscopes et de leur usage. The rapid progress of improvement in the manufacture of the achromatic compound microscope in this country is con- siderably indebted to the spirit of liberality evinced by the late Dr. G-oring and R. H. Solly, Esq. To the patronage of the former we owe the construction, by Tulley, of the first triplet achromatic object-glass, that of the diamond lens, by Varley and Pritchard, and of the improved reflecting instrument of Ajnici by Cuthbert. To Mr. Solly is due the credit of bringing before the public the improved microscope of Mr. Valentine, the exquisite workmanship of Mr. Koss, and by his intimate connection with the Society of Axts, and his well-known liberality, he has been the means of making its Transactions, since 1831, the vehicle through which nearly all the improvements in the construction of telescopes and microscopes, by Mr. Varley especially, have been made known to the world. The late Dr. Goring, at whose instigation Tulley, in 1824, constructed the first achromatic object-glass in this country, said,* in 1829, "That microscopes are now placed completely on a level with telescopes, and, like them, must remain stationary in their construction." "Happily for us," says Mr. Bowerbank,t "this prediction has not been fulfilled. Admirable as were the combinations alluded to by Dr. Goring, they were very far inferior to those which we now possess, and which we, like the worthy doctor, are, perhaps, inclined to believe are scarcely capable of being surpassed ; but however beautifijl the combinations around us, let us hope that the same skiU and talent which have wrought these * Exordium to Microscopic lUustrations, 1829. t Address to the Microscopical Society, February 10th, 1847. 46 PRACTICAL TREATISE ON THE MICROSCOPE. great and valuable improvements in the instrument will con- tinue to aid and assist the scientific world, by aiming at and achieving a still further degree of perfection." The great advances which have been made in microscopic science within the last few years, and the immense number of valuable contributions to animal and vegetable physiology alone, with which the scientific journals of this and other countries are more or less filled, all tend to show with what rapid strides accurate knowledge is being advanced; and the great demand for achromatic microscopes has been such, that since the year 1836, in this metropolis alone, upwards of 1000 first-rate instruments have been manufactured by our three great makers, Messrs. Powell, Ross, and Smith, to whom with Mr. Lister should be awarded no small share of the honour reaped by those who, through their instrumentahty, have successfully laboured in the field of microscopic investi- gation. THE SIMPLE MICROSCOPE. 47 CHAPTER I. THE SIMPLE MICROSCOPE. The simple microscopes in general use may be divided into two classes; first, those used in the hand; and, se- condly, those provided with a stand or apparatus for supporting the object to be viewed, together with an ad- justment of the magnifying power to and from that object, vrith a mirror or speculum for reflecting the light through such objects as are transparent, and a condenser for such as are opaque. To the first class, or those microscopes used in the hand, belong the various kinds of pocket lenses, or magnifying glasses so commonly used ; they consist for the most part of double convex or planoconvex lenses of glass, varying in focal length from the quarter of an inch to two inches; one or more of these is set in a frame of metal, horn, or tortoisesheU, and is made to shut up between two other plates of the same material, which, besides forming a handle for it, serve to keep it free from dust and scratches ; the shutting up is similar to that of a knife-blade into its handle. Sometimes these lenses are set in pairs, with a thin piece of horn or tortoisesheU between them, having a hole in its centre corresponding to the centre or axis of the two lenses ; this serves as a stop to cut oflT all the outer rays of light, so that when an object is viewed by the combined power of the two lenses, it is not only more magnified, but the defining power of the instrument is increased in a like proportion, so that we might almost call it a doublet. These magnifying glasses are extremely useful for all pur- poses where a high power is not required ; to the anatomist they are essential for examining preparations either in or out of bottles, and for dissections and injections. For the latter purpose, a lens of half-an-inch focus wiU magnify sufficiently to enable an observer to pronounce whether the vessels of most tissues be perfectly filled, or whether extravasations have taken place. In short, no person in the pursuit of any branch 48 PRACTICAL TREATISE ON THE MICROSCOPE. of natural history should be without one ; its aid is hourly required. There are two forms of these pocket magnifiers in general use ; the most common form, represented by fig. 23, carries one, two, or three mag- nifiers, whilst a much larger and more convenient form is repre- sented by fig. 24, in which there are two sets of lenses, varying in their focal length from two inches to a quarter of an inch; between the lenses may be seen ^'S-^^- in both figures the diaphragm or stop, which enables us to use the two lenses as a doublet. Fig. 24. A square hole is made in the end of the handle of fig. 23, and a round one in the middle of that of fig. 24, for the pur- pose of attaching them to a stand, as wiU be subsequently shown. Mr. Smith generally puts three lenses into one handle, the highest power is a planoconvex, the next a crossed lens, and the lowest a double convex lens ; these, when com- bined, perform uncommonly weU. When a higher magnifying power is required, the form generally used is that known as the Coddington lens, con- sisting of a sphere of glass, around the equator of which a triangular groove has been cut, and the groove itself subse- quently fiUed up with opaque matter, as represented in section by fig. 25. The great advantage of this form of lens is, that however obliquely pencils of light, B A, may fall upon it, they, like the central ones, pass at right angles with the surface, THE SIMPLE MICROSCOPE. 49 and, consequently, the aberration is trifling. This lens gives a large field of view, equally good in all directions, and it B A little matters in what position it is held, hence it is peculiarly applicable as a hand magnifier. The lens is generally set in silver or German silver, as represented by fig. 26, and the handle is so contrived, that it occupies but little room in the waistcoat pocket. It may be as well here to mention that many of the lenses sold as Coddington lenses are not constructed after this manner, but are made up of two convex lenses, not por- tions of spheres, hence they are destitute of many of the advan- '^' tages of the true Coddington lens. Another lens, somewhat of the same description as the Fig. 23. last, is much boasted of by its manufacturers, and is puffed off at every toy-shop as the Stanhope lens ; it consists of nothing more than a double convex lens of great thickness, on one side of which the convex surface is greater than on the other; and when the most convex is turned towards the eye, an object placed upon the other convex sur- face is in the proper focus of the lens ; it is, in consequence, generally used more as a toy. than as a philosophical instru- ment, for viewing the scales of butterflies' wings and other flat objects which can readily be attached to it, or for showing the eels in paste, and the wonders in a drop of water. If, how- ever, the flattest side be turned towards the eye, this form of lens may also be used as a magnifier, its focus being then from 1^ to ^ of an inch. When any of these lenses have to be held for a long time in the hand, much inconvenience wiU be felt, hence various stands or supports have been contrived by which the magnify- ing power may be kept in a fixed position over the desired object. The engraver, the watchmaker, the jeweller, and the artist, all require some form of lens, and each has an appa- 4 50 PRACTICAL TREATISE ON THE MICROSCOPE. ratus by means of which it may be supported and adjusted, making it, in fact, a single microscope ; and as it would be foreign to our purpose here to enter into the details of the various contrivances which have been adopted, from time to time, we shall merely make mention of those useful in micro- scopical investigations. The mo9t simple, but not the least useful of the single microscopes, is represented by fig. 27. It is principally used by watchmakers and wood- engravers, and consists of a loaded stand, of metal or wood, from which rises a circular stem of stout wire or tube ; upon this slides another piece of tube, carrying an arm also of stout wire, having at its end a ball and socket joint, and to the ball of this joint is attached a second smaller arm, to the end of which last, is fitted either a spring or else a ring, serving the purpose of carrying the lens; when the spring is used, the magnifier generally employed is the one the watchmaker adapts to his eye, it is represented by fig. 28, and is nothing more than a lens of an inch focus, set in a long cell of horn, enlarged at one end like a trumpet, this enables it to be grasped firmly by the muscles around the orbit, or if the ring be used, the lens may drop into it. The coarse adjustment is made by sliding the tube up or down the stem, whilst a finer adjustment is secured by means of the small arm and the ball and socket joint ; but it will be seen that if this last be used, and the arm be moved into any other position than a horizontal one, the lens wiU not be in a plane at right angles Fig. 27. Fig. 28. THE SIMPLE MICROSCOPE. 51 to the object. To remedy this inconvenience, the author has found the following contrivance extremely useful, a section of the lens and cell in which it is contained being represented by fig. 29. The semicircular spring is retained, its ends are seen in section at b b, and entire in fig. 30 at d, land a ring, a a, is adapted to it, rather less in diameter than the spring, and three-eighths of an inch in depth ; it has a shoulder or rim at one end. m •Fig. 29. and also two steel pins, c c, screwed in near the top edge, exactly opposite each other ; these pins are received by two holes made in the semicircular spring, so that the cell may turn or swing upon the pins just as a compass on its gimbals. The lenses are made to drop into this cell, and it will be readily seen by fig. 30, which is a representation of the arm Fig. 30. and cell, just one-half its real size, that in whatever position the arm is placed, the cell carrying the lens will be always hori- zontal : — a exhibits the piece of brass forming the connection between the two parts of the arm, it has a socket at one end, in which the ball, b, works ; c is the small wire arm supporting the spring, d; e is the cell which carries the lenses; 2 repre- sents the situation of the cell when the arm, c, is horizontal; 1 the same when the arm is elevated ; and 3 when depressed, in both these places the cell maintains its horizontal position. 4* 52 PRACTICAL TREATISE ON THE MICROSCOPE. The lenses are set in brass frames, which easily fall into the cell, as seen in section in fig. 29, where e represents the lens, and d the frame in which it is set ; and when it is required to change the power, we have merely to turn the cell upside down, the lens wiU drop out and another can be substituted. It may be as well here to state, that the form of the low power lenses employed for the purpose of dissecting should be double-convex, a planoconvex, with its convex side towards the eye, gives a flat field, perfect in the centre, but not at the margins. This form of microscope is exceedingly useful for minute dissections of nerves that are carried on under water in troughs or other vessels, and will be found sufficiently steady for the purpose, the length of the arm allowing the lens to be brought over any part of the trough or vessel in which the dissection is contained, so that the size of the subject to be examined need not be considered. When a much more steady instrument is required for the purposes above described, Messrs. Powell and Lealand have contrived a form represented bv fig. 31 ; it consists of a brass foot, or base, h, about five inches in diameter, and an inch and a half thick ; to make it more steady, it may be loaded with lead ; from this foot rises a trian- gular stem, a, about twelve inches in length, having a rack, d, on one of its sides ; upon this stem, a square box, c, carrying a pinion and two milled heads, is made to move up and down by the rack. To the box is attached a strong tubular, but conical. Fig. 31. THE SIMPLE MICROSCOPE. 53 arm, /, nine inches long, provided at its free end, a, with a stout ring, ff, into which either a compound body may be screwed, as seen in fig. 32, or a lens, ^, set in a large cell may drop. The com- pound body, it will be seen, has also a rack and pinion mo- tion of one inch in extent for a fine ad- justment, and the body itself may be inclined at any angle by means of a swivel joint to the ring. This instrument is particularly useM for minute dissections carried on in large troughs imder water; and when the opera- tor wishes to view his dissection with a Fig. 32. high power, he may remove the single lens under which he has been at work, and substitute for it the compound body, which is usually supplied with three eye-pieces, and an inch and two inch object-glass; but in no case is he required to move his dissection, as the compound body can be appKed to the same objects as the single lens. To make this instrument available for the general purposes of a com- pound microscope, it is provided with an oblong frame or box, open at the sides, and in the bottom of which is con- tained a mirror; the top of a box having a hole in it about an inch and a half in diameter, answers the purpose of a stage, and into it a pair of forceps, a frog plate, and other apparatus may be fitted, as into the stage of an ordinary compound microscope. To the ring, also, may be adapted a small arm, capable of carrying a Coddington or other lens of high power. 54 PEACTICAL TREATISE ON THE MICKOSCOPE. >-< When portability is studied, a very convenient and useful microscope, for many purposes, can be readily made with one or both of the pocket magnifiers, before described at page 48, if either of the two forms, as there represented, have a hole in the handle. These being provided with a stand, as repre- sented by fig. 33, of any convenient size, from which a small stem rises, the pocket lens may be made to — slide up and down this ' stem, and if required to be fixed at any given point, a small screw wiU suffice for the purpose. This method of mounting the pocket lens on a stand was first suggested by Mr. Lister, and has been carried out by Messrs. Smith and ^ig 33. Beck ; but as the plan adopted by them, represented by fig. 33, is rather dififerent from that just described, it will be requisite here to give an account of their improvements.* Their pocket magnifiers have a square hole in the end, and they use a circular stand, and on the stem, which is round, a piece of brass is made to slide up and down, carrying a binding screw on one side, and a small arm on the other ; this arm is straight for about a fourth of an inch, and then is bent at a right angle for about the same length, the last part is square, and upon the square, the magnifier is made to fit ; this is a much better plan than the former one, in which the screw for tightening is in the end of the handle of the magnifier, as less trouble is required in * By an error of the artist, the magnifier is represented the wrong f:'ide upwards. THE SIMPLE MICROSCOPE. 55 fixing, and the magnifier itself can be taken off or put on with the greatest facility. Mr. Eoss has contrived a small but exceedingly useful in^ strument, answering the same purposes as the preceding; it is represented by fig. 34, and consists of a circular foot, e, about Fig. 34. an inch and a half in diameter, from which rises a short tubtdar stem, d, into this shdes another short tube, c, carrying at its top a joint, f;to the joint is fixed a square tube, a, through which a square rod, b, slides; this rod has at one end another but smaller joint, ff, having attached to it a lens holder, h. By means of the joint at f, the square rod can be moved up and down, so as to bring the lens close to an object, or remove it from it, and by the rod sliding through the square tube, a, the distance between the stand and the lens may either be increased or diminished ; the joint, ff, at the end of the rod, is for the purpose of allowing the lens to be brought either perfectly horizontal, or to be inchned at any angle with the subject to be investigated. By means of the sliding tube, c, the distance between the table and the jointed arm can be 'increased or diminished. This microscope is provided with 56 PRACTICAL TEEATISE ON THE MICROSCOPE. lenses of one inch and one half-inch focal length for the dis- section and examination of opaque objects ; but by means of a dissecting table or platform, with a mirror underneath, as described with Mr. Powell's instrument, page 53, it wiU answer equally weU for transparent objects, especially if the dissecting rests, subsequently to be described, be used at the same time ; the joint at f allows of the lens being adjusted with very great nicety. This apparatus is also readily taken to pieces, by unscrew- ing the pillar, d, from the stand, and, with the lenses, dis- secting instrmnents, and forceps, is packed in a small case, which can be carried in the pocket. These little instruments the Author has found extremely useful for the examination and selection from sand of many of the smaller kinds of foraminiferous shells. A small quantity of the sand supposed to contain them, may be spread on a piece of black paper on the table, and by means of this simple microscope, and a sable or other pencil brush capable of being brought to a fine point, a great deal of work may be performed in a short space of time, and with much more ease than with a compound instrument, in which all the objects are reversed; and as the cost of these microscopes is comparatively trifling, and the uses to which they are applicable so extremely various and important, no student of natural history should be without one. The instrument best suited for dissection is one which was described in the forty-ninth volume of the Transactions of the Society of Arts, by Mr. Slack. It consists of a box or case, seven inches high and four inches broad, represented open in fig. 35. The upper siurfaces, r r, are sloped ofi" to four inches square to form arm rests, and the top is left six inches by four. The front of the case is provided with a flap or door, having hinges at the bottom and a lock at the top; the mirror is situated in the bottom of the case, and is of large size, and directly over it, in the top, is an opening, g, an inch and a quarter in diameter, which may be closed, if required, by a brass cap. THE SIMPLE MICROSCOPE. 3 51 Fig. 35. Fig. 36, is a back view of the instrument, arranged for use. The stage, h, is screwed into the top of the box, and is raised one inch above it, by means of a tube, in which it is made to Fig. 36. revolve, so that an object placed on it may be turned into any convenient position. The apparatus for carrying the lenses and for the adjustment of the same, is represented as it is attached to the back of the case. A vertical stem, six inches long and four-tenths square, with a rack on one side, carries the lens holder, m n, which may be moved backwards and forwards by a rack and pinion at m., and is made to turn hori- 58 PKACTICAL TREATISE ON THE MICROSCOPE. zontally upon a steel pin at the top of the square stem. The stem is lowered and raised by a pinion with a large milled head, I, two inches in diameter, by which tolerably fine adjust- ments may be made, but finer still may be effected by the lever, o, which fits into a series of holes drilled in the circum- ference of the same milled head, /. The whole of this adjusting apparatus is attached to a plate of brass, jj, and is made to shde into another plate, i i, fixed to the back of the case by screws. When not in use, the entire apparatus on the top of the case may be removed and placed in a box or drawer in its interior. When transparent objects are being dissected, the screen, q, made of black cloth, may be attached in front of the stage by two brass pins, pp; this screen or curtain has a two- fold use, the one to intercept all extraneous light save that reflected from the mirror below, the other to keep the light of the lamp or candle employed in the illumination from the eyes of the observer. The pins, p p, are bent a little forwards, that the curtain may not be in the way of the head. The microscope is thus arranged for the dissection of transparent bodies, such as the vessels or other tissues of plants, for which the inventor, Mr. Slack, was so celebrated ,■ but when opaque objects are under examination, the condensing lens must be employed ; this may either be fixed on a separate stand, or to some part of the top of the case. An improvement has been made by Mr. Goadby in this dissecting microscope of Mr. Slack : he places the stem for the adjustment, in the interior of the case, and the milled head only is allowed to project on the outside ; this can be put on or taken off at will, as the end of the pinion is made square to receive it. The case is on rather a larger scale than Mr. Slack's, but in shape is precisely similar. As most of Mr. Goadby's dissections are carried on under water, square tin troughs are used for the purpose, each of which has a circular ring fastened to the bottom, to fit into the aperture, g, of the stage, and by this means they are prevented from shifting their position. A very useful single microscope is that made by Mr. Ross, and described by him in the Penny Cyclopedia, article, " Micro- scope." It is represented by fig. 37, and consists of a brass THE SIMPLE MICEOSCOPE. 59 pillar, about six inches long, screwed into a tripod base ; .to the upper part of the pillar is attached, by screws with milled Fig. 37. heads, a large flat stage, provided with a spring clip, and other apparatus for holding the objects. By means of the large milled head, a triangular bar, having a rack, is raised out of the pillar; this bar carries a lens-holder, having a horizontal movement in one direction, effected by a rack and pinion, and a circular one, by turning on a pin. It is also provided with a concave mirror, for reflecting the light through the hole in the stage; a condensing lens, for the purpose of illuminating opaque objects, and a pair of forceps for holding small objects, may be applied to either of the holes in the stage. This microscope is usually supplied with lenses of one inch and one half inch in focal length for dissecting; but the higher powers generally employed are either doublets or triplets; or it may be converted into a 60 PRACTICAL TREATISE ON THE MICROSCOPE. compound microscope by taking away the lens-holder and substituting for it a compoimd body, and when provided with a cradle joint, either at the top or bottom of the pillar, may be inclined after the manner of the larger instruments pre- sently to be described. This microscope, with its broad stage, is well adapted for minute dissections, and is rendered more convenient for the purpose if placed between two inclined planes, to be here- after mentioned, which form what is called the dissecting rest. This apparatus gives support to the arms, and brings the wrist on a level with the stage, whereby small cutting instru- ments can be managed with the greatest nicety. Another highly useful, and far more complete stand of a simple microscope, for the dissection of minute botanical and other objects, was contrived by Mr. Wm. Valentine, and constructed for him by Mr. Andrew Koss, in 1831 ; it is fuUy described in the forty-eighth volume of the Transactions of the Society of Arts, and was one of the first simple microscopes provided with a moveable stage, and with coarse and fine adjustments, as represented by fig. 38. It is supported on a firm tripod, made of bell-metal, the feet of which, a a a, are made to close up together. A strong pillar, b, rises from the tripod, and carries the stage, e, this is fiirther strengthened by two brackets, r r. From the tube or pillar, a triangular bar, d, and a triangular tube, c, slide, the one within the other ; the outer or triangular tube, c, is moved up and down by a screw, having fifty threads in the inch, turned by a large mUled head, v, situated at the base of the piUar, this is the fine adjustment. The small triangular bar, d, is moved up and down within the triangular tube, c, just described, by means of a rack and pinion, turned by the milled head, t, forming the coarse adjustment: this bar carries the lens-holder, mnop. The stage, e, consists of three plates, the lowest one is firmly attached to the pillar, and upon this the other two work. The upper one carries a small elevated stage, g, on which the objects are placed ; this stage is mounted on a tube, f and has a spring clip, h, for holding, if necessary, the objects under examination. By means of two screws, placed diago- THE SIMPLE MICROSCOPE. 61 nally, one of which is seen at s, this elevated stage can be moved in two directions, at right angles to each other, and 4'l[&=^=^ Fig. 38. the different parts of any object can be brought successively into the field of view. The arm, n p, which carries the lenses, is attached to the triangular bar, rf, by a conical pin, on which it can turn hori- zontally, and the arm itself can be made longer or shorter by means of a rack and pinion, m o, attached to it, hence the lens, q, may be applied to all parts of an object without interfering in any way with the stage. The mirror, I, is placed upon the largest of the three legs forming the tripod, and consists of a concave and plane glass reflector. To the under side of the stage is fitted a Wol- laston's condenser, k, and the lens is made to slide up and 62 PRACTICAL TREATISE ON THE MICROSCOPE. down by means of two small handles projecting from the cell containing the lens. Two small tubes, i, into which either a condensing lens or a pair of forceps may be fitted, are attached to the under side of the stage. The magnifiers employed in this instrument were either single lenses or doublets, and Mr. Valentine, who is so well known as a most skilful vegetable anatomist, has managed to dissect under a lens of one-twentieth of an inch focus. To make it a compound microscope, the arm carrying the lenses can be removed, and a compound body, supported on a bent arm and provided with a conical pin, at its end, can be substituted, and the coarse and fine adjustments in the piUar will answer the purpose of focussing the compound instrument, as well as the simple magnifiers. This microscope, the first of the kind ever made by Mr. Eoss, was remarkable for the excellence of its workmanship, and may be said to have paved the way for a new era in the forms of these instruments. A very useful microscope for dissecting is that made by Messrs. Smith and Beck, represented by fig. 39 ; it may be supported upon a heavy circular brass foot, or be screwed to the cover of a box, or block of hard wood. The central pillar is circular, about six inches long and three-fourths of an inch in diameter, and from it may be raised a triangular bar, by a rack and pinion, turned by two large milled heads. The lens-holder has two movements like that of Mr. Ross, the one by a pin fitting into the top of the triangular bar, the other by a rack and pinion. The mirror is of the usual construction. The stage is of a circular figure, three inches in diameter; into it may be fitted dissecting troughs, com- posed of a ring of brass, with a glass bottom, or a similar ring with an ebony bottom, and others equally useful, which are covered or lined with cork. This instrument is suppUed with single lenses and with doublets, and has proved a very useful working tool in the hands of Mr. Darwin, who suggested many ingenious pieces of apparatus to fit into the hole in the stage for holding subjects under examination. Besides this single microscope of Messrs. Smith and Beck, and those THE SIMPLE MICROSCOPE. 63 previously noticed, there are many very useful forms sold by some of our other opticians in this metropolis, and in the Fig. 39. provinces ; those of Mr. Pritchard, which are described in his works, require especial mention. The author of a little tract, entitled The Wonders of the Microscope, recommends strongly an instrument invented by Easpail, which can be bought in Paris for thirty francs, or about twenty-five shillings English : it is provided with four lenses, varying in magnifying power from fifty to three hundred diameters. The author was lately shown one of these instnunents, by his friend, Mr. H. W. Diamond, and can speak very favourably of its performance. As the single microscope is principally used for dis- section, the most essential part, next to good glasses, is 64 PRACTICAL TREATISE ON THE MICROSCOPE. a large firm stage for supporting the objects under exami- nation; and as it is found that, after a little practice, an object can be moved about on the stage with very great nicety, the stage movements may be dispensed with where low powers only are employed ; but with doublets and trip- lets some more delicate adjustment than that of the hand becomes necessary, and such an instrument as that described by fig. 38 should be had recourse to, where both fine and coarse movements for the magnifiers are proAdded, and all parts of the object can be carried under the lens by the ad- justable stage. The magnifying powers generally employed with single microscopes, may be divided into those consisting of one lens only, and those of two or three lenses combined, from which circumstance they are termed doublets or triplets. In the first class are included all the powers, from two inches up to one quarter, and sometimes one-tenth of an inch; these should be set in flat cells, hke that seen in fig. 31, and be made to drop easily into the lens-holder. Some persons use planoconvex lenses for the very low powers, in these the centre of the field will be perfect and well defined, but the margins not so ; hence, both theoretically and practically, it wiU be found that double convex lenses are the best for low powers, especially for dissecting; those who are in possession of a two-inch achromatic object-glass, wiU soon learn that where very careful work is required, a glass of that description will be by far the most pleasant to use. Mr. Powell supplies, with his dissecting microscope, represented by fig. 31, sometimes as many as seven lenses, the four lowest range in focal length from two inches to half-an-inch, and the fifth, is a Coddington lens, of a quarter of an inch focus, the remaining two being doublets, one of one-tenth, the other of one-twentieth of an inch focus. Two of these largest lenses are double convex, the other two either crossed or plano- convex. After the ordinary lens of half-an-inch in focus, the next increase in the magnifying power should be supplied by the Coddington lens, represented by figs. 25 and 26; this affords a large field, equally good in all directions, and its THE SIMPLE MICROSCOPE. 65 value is intermediate between that of a double convex lens of the best form and a doublet or achromatic lens. The doublet m general use is that before alluded to as the invention of Dr. Wollaston, and represented in section by fig. 40. It consists of two planoconvex lenses, having their focal lengths in the proportion of one to three, or nearly so ; these are set in two separate cells, a c, the upper one, a, is ca- Pig. 40. pable of being moved up and down in c, by means of the screw, as represented by the figure ; this enables the optician to adjust them to perform accurately. The lenses are placed with their flat sides towards the object, and the one of longest focus, which is also the largest, is placed nearest the eye. Between the two lenses there is a stop or diaphragm, b, which, for accurate definition, should also be carefully adjusted. The doublet, as described by WoUaston, in the Philosophical Transactions, was not pro- vided with a stop, nor does he even allude to the introduction of one; it is not certain, therefore, whether he was at all aware of its value, and his bright career having terminated in so short a time after the publication of the paper, the omission may, in some measure, be accounted for. The form of doublet described, at page 29, as the invention of Sir John Herschell, although free from aberration in the centre of the field, has a great deal towards the margin, and is therefore seldom used as a magnifier. When a triplet is required, it should be constructed on the plan of that of Mr. Holland, first described in the forty- ninth volume of the Transactions of the Society of Arts, and before alluded to at page 31 ; it consists, as is shown in sec- tion by fig. 41, of three planoconvex lenses, ab c, the first two, a b, being placed close together, and the stop or diaphragm between them and the third lens, c; this combination of three lenses was used by Mr. HoUand, either as a simple microscope, or as an object-glass to a com- Fig- 41. pound one ; and, although termed a triplet, it is essentially a doublet, having its front lens made up of 66 PRACTICAL TREATISE ON THE MICROSCOPE. two. A glass of this form Is capable of transmitting as large an angvilar pencil as 65° with perfect distinctness. The above described combination of three lenses approaches so very closely to the objects to be examined, that they re- quire to be covered with the very thinnest mica, which is objectionable, and no more than three lenses can possibly, be employed to form a single microscope ; hence the limit to the improvement of this instrument. Mr. Holland states, that for a triplet to be efficient for the podura, &c., it should be equivalent in power to a single lens of one-twenty-fifth of an inch focus; and in answer to those who object to the use of the triplet, on account of its approaching so closely to the object, he states that some of his preparations are covered with mica so thin, that they can be examined by a spherule of one-three -hundredth of an inch focus. " It was at one time hoped," says Mr. Koss, " as the precious stones are more refractive than glass, and as the increased refractive power is unaccompanied by a corresponding increase in chromatic dis- persion, that they would furnish valuable materials for lenses, inasmuch as the refractions would be accomplished by shal- lower curves, and, consequently, with diminished spherical aberration."* But these hopes were disappointed: every- thing that ingenuity and perseverance could accomphsh was tried by Mr. Varley and Mr. Pritchard, under the patronage of Dr. Goring., It appeared, however, that the great reflective power, the doubly-refracting property, the colour, and the hete- rogeneous structure of the jewels which were tried, much more than counterbalanced the benefits arising from their greater re- fractive power, and left no doubt of the superiority of skilfully made glass doublets and triplets. The idea is now, in fact, aban- doned ; and the same remark is applicable to the attempts at constructing fluid lenses, and to the projects for giving to glass other than spherical sin-faces, — ^none of which have come into extensive use. * Art., Microscope, Penny Cychpisdia. THE COMPOUND MICROSCOPE. ' 67 CHAPTER II. COMPOUND MICROSCOPE. A COMPOUND microscope differs from a simple cue in havino- the image of an object formed by an object-glass further magnified by one or more lenses forming an eye-glass ; or, in other words, the rays of light from an object being brought into a new focus, there form an image, which image being treated as an original object by the eye-piece, is magnified in the same way as the simple microscope magnified the object itself. For a microscope to be a compound one, it is, there- fore, necessary that it should have an object-glass and an eye- glass; in some of the old microscopes there were only two lenses, but it has been stated that, in the simple microscope, as many as three are employed to form a triplet, and yet, with this number of lenses, the microscope is still a simple one. This is easily explained: the first two lenses of the triplet only effect what might have been accomplished, but not so well, by one ; and the third lens is only useful for modifying the light before it enters the eye. As the object of this work is entirely practical, no mention will be made of the compound microscopes that have been heretofore, or are even now, manufactured in this country, that are not achromatic, and which, therefore, are unfitted for scientific investigation, and attention will be principally directed to those made by our first-rate opticians, Messrs. Powell, lloss, and Smith, all of whose object-glasses will stand the severe tests hereafter to be described, approaching, as far as we can judge at present, the limits of conceivable perfection, and the stands or supports for which ar& con- structed on the most approved mechanical principles, to prevent tremor, and to afford the greatest facility for using the various movements, and, in point of workmanship, are also unequalled. Every compound microscope may be said to consist, like the simple one, of two essential parts, viz., the stand and the 5* 68 PRACTICAL TREATISE ON THE MICROSCOPE. optical apparatus, both of which are very much more com- plicated than they are in the former instrvunents. The stand is made up of the compound body (or tube for carrying the optical apparatus), and the stage with the supports and ad- justments for each; whilst the optical part consists of the object-glasses or magnifying powers, the eye-pieces, and the mirror. It Httle matters what the shape or size of the instru- ment may be ; for whatever plan is adopted, or in whatever country it may be made, the parts above described are strictly essential, and must be present in each. The compound body is generally a tube of brass, from eight to ten inches in length, and from an inch to an inch and a half in diameter ; to its upper end the eye-pieces are adapted, to its lower the object- glasses; as these latter are of different magnifying powers, and as no two objects under examination are of the same thickness, it is highly requisite that there should be some mode of focal adjustment applicable to every condition. This is effected in two ways, one of which is termed the coarse, the other the fine adjustment; the first is generally accom- phshed by rack and pinion, by which the whole of the tube carrying the eye-piece and object-glass is made to approach or recede from the object by turning a large milled head connected with the pinion ; whilst in the second or fine ad- justment, the object-glass only is moved, and that by means of a very dehcate screw, acting either on the long end of a lever or in some of the modes Ijereafter to be noticed, whereby the same result is obtained. In the best constructed stands, the entire compound body containing the magnifiers is moved up and down by the coarse adjustment, but in many of the older microscopes, as represented by fig. 21, there were two or more tubes to make up the compound body. When this was the case, the outer tube was firmly attached to the other part of the stand, and formed the guide for the inner one carrying the optical apparatus to sUde through; under these circumstances the rack-work was placed in the com- pound body itself; but much greater stability is ensured by the adoption of the former method. In some instruments there is a tube connected with the compound body, capable THE COMPOUND MICROSCOPE. 69 of being drawn out to the extent of jBve or six inches: this is termed the draw-tube, into one end the eye-pieces fit, and into the other an erecting-glass is made to screw. This draw-tube has a scale of inches and parts engraved on its outer side, as represented by fig. 42, where a is the eye- piece, b the upper end of the com- pound body, and c the draw-tube, with the scale of inches and parts on it. The many uses of this tube, and of the erecting-glass also, will be fully described hereafter. The inner side of the tube carrying the magnifiers is, in all cases, provided with one or more stops or dia- phragms for cutting off the extra- neous pencils of light. The next part of the stand in importance is the stage or appara- tus on which the objects are to be placed for examination ; this, in the most complete microscopes, consists of two or more plates of brass, one of which, termed the stage-plate, is capable of being moved in two directions, at right angles to each other, either by screws, racks and pinions, by a combination of the two, or, more simply, by a lever. Upon the stage-plate another plate is adapted, termed the object-plate ; this last, for the more ready adjustment of the object to be examined. Is made to slide up and down upon the stage-plate, and is generally supplied with a raised ledge at its lower part, against which the objects themselves may rest when the stage is in an inclined position ; and sometimes another piece of brass, termed a clip, with a weak spring in its front part, is made to slide upon it, so that any object, if necessary, may be firmly secured between the clip and the raised ledge. The object-plate, besides the move- ment up and down on the stage-plate, and that in two direc- tions at right angles to each other, effected by the screws or rack-work, has also a circular one in a horizontal plane, which Fig. 42. 70 PRACTICAL TREATISE ON THE MICROSCOPE. is accomplished by mounting it upon a short piece of tube^ capable of fitting into another tube in the stage-plate; on this tube it turns, and by it the object-plate is also raised above the working parts of the stage-plate itself. The stage movements generally extend from half-an-inch to two inches, so that by the sliding up and down of the object-plate, and the distance the same plate is capable of being traversed over by the rack-work, aU parts of an object of considerable size can be brought in succession into the field of view. The different methods of effecting these stage movements will be described with the instruments to which they are severally adapted. To the under side of the stage a number of other pieces of apparatus can be fitted, viz., the diaphragm plate, the achromatic and other condensers, the lower prism of the polarizing apparatus, and the dark stops or wells, all of which wiU hereafter be described. To the object- plate should also be fitted the forceps for holding opaque objects. The methods of mounting the compound body and the stage are exceedingly various, the most improved plans are represented in the following figures, and for our present purpose it will be merely necessary to divide them into two classes ; first into those in which the compound body is sup- ported at its lower end, on an arm capable of being moved up or down by a rack and pinion ; and, secondly, into those in which the compound body is supported, either by an arm firmly attached to the back of it, as seen in fig. 21, when it is necessary that the body should be composed of more than one tube, or where a large portion of its length is supported, after the plan of Mr. George Jackson. The remaining portion of the stand of the compound micro- scope consists of the foot or basis, and of one or more pillars or supports rising from it, to which the compound body and stage are attached. The foot is generally a stout tripod of brass, cast in one piece, or, for convenience of package, it may be composed of three flat feet, capable of being folded to- gether, as in fig. 21, or as in two of Mr. Powell's instruments, of three longer legs, standing in an inclined position, like THE COMPOUND MICROSCOPE. 71 those in a three-legged stool. Some makers even use a heavy circular foot instead of a tripod, but this, although steady when the instrument is upright, is not so when it is inclined. From a foot of one of the above forms a stout pillar rises, having at its upper part a cradle-joint, to which both compound body and stage are firmly attached, so that when the joint is used, both these parts move together. Mr. George Jackson, having anticipated some difficulty in making a good cradle-joint, was induced to use two pillars instead of one, by which a greater degree of steadiness was obtained ; his com- pound body and stage were connected to both pillars by trunnions, on which they were made to turn. Mr. Jackson also mounted his compoimd body on a grooved beU-metal arm, — a plan somewhat similar to that adopted by Mr. Ross, in one of his early microscopes,* and which wiU be more fuUy described in the instruments of Messrs. Smith and Beck, who have adopted it. Mr. Eoss uses the tripod foot, and two flat supports, by which the same end is accompKshed as by pillars; but the supports, he considers, are much more free from vibration. In some of the recently constructed portable instruments, the stage is mounted on a strong pivot, on which it can be turned in the same direction as the compound body, for convenience of package. The smallest instru- ments of Messrs. Powell and Koss are constructed on this principle. The optical part of the compound microscope consists of the object-glasses, the eye-pieces, and the mirror. The object- glasses supplied with the best instruments are generally either five, six, or seven in number, and vary in their magnifying power from 20 to 2,500 diameters ; they are called two-inch, one -inch, half-inch, one-quarter, one-eighth, one-twelfth, and one-sixteenth; but it must be understood that these names are not derived from the distance the bottom-glass of each combination is from the object, but from a fact found in practice, that a thin single lens, to magnify the same number of diameters as any of the preceding achromatic combinations, * Art., Microscope, Penny Cyclopcedia. 72 PRACTICAL TREATISE ON THE MICROSCOPE. would be required to be of the same focal distance as that given to the others by name. In other words, if a single lens were made the object-glass of a compound microscope, and if it were necessary to employ a power equal to that of the one-fourth achromatic combination, with the same com pound body, it would be found that a thin single lens of one- quarter of an inch focus would be required to give that power. It would be more useful in practice if the name given to each of the object-glasses were expressive of the magnifying power instead of being derived in the manner above described; if we take, for instance, the glasses called the half-inch, as constructed by each of our three eminent makers, and compare them together, we shall find that all three wiU differ, more or less, in their magnifying power, but stiU they all bear the name of half-inch ; neither do two glasses, similar in name, even of the same maker, always agree exactly ; hence it would be very desirable in practice to apply a term to them which should express their magnifying power, but such a nomenclature could not at this advanced period be easily carried out. The eye-pieces supplied with the compound achromatic microscopes are generally three in number, and the form employed is that known by the name of the Huyghenian, it having been first employed by Huyghens for his telescopes. Each one consists of two planoconvex lenses placed at a dis- tance from each other equal to half the sum of their focal lengths ; the plane surfaces of the lenses are towards the eye, and that nearest the eye is termed the eye-glass, whilst that most distant is termed the field-glass. A stop, or diaphragm, is placed about half-way between the two lenses, this arrange- ment was adopted by Huyghens for the purpose of diminishing the spherical aberration, by producing the refraction at two glasses instead of one, and of materially increasing the field of view; but it was reserved for Boscovich to point out that another valuable property of this eye-piece was the correction of a great part of the chromatic aberration as well. This subject has been since critically examined by Mr. Varley, and to his paper, in the fifty-first volume of the Transactions THE COMPOUND MICKOSCOPE. 73 ©f the Society of Arts, the author would refer those of his readers who would wish to gain more information upon the matter. Another eye-piece sometimes employed is the in- vention of Kamsden ; it consists of two planoconvex lenses as in that by Huyghens, but the field-glass is reversed, or its plane smrface is placed farthest from the eye-glass ; this in- strument, which wiU be again alluded to in the chapter on micrometers, is chiefly used when it is required to measure the magnified image of any object, hence it has been fre- quently called the micrometer eye-piece, the divided glass being placed immediately in front of the field-lens. When this eye-piece is used, the image is formed in front of the field-glass, and, consequently, the focal point of the eye-piece is outside the field-glass; but in the Huyghenian form, the image of the object is formed at the diaphragm between the field and eye-glass; hence the former has been termed the positive, and the latter the negative, eye-piece. The mirror generally consists of a frame of brass, in which are set two silvered glasses, one concave the other plane, which should not be less than two inches in diameter; the former reflects the light in converging the latter in parallel rays. For facility of adjustment, the frame carrying the glasses is made to turn in every direction, by means of joints, and in the best microscopes it is adapted to a tube on which it can be slid either up or down, and so be approximated to the under surface of the stage, in order that the rays reflected from the concave surface may be brought into a focus or not upon any given object on the stage. In some microscopes the plane mirror is replaced by one made of plaster of Paris, which reflects a soft white light, or by a prism of glass, the invention of M. Dujardin. Mr. Varley has suggested a plan of covering the plane mirror with pounded glass, or carbonate of soda, by which means the light of a bright cloud opposite the sun may be artificially imitated, and even the rays of the sun itself may be reflected, and so produce a soft white light. The different modes of using the mirror wiU be alluded to in the chapter devoted to the illumination of microscopic objects, where, also, will be described several other kinds of appa^ 74 PEACTICAL TREATISE ON THE MICROSCOPE. ratus, whereby the quality of the light may be materially modified. All the parts essential to a compound achromatic micro- scope having now been described, attention wiU next be directed to the different arrangements adopted by the prin- cipal makers to render the mechanical part most effective, and, as in all other cases, the names of the manufacturers win, as far as practicable, be taken in alphabetical order, no preference being given to the workmanship of one over that of another, but credit always awarded wherever it may be due. MESSRS. POWELL AND LEALAND'S ACHROMATIC COMPOUND MICROSCOPE. This instrument, first described in the Microscopical Journal, Vol. I., page 177, is represented by fig. 43 ; it stands on a firm tripod base of brass, on which is a circular plate ; to this two stout pillars are attached, bearing at their upper extremi- ties the ends of the trunnions, upon which a strong piece of metal, giving attachment to the compound body and the stage, is supported; by means of the circular plate, the pillars can be turned upon the tripod, and the weight of the com- pound body and stage brought over one or more of the feet of the tripod, and the instrument, therefore, rendered more steady. This plan of using the double pillar was first adopted by Mr. George Jackson, in 1838, and possesses the advantage of being light and of distributing the weight of the superin- cumbent parts more equally on the tripod than where only one piUar is employed. The compound body is supported nearly the whole of its length on a strong arm, having a hoUow frame at its top, after a plan first described by Mr. Powell, in the fifty-third volmne of the Transactions of the Society of Arts. The coarse adjustment is made by a rack and pinion contained within the frame above noticed; and the latter turned by the large milled head A. In order that the compound body may be moved easily and still be very steady, it is attached to a cradle resting upon two rollers, one inch-and-a-quarter THE COMPOUND MICROSCOPE. 75 76 PEACTICAL TEEATISE ON THE MICROSCOPE. wide, and three-and-a-half inches apart, this Tjeing equivalent to a triangular bar of the same size. The fine adjustment is made by a screw with a cone, against which the cradle, or portion of brass attached to the body, is firmly pressed by means of a spring ; one of the milled heads of the fine adjust- ment is seen at B. By this method of mounting the com- pound body, all tendency to run down by its own weight is prevented, in consequence of its motion being that of a sliding combined with a rolling one. The lower part of the arm carrying the compound body at I is provided with a conical pin fitting into the piece of metal supporting the stage; by this a circular motion is obtained, and the body can be turned away from the stage, so that an object placed upon it can be properly adjusted before the body is brought over it. The stage is of the form first constructed by Mr. TurreU, and described by him in the forty-ninth volume of the Transactions of the Society of Arts. It has a motion each way of three-quarters of an inch in extent, that from side to side being effected by a screw turned by the milled head C, whilst the up and down motion is performed by a rack and pinion in connection with the milled head D. The stage- plate has a circular motion, and on it is a spring clip H for seciuiag the objects when the instrument is inclined. A small arm E is seen underneath the stage; this carries the dark weUs, to be used when minute opaque objects are iUu- minated by the Lieberkuhn. The mirror is mounted rather differently from those supplied with other microscopes ; instead of a semicircle of brass with two pins, on which the frame containing the reflectors may turn, there is a quadrant of brass, having at one end a strong pin on which the frame is turned up or down, and at the other end a still stronger one, on which the quadrant and the frame together are capable of being revolved; this last fits into a short piece of tube, made to slide either up or down the long tube attached to the bottom of the stage, by which the mirror is connected with the other part of the stand ; the reflectors themselves are both plane and concave, as in other instruments. With this microscope are supplied an achromatic condenser, a THE COMPOUND MICEOSCOPE. 77 micrometer, frog-plate, vial-holder, small and large condens- ing lens, steel disc a substitute for the camera lucida, polarizing prisms, and many other important pieces of appa- ratus, and the price varies from forty to seventy guineas, depending upon the number of the powers and the apparatus attached thereto; the powers themselves range from the two inch to the one-sixteenth, and magnify from 20 to 2,500 diameters. A second microscope, constructed by Messrs. Powell and Lealand, and known by its being mounted on three legs, is described in the London Physiological Journal, page 63, and is represented in Plate 2. The three legs inclined, as seen in the figure, support, at their upper part, the trunnions to which the tube, J, and the stage are attached. From out the tube, J, a triangular bar is raised by a rack and pinion con- nected with the large miUed head, A. To the upper part of the triangular bar a broad arm is fixed, bearing the compound body ; this arm is hoUow and contains the mechanism for the fine adjustment, which is effected by turning the milled head, B. The arm is connected with the triangular bar by a strong conical pin, on which it turns, so that the compound body may be moved aside from the stage when necessary. The stage is similar to that described in the preceding instrument, and is capable of being moved from side to side by the milled head, C, and up and down by that at D. When both are turned together,' a diagonal movement is produced, the axis of D is carried through to the opposite side of the stage, where there is another milled head, so that, if necessary, both hands may be employed at the same time. The achromatic con- denser is represented as fixed into its place at the bottom of the stage, where also may be seen an arm, E, for the stops or dark wells. The mirror, G, and the spring clip to the stage, H, are aU similar to those described in the former instrument. In order to render the compound body exceedingly steady, two small rods, springing from the arm, are attached to the back part of the body a Httle above the centre. To this microscope, as well as to the preceding, aU the apparatus there mentioned can be fitted. The stand itself is not so 78 PRACTICAL TREATISE ON THE MICROSCOPE. oostly as the first described ; and, although much lighter and more portable than it, is nevertheless exceedingly steady, from all the parts being accurately balanced. MESSES. POWELL AND LEALAND'S PORTABLE MICROSCOPE. One of the most portable and convenient forms of compound microscope is that made by Messrs. Powell and Lealand, and represented by fig. 44, about one-third of its actual size. It is supported on three legs, ABC, capable of being folded one upon the other, and when so folded can be brought in a line with the tube D E, supporting the stage, G, and the mirror, F, and through which slides the triangular bar, H, having at- tached to it the arm, I, carrying the compound body, L K ; this last, for convenience of package, is made to unscrew at K, and the eye-piece, L, being removed, the folded legs can be passed through the tubular part of the body, and both together laid parallel with the tube D E. The legs are connected with the tube D E by a strong ciirved piece of brass, M, which winds round to the opposite side of the tube ; a stout pin, with a screw nut, serves as an axis upon which the tube, D E, and all that is attached to it, can be turned from a vertical to a hori- zontal position. The stage, G, presenting a box-like appear- ance, is also capable of being turned into a position parallel with the folded legs, by drawing back the sliding-piece, P, which, when in use, keeps it in a horizontal position. The apparatus for moving the stage is contained within the box, G, and is similar to that employed by Messrs. Powell and Lealand in their larger instruments, the up and down move- ment being performed by turning the milled head, N, and that from side to side by the larger milled head, O. The slide, Q, is for the purpose of supporting the object when the microscope is inclined, and in it are two sockets for receiving the forceps for holding opaque objects. To the under-side of the stage are attached a diaphragm and a small arm for carry- ing the dark stops or wells ; and, should it be required, an achromatic condenser, or polarizing prism, may be fitted into the place occupied by the diaphragm. The coarse adjustment THE COMPOUND MICROSCOPE. 79 of the instrument is efFected by rack and pinion, by which the triangular bar, H, and with it the arm, I, carrying the com- Fig. 44. pound body, K L, are moved up or down, the rack being situated at the back of the triangular bar and the pinion con- nected with the large miUed head, E. The fine adjustment is made by turning the screw, S ; this acts on the end of a lever contained in the hollow arm, I, by which the short tube, T, to 80 PRACTICAL TREATISE ON THE MICROSCOPE, which the object-glasses are attached, is slowly raised or de- pressed. The mirror, F, is made to slide up and down the tube, D E, and being mounted on a semi-circular arm, can be turned in every possible direction. One great value of this microscope is its extreme portability, as the whole apparatus, consisting of the above described instrument, together with four object-glasses, two eye-pieces, animalcule cage, dark stops, forceps, &c., can be packed in a box, the internal measurement of which is nine inches long, five broad, and two deep. Besides the three preceding microscopes of Messrs. Powell and Lealand, there are two others made by them requiring especial mention. The first of these is of large size, and consists of a heavy tripod base, from which rises a short stout pillar, having a cradle joint at its summit, to which is attached a triangular bar, fifteen inches in length, and each of its sides one-inch-and-a-quarter broad. To the middle of this bar a stage, seven inches square, is fixed ; it has the same kind of adjustments as those of the smaller instruments, being made also on Mr. Turrell's plan, and, from being of large size, the hands of the operator do not interfere with the object when adjusting it. The milled heads for effecting the adjust- ment are placed in a line, so that one hand only is required to move the stage in two directions. The compound body is firmly supported on the upper half of the triangular bar by a frame which fits the bar accurately and is made to move smoothly up or down by rack and pinion turned by two milled heads; the compound body is also capable of being turned away from over the stage by means of a joint in the frame supporting it. The fine adjustment is made by an endless screw and two inclined planes ; it has also two milled heads, only two inches apart from the coarse, but both horizontal and parallel with them; by this means, the hand may be passed from one to the other very readily. To the lower part of the triangular bar is adapted the mirror, which is of the same construction as that previously described, but capable of being moved up and down on the triangular bar by rack and pinion; the achromatic condenser is attached THE COMPOUND MICROSCOPE. 81 to the mirror, and is moved with it on the bar, so that the axes of its lenses may coincide with those of the object- glasses. To the stage may be fixed all the usual apparatus, and even a frame of large size for holding such objects as are three or four inches broad. The weight of this instrument is very great, and it is remarkable for its steadiness and the excellence of the workmanship ; its price, with all the appara- tus complete, approaches nearly to one hundred pounds. In consequence of the great amount of labour expended in its construction, and its necessarily high price, the demand for this microscope has not been great for the last few years, the three previously described having in a measure superseded it. Another very useful microscope for general purposes, made by Messrs. Powell and Lealand, is much less costly than any of the others, the tripod and supports for the compound body and stage being made of cast-iron ; the stage is of large size, and they have lately effected a great improvement in it by making it adjustable by a lever ; in the stages hereafter to be described with the lever movement, two or more plates are employed, but in this instrument one only is used, and it per- forms exceedingly well, being very steady even with the highest powers. The compound body is supported on an arm fixed to the back of it, and the coarse adjustment is made by rack and pinion, the fine by a screw acting on the end of a lever. This microscope is available for all the purposes to which the more costly ones are apphed, and is particularly- useful to the medical student, to whom its low price is also a great recommendation. MB. boss's COMPOUND AND SIMPLE MICROSCOPE. This instrument, first described by Mr. Ross, in the London Physiological Journal, in 1843, is represented by Plate 1 ; and as no language of the author could convey so good an idea of its construction as that given by Mr. Eoss himself, he wiU here take advantage of it, and quote his own words: — « The mechanical construction represented in Plate 1, is derived from a practical acquaintance with the various im- 6 82 PRACTICAL TREATISE ON THE MICROSCOPE. provements made in the microscope for the last twelve years. The general arrangement, which is properly the province of the mechanic, has been contrived to obtain the utmost freedom from tremor, and to afford the greatest facUity in using the various movements, while the extent, direction, and number of these have been collected from the experience of the most indefatigable observers in all the various branches of micro- scopic inquiry. Nearly five hundred instruments have been made on the plan here represented, and as no alteration or addition has been found necessary for the accomplishment of all the modes of microscopic investigation at present employed, the mechanical structure of the microscope stand may be con- sidered thus far established. " The optical part also has arrived at such perfection, that points or lines, whose distance is such that their separation is bordering on interfering with the physical constitution of light, can be distinctly separated; thus ensuring a reality in the appearance of objects, where the minuteness of their detail approaches the natural limit of microscopic vision. "Description of the Instrument, (Plate 1.) "A A are two uprights, strengthened by internal but- tresses, mounted on a strong tripod, B, at the upper part, and between the uprights is an axis, C, upon which the whole of the upper part of the instrument turns, so as to enable it to take a horizontal or vertical position, or any intermediate inclination, such, for instance, as that shown in the plate. This moveable part is fixed to the axis near its centre of gravity, and consists of the stage, D D, the triangular bar and its socket, E and F, the arm, Gr, which carries the microscope tube, H, and the mirror, I. The stage, D D, has rectangular movements, one inch in extent, on the racked cylinders, a a, and are moved by pinions connected with the milled heads, b b' ; it also has the usual appendages of forceps to hold minute objects, and lens to condense the light upon them. The triangular bar, together with the arm and micro- scope tube, is moved by the milled heads, e e, and a more delicate adjustment of this optical part is effected by the THE COMPOUND MICEOSCOPE. 83 milled head, /. The other miUed head, g, fixes the arm, G, to the triangular bar. " The outline of the structure, as before observed, has been arranged to obtain, first, the utmost freedom from tremor, and secondly, to afibrd the greatest facility in using the various movements, "In experimenting to obtain the first of these conditions, I suspended the moveable part of the instrument near the centre of gravity, and employed the inverted pendulum (an instru- ment contrived to indicate otherwise insensible vibrations) to arrange the form and quantity of material so as to produce, as nearly as possible, an equality of vibration throughout the whole instrument; hence the object upon the stage and the optical part vibrating equally, no visible vibration is caused. The arrangement for accomphshing the second condition is, first, that the whole movements should be as near the base of the instrument as is consistent with the greatest proximity among themselves ; then the milled heads, e and/, for moving the triangular bar, and the fine adjustment for the optical part, should be moved by the left hand, while the heads, h h', for the movement of the stage, should be worked by the right hand. The other milled head, e, is convenient ■when the right hand may be unemployed with the stage movements. The positions of the milled heads, h V, are ex- tremely convenient, as the middle finger may be placed under h, and the fore-finger under U, and the thumb passed from the one to the other in the most natural and easy manner. The left hand is also readily shifted from the milled head, e, to employ the fore or middle finger to move the screw head, /. This head is connected with a screw and lever, which makes one revolution of it move the optical part one-three-hundredth of an inch. This arrangement aflTords an elastic movement to the end of the tube, as a guard against injuring the glasses or the object under examination." In consequence of the improvements at this time being made by Mr. Eoss in the illuminating part of the microscope, which will considerably modify the general form of the instrument, a separate description will be given in the Appendix. 6* 84 PRACTICAL TREATISE ON THE MICROSCOPE. MR. ROSS'S PORTABLE ACHROMATIC COMPOUND MICROSCOPE. This instrument is represented by fig. 45, and, like the larger one, is supported on a firm tripod base, a, from which rise two strong uprights, h, supporting at their upper parts the trunnions to which the square frame, c, carrying the stage and the tube, d, are attached. Within the tube, d, a smaller tube is made to slide up and down by rack and pinion, the former is seen at n, the latter being turned by the milled head, o; this forms the coarse adjustment. To the upper part of the inner tube a very stout arm, e, is attached by the screw, f, on which the arm may be turned ; into the opposite end of the arm, the compound body, g, is screwed. The fine adjustment consists of a conical-pointed steel screw pressing against the top of a slit in an inner tube, to the end of which the adapter for receiving the object-glasses is fixed. The stage has the usual rectangular motions, that from the side being performed by a screw and nut, by tiu-ning the milled head, i, whilst the up and down movement is effected by rack and pinion, by turning the milled head, h. The stage-plate is provided with a sHdiog-rest, I, by which the distance of an object from the central hole in the plate may be regulated before focussing ; this answers the purpose of the complicated sliding frame in the more expensive instruments. At the upper part of this stage-plate there are two holes for the reception of the forceps and side refiector. To the under part of the stage the achro- matic condenser, the diaphragm-plate, the dark wells, and polarizing prism, may all be adapted as in the larger instru- ments ; and, for convenience of package, the stage itself may be turned on a pivot, so as to be at right angles with the tube, d. The mirror, m, is mounted in the usual manner, and is capable of being raised up or down the tube, d, on which it is supported. This stand, like the preceding, is constructed on a plan ascertained by Mr. Ross, after a lengthened series of investi- gations, to be the most steady, and is particularly to be recommended to those whose means are limited, in conse^ THE COMPOUND MICKOSCOPE. 8^ Fig. 45. 86 PRACTICAL TREATISE ON THE MICROSCOPE. quence of its low price, it being of a form which may be added to from time to time, according to the wants of the employer; thus, for instance, a vertical stand with two eye- pieces, exclusive of the object-glasses, may be procured with- out the stage movements or the fine adjustment, at the small cost of £4 10s.; and as both the stage and the compound body are of the same size in the vertical as in the perfect instrument, the fine adjustment and stage movements may be added to the former at any time, and render it as complete as that represented by fig. 45. Tor convenience of package, the compound body may be unscrewed from the arm, e, and the entire instrument, together with condensing lens, forceps, animalcule cages, &c., be fitted into a case seven-and-a-half inches high, six-and-a-half inches broad, and five-and-a-half inches deep; or, if preferred, the foot, a, may be removed from the uprights, 6, and the stage being turned parallel with the axis of the tube, d, the whole will pack in a flat box seven-and-a-half inches long, five-and-a- half inches broad, and two-and-a-half inches deep. Mr. Ross has also made a small complete microscope stand, which is a perfect model of the larger instrument. This, to- gether with all the apparatus, is packed in a case nine inches long, six-and-a-half inches wide, and three inches deep, and forms a very compact travelling microscope. Besides the preceding instruments, Mr. Eoss has made many other kinds. One of the best of these is described and figured in the article " Microscope," in the Penny Cyclopcedia ; this was the instrument having the middle-third of the com- pound body supported by a triangular cradle on a beU-metal arm, which suggested to Mr. Jackson the plan of attaching the entire length of the body to an arm somewhat of the same kind, but with dove-tailed slides, for it to move up and down on. The stage which Mr. Eoss adapts to his microscopes differs in some few respects from those employed either by Mr. Powell or Mr. Smith: the movements are effected by two racks and pinions placed at right angles to each other, and either worked by milled heads placed underneath the stage at THE COMPOUND MICROSCOPE. 87 right angles to the movements, or else as seen in Plate 1, where they are both in the same plane with them; in the portable instrument there is, however, a screw introduced instead of a rack, by which the movement from side to side is effected ; but the screw is a fixture, and the stage-plate, with the miUed head attached, is moved backwards and forwards on the screw. To all Mr. Eoss's instruments the achromatic condenser, the polarizing prism, and other apparatus, are capable of being adapted, but there is no draw-tube to the compound body in either of them for the erecting-glass or micrometer eye-piece, as in the microscopes of Messrs. Smith and Beck ; the form of eye-piece employed by Mr. Ross not requiring such an addition in the use of the micrometer. MESSES. SMITH AKD BECK'S LARGE ACHROMATIC COMPOXIND MICROSCOPE. This instrument is represented by Plate 3, and consists of a firm tripod base, AAA, upon which two strong pUlars, B B, are screwed : these at their upper parts support the trunnions, to which the beU-metal arm, C, and the stage, E, are attached, and by means of which this part of the iastru- ment can be inchned at any angle. The arm supports the entire length of the compound body, F, on its inner edge, which is ploughed out in such a manner as to receive two brass rods or guides attached to the compound body ; one of these, which is soldered to the whole length of the body, is of a triangular figure, and to its apex is screwed a thin flat piece of metal of corresponding length, about five-eighths of an inch broad, and one-eighth of an inch thick, having a rack, or sometimes, two racks, cut on its oiiter or unattached side; the former guide fits into a triangular channel ploughed out of the arm, and the latter slides into a channel of the same shape as itself immediately at the back of the triangular one ; the triangular gxiide forms a firm support for the body to rest upon, and the flat guide answers the purpose of keeping the first in close apposition with the channel, whilst by the rack 88 PRACTICAL TREATISE ON THE MICROSCOPE. at its back, the movement of the body up and down the arm is effected by the pinion connected with the milled heads, Gr G, which form the coarse adjustment. There is a draw-tube at the upper end of the body, into which the eye-pieces and erecting-glass fit, and to the lower end there is added a short tube, to carry the object-glasses ; this is moved up and down slowly by the nut, K, acting on the end of a lever, and so forms the fine adjustment. The stage adapted to this instru- ment may be one of two forms, either one whose movements are effected by a lever, or else so constructed that the up and down motion is produced by a rack and pinion, and that from side to side by a screw, whose axis is carried across to the opposite side of the stage, and there can be turned by the left hand. The lever stage is represented as attached to the instrument ; this is constructed after the plan of that of Mr. Alfred White, and described by him in VoL I. of the Trans- actions of the Microscopical Society. It consists of three plates of brass, the lower one of which is fixed, and the other two provided with certain dove-tailed guides and slides, so that the upper one may be moved by a lever, either independently of the middle one, or else be carried along with it. The lever is seen at O ; it is about five inches long, and is loaded with metal at its upper part, so as to balance the weight of the stage-plate, and at its lower end is provided with a baU work- ing in a socket connected with the upper plate; about an inch higher up is another ball working in a socket, P, in a small arm connected with the support of the compound body, C C. The dove-tail guides of the middle stage-plate are arranged horizontally, whilst those of the upper plate are placed vertically; when, therefore, the lever, o, is moved either to or from the support of the compound body, both stage-plates will move horizontally in the opposite direction but when the lever is moved in a hue parallel with the side of the same support, then only the upper one is moved ; and as the end of the lever to which the hand is applied moves in all cases in an opposite direction to that of the ball, a, and as the compound microscope always inverts the image of the object under examination, the object wUl appear to move in the THE COMPOTIND MICROSCOPE. 89 direction of the hand. The object-plate is provided with a spring cKp, N, capable of being slid up and down, and of being turned upon the upper plate of the stage, and is always moved with it. To the under side of the stage, the diaphragm, E, is seen attached. Mr. White's lever stage, of which the above described is a modification, is represented by fig. 46, as fixed for use to the lower end of the arm supporting the compound body. The mirror, S, is of large size, and is mounted on a tube, W; it has plane and concave reflecting surfaces; the frame is supported by a semicircular piece of brass, T, with two pins for it to turn on, at U is a joint on which it can be moved horizontally, and at V another joint for turning it away from the axis of the instrument, so that very ob- lique light may be sent through the hole in the stage, and by means of the short tube it may be sHd up and down on the support, W. In the arm, C C, may be^seen two square holes, d d, into these the supports of the side reflector and of the small condensing lens are made to fit, and are kept firmly fixed by the screws, D D. To this instrument, if preferred, another stage may be fitted, as exhibited in Plate 3, fig. 2, where A represents part of the large arm for supporting the compoimd body, B one of the pillars, C the joint, and D the tube for the mirror. The stage-plate, E, carrying the object-plate, F, is moved from side to side by the miUed head, G, connected with a screw, whose axis passes through to the opposite side of the stage, where there is another nulled head, and up and down by a rack and pinion connected with the milled head, H. Fig. 46. 90 PEACTICAL TREATISE ON THE MICROSCOPE. MESSES. SMITH AKD BECk'S SMALLER ACHROMATIC COMPOTJND MICROSCOPE. This instrument is represented in Plate 4, fig. 1 ; it is mounted on three feet, AAA, capable of being closed to- gether ; into a circular plate attached to these feet is screwed a pillar, B, having a cradle joint, C, at its upper part ; to the joint is attached a bent arm, D, grooved like that of the larger instrument, and supporting in a sicnilar manner the compound body, G, the triangular guide and rack being seen at E. The milled heads, F F, are for the coarse adjustment. To the bottom of the compound body there is attached a small tube, into the lower end of which the object-glasses are screwed, and to its upper end a lever, the short extremity of whose long end, I, is capable of being moved up and down by the nut, K, working on the screw, H, and so forming the fine ad- justment ; the tube. is kept tight against the nut by a spring, which, when the object-glass is accidentally brought in contact with any object on the stage, allows of its retreating for a short distance, and in most cases, prevents either the object or the object-glass itself from being fractured, hence it has obtained the na,me of the safety-tube. To the lower end of the arm, D, the stage, K, is screwed; this consists of two plates, which are capable of being moved in two directions at right angles to each other, by racks and pinions connected with the milled heads, M M. The object-plate, L, is pre- cisely similar to that in the preceding instrument, and to the under side of the stage aU the usual apparatus may be fixed. The mirror, N, is mounted in the ordinary way upon a semi- circular frame, O, having a pin passing through a piece of cork in the end of the tube, P, on this it can be turned hori- zontally. To render this a cheaper instrument, the stage shown at fig. 2 may be substituted for the adjustable one. A represents part of the arm supporting the compound body, B a plate of brass attached by screws to the lower end of the same arm, C the joint at the upper part of the pillar. Upon the plate B is supported the object-plate, D, capable only of being moved by the fingers in two directions, the one verti- THE COMPOUND MICEOSCOPE. 91 cal and the other circular. On account of the feet folding together, this microscope can be packed in a flat box, the thickness of which is regulated by the breadth of the stage. When more portable and less expensive stands are required, the two following deserve especial notice. MESSES. SMITH AND BECK'S ACHROMATIC COMPOUND MICEOSCOPES FOR STUDENTS. Fig. 1, Plate 5, represents the largest of these instruments. The base is composed of brass, cast in one piece ; it stands on three feet, AAA, from which proceed the two flat, upright cheeks, B B, having a trunnion joint at C, on which the stage and the compound body are capable of being turned. Into the plate, H, is screwed a stout tube, L, upon which slides another tube supporting the straight arm, M. This last is ploughed out in the same manner as the arm in the larger instruments, and the compound body, N, resting on the guides, O, is moved up and down it by turning the miUed head, P. Within the tube at L, to which also the arm, M, is attached, is situated a spiral spring, that keeps the arm, M, always firmly in contact with the plate, I ; against this last the fine screw, K, with a graduated miUed head, presses ; when the screw Is turned, both the arm, M, and the compound body are moved slowly up or down, forming the fine adjustment. The spring is prevented from forcing the arm, M, out of the tube, L, by a stop situated just above the milled head, K, which is not represented in the figure. The stage is a plate of brass, about four inches long and two inches wide, having dove-tail grooves, in which the frame, G, for holding the ob- jects, slides up or down, it being readily moved by two small handles projecting from it ; one of the ends of the frame is provided with a socket, F, for the reception of the forceps and other instruments. The mirror, D, is mounted in the usual manner on the semicircle of brass, E, and is capable of being turned on a large pin fitting into the end of the tube which is attached to the under surface of the stage. The second microscope is constructed much on the same 92 PRACTICAL TREATISE ON THE MICROSCOPE. plan as the last described, but is much smaller, and only capable of being used in the upright position; it is repre- sented by fig. 2. The stand is supported on three feet, AAA, having two flat upright cheeks, B B, connected with them, to the top of these the stage-plate, D, is fixed. The tube, G, is screwed into the upper surface of the stage-plate. Within it, as in the larger instruments, a smaller one slides, having the arm, H, supporting the tube, I, connected with it. Through the tube, I, slides very smoothly up and down the compound body, L, carrying the eye-pieces and object- glasses ; this forms the coarse adjustment, whereas the fine adjustment is made by turning the screw with milled head, E, which either raises or depresses the arm, H, and the entire compound body, L I, with it, in the same manner as was described in the preceding instrument. A diaphragm, K, is fitted into the bottom of the stage-plate. The mirror, C, is supported on trunnions working in the front part of the cheeks, B B ; but having only a circular movement, hence it is required that the fight to illuminate objects should be always in front of it. A stand of this description is ex- ceedingly useful for keeping on the table where dissections are going on, as small portions of the different tissues can readily be placed under a quarter-of-an-inch object-glass, and be examined as they are removed, the shortness of the stand allowing of its being used without much trouble ; and almost aU objects, for temporary purposes, being mounted in fluid between glasses, they are apt to sfip down when placed on the stage of an inclined instrument; and as aU the large microscopes are too high to be used on a table at which dis- sections are carried on, without either being inclined or the dissector being obhged to get up from his seat every time an object placed between glasses, with or without fluid, is re- quired to be examined in the horizontal position, this little instrument is extremely useful for these purposes, and two such, one provided with a power of forty, the other with that of two hundred, should be always at hand; they are most efficient working tools, the cost of each without glasses not exceeding £3. The sliding up and THE COMPOUND MICROSCOPE. 93 down of the body, L, in the outer tube, I, forms a very good coarse adjustment, whilst, after the object-glass has been brought sufficiently near the object by this means, the fine will answer for the remainder. The height of this instru- ment, when the compound body and draw-tube are shut down, is not more than eight inches, and it is not much too large to be carried in the coat pocket. With all these micro- scopes the usual accessory instruments are snppHed if re- quired ; many of them differ in some points of construction from those both of Messrs. Powell and Ross, and with them wiU be fully described in the chapter devoted especially to the consideration of these subjects. Before concluding this chapter, the author would direct the attention of his readers to the compound microscopes of Mr. Pritchard, fully described in the last edition of his Microscopic Illustrations, where will also be found full directions for the construction of proper stands, and the methods of using the various microscopes and the pieces of apparatus supplied with them, with numerous illustrations to explain the same, aU of which subjects wiU repay an attentive perusal. A compound microscope constructed by Mr. Varley, and described by him in the fifty-fifth volume of the Trans- actions of the Society of Arts, as the Single Lever Microscope, here also requires especial notice. This instrument is repre- sented by fig. 47, one-third of the real size, and consists of a hollow foot, somewhat like that of a bird in shape, from which a stout pillar rises, having at its top a thick, flat disc of brass, a, with a central hole; to this the microscope is joined by means of a strong block, b, whose face is turned to fit against it ; a central screw passes through the hole, and all the important parts of the instrument are kept fast to the block by the screw nut, c. Through the block, h, slides the long rod, d, against which a saddle is placed for the screw, e, to bind it fast at any height. To the same block, b, the back plate of the stage, g, is fastened; from this is given off the arm, r, which, in connection with the shorter arms, q q, sup- ports the fulcrum of the lever, s, having attached to it two balls, the lower one of which works between two plates at p, 94 PRACTICAL TREATISE ON THE MICROSCOPE, and the upper one between two others at t; to the upper of these last the stage-plate, h, carrying the object-plate, y, is Fig. 47. joined. The lever descends sufficiently near to the table to enable the hand, whilst resting thereon, to puU or push it in THE COMPOUND MICEOSCOPE. 95 any direction, and so move the stage the reduced quantity, which, in this case, is as one to six. To enable both sides of the stage- plate, h, to move simultaneously, a parallel motion is added, one of the rods of which is seen at w. Whichever way the baUs and sockets move, the stage-plate, h, obeys their mo- tions, and an observer, with the lever in his hand, may foUow the course of any living object. By an error on the part of the artist, fig. 47 is reversed; the lever should be on the right hand. To the lower part of the stage is fitted either one of Mr. Varley's dark chambers, or a Wollaston condenser: Mr. Varley prefers the former, as it is more free from colour. At the lower part of the tube, z, into which the rod, d, slides, is seen the mirror; this, as in Mr. Powell's microscopes, is mounted on a bent arm, and, if necessary, by means of a sliding tube, may be moved up or down the tube, z. The tube of the compound body, 1, is mounted by means of a hollow case or trough, 2, having two arms, 7, upon the rod, d, and is kept firmly fixed in any position by means of a screw with a milled head and a bent spring. To the back of the tube is soldered a rack, this is connected by two saddle-pieces, 3, with a bar, 4. A pinion, held in a spring, made of plate-brass, as wide as the trough, is attached by a screw to its inner side, and the milled heads which turn the pinion are seen on each side of the same trough ; by either of these the coarse adjust- ment is efiected. Through the upper part of the tube, 1, slides that part of the compound body which supports the eye-piece, and to the lower end is attached a bent arm, through which works the milled head-screw, 12 ; above this is another bent arm connected with a smaller sliding tube bearing the object- glasses ; within this tube is a spiral spring, the action of which causes the tube to be pushed out, but this is prevented by the long arm of a lever, 11, against which the screw presses. When the screw therefore is turned, the arm, 11, is either raised or depressed slowly, and by this the fine adjustment is accomplished. A condensing lens, 27, is most conveniently held by a moveable arm ; the curve, 29, and joint, 30, allow it to be moved to or from the stage, either vertically or horizontally. 96 PRACTICAL TREATISE ON THE MICROSCOPE. SO as to suit every piirpose. For convenience of package, or for applying Mr. Varley's graphic eye-piece to this instru- ment, the compound body and its supports, 7, may be removed from the rod, d, and the rod itself may be drawn out of the tube, z, so as to allow of any object not more than three inches thick being examined under a lens of two inches focus. Amongst other advantages in this microscope, there is added to it a small piece of apparatus, by which a phial having chara growing in it, or animalcules adhering to its' inner sur- face, may be examined in a vertical position : many of these last would, in all probability, be shaken off if the phial were turned about when inclined. Also, by the addition of the graphic eye-piece, the tracing of aU kinds of objects, whether magnified much or little, can be readily accomplished. The price of this microscope, exclusive of the object-glasses, varies from £20 to £30. Another very excellent form of micro- scope is that constructed by Mr. Dancer, of 43, Cross-street, Manchester; it is represented by fig. 48, and consists of a firm tripod of brass, from which rise two stout pUlars, bearing at their upper extremities the trunnions that support a slightly curved arm, to which the stage and compound body are at- tached, somewhat after the plan of that of Mr. George Jackson. The compound body itself consists of two tubes, the outer one being attached to the arm by two saddle-pieces with screws ; this tube is sprung at either end, and within it a smaller one can be moved up and down by rack and pinion, turned by a large mUled head ; this forms the coarse adjust- ment, whereas the fine is effected by a plan of Mr. Hoss, viz., by a lever attached to the small tube carrying the object-glasses, which is moved either up or down by a fine- threaded screw. The stage is about four inches long, and two- and-a-half broad, and on it slides an object-plate longer than the stage-plate, but about half its breadth. To the front of the stage may be fixed the forceps and a large condensing lens, if necessary. The mirror is of the usual form, and is capable of being moved up or down the tube that connects it with the under surface of the stage, and can also be inclined at any angle. With this microscope, Mr. Dancer supplies the THE COMPOUND MICROSCOPE. 97 usual amount of object-glaeses and other apparatus, and to the correct performance of the former the author is happy to add his wiUmg testimony; although they do not surpass those of the three principal makers in this metropolis in their defining and penetrating power, they are, nevertheless, capable of Fig. 48. exhibiting remarkably well the usual test objects, and are, on account of their cheapness, highly to be recommended. The stand itself is very well planned, and the manner in which the workmanship is executed reflects very great credit on the manufacturer. Mr. Dancer has lately made two or three im- 7 98 PEACTICAL TREATISE ON THE MICEOSCOPE. provements in the stand of his instruments ; the compound body is now mounted upon a plan somewhat like that of Messrs. Smith and Beck ; it slides up and down in a dove- tailed groove in the arm, but the dove-tail is turned the reverse way ; its extent of motion is much increased, so that the lowest powers may be employed, and two milled heads instead of one, as heretofore, have been adapted to the rack movement. He has also added the moveable stage, represented in fig. 48, and increased the angle of aperture of the two inch, one inch, and half- inch object-glass to the same extent as those of Messrs. Powell, Ross, and Smith, so as to render them capable of being used with the ordinary long compound bodies and eye-pieces of high power. Mr. PUlischer has been a manufacturer of microscopes in this metropolis for the last four years ; he supplies three kinds of stands. The first and most complete of these is represented by fig. 49; it consists of a firm tripod of brass, A, similar to that of Mr. Ross, in Plate 1 ; to this are fixed the two curved supports, B B, of a stout plate, capable of being turned on two trunnions, one of which is seen at C. This plate forms the under surface of the stage, I, and to it is firmly fixed the bent arm, D, supporting the compound body, which last slides in a dove-tailed groove, after the plan of Mr. Jackson, and is moved up and down by rack and pinion. There is a draw- tube in the compound body at F, immediately below the eye- piece, G ; the coarse adjustment is made by two large milled heads, one of which is seen at E ; the fine, as seen at H, by a screw acting on the end of a lever, a plan first adopted by Mr. Ross. The stage, I, is on Turrell's plan, but by a con- trivance of Mr. Pillischer's, it is considerably reduced in thick- ness ; the two rectangular movements being effected by turning the milled heads, K L, the latter having a corre- sponding miUed head on the opposite side of the stage. The mirror, M, is of the usual construction, and slides up and down a tube attached to the under surface of the stage-plate. A second microscope for students has a foot and uprights, the same as the larger microscopes ; the support for the com- pound body is a bent arm, to which is attached a tube, about THE COMPOUND MICEOSCOPE. Fig 49. four inches in length, through which the compound body is moved by a rack and pinion, as in the microscope represented in fig. 48, but the rack is not exposed. There is no fine ad- 100 PRACTICAL TREATISE ON THE MICROSCOPE. justment ; the stage consists of the usual sliding-plate, which may be moved up and down by the fingers, or by a lever, after the author's plan; the under surface of the stage is suppHed with a diaphragm. Another microscope, made by Mr. Pillischer, is contained in a box seven inches long, by four inches broad, and two-and- a-half inches deep ; this box forms the foot, and into the cover screws a tube, three quarters of an inch in diameter and six inches in length, having another tube sliding within it capable of being moved up and down slowly by means of a fine screw ; to this tube is attached a strong arm, to which the compound body, six inches in length, is screwed ; the compound body is also composed of two tubes, and before being used, the inner one is drawn out two inches, to make it the usual length. The stage is of an oblong square figure, and one of its edges is furnished with a tongue-piece to slip into a slot attached to a short piece of tube which slides up and down the main stem, and so forms the coarse adjustment. The mirror is situated at the bottom of the stem, and is mounted in the usual manner. This instrument will answer for all the pur- poses for which an ordinary microscope can be used, either in the inclined or vertical position, the inclination being given by opening the cover and keeping it in one place, by means of two long hooks. The low price and portability of this instrument are its principal recommendations. Although the microscope stands of Mr. Pillischer do not differ very materially in external form from 1':ose previously described, yet, from being very simple in their construction, he is enabled to furnish them at a rather cheaper rate than those manufactured by the more celebrated opticians. The author, however, from what he has seen, can speak well of the manner in which the work is executed, and thinks that of the microscopes manufactured in this metropolis, the stands of Mr. Pillischer are next in point of merit to those of Messrs. Powell, Ross, and Smith. Mr. King, of Bristol, has been for some years a maker of the stands of achromatic microscopes ; he usually supplies two kinds, one very similar to that of Messrs. Powell and Lea- THE COMPOUND MICROSCOPE. 101 land, represented in Plate 2, the other somewhat like that of Mr. Eoss, in fig. 45. The first is the largest and most com- plete ; it is supported on three inclined legs, as in Plate 2, but the mode of mounting the compound body is like that in Plate 1, and the fine adjustment is placed on the top of the arm, not on one side as in Plate 2. To this instrument may- be applied all the usual apparatus, moveable stage, achromatic condenser, &c., with which other first-rate microscopes are furnished. The second microscope is smaller than the pre- ceding, and is intended chiefly for students. In form it is very similar to that represented in fig. 45, but the uprights to support the stage, which is of large size, are shorter ; and it is not generally provided with stage movements or fine adjustment. Mr. King makes no claim to originality in the form of stands he adopts, but has selected what he deems the best points of construction in those of the first London makers. The author can, however, highly commend the manner in which the work is executed. Report speaks well of the stand of the achromatic microscope constructed by Mr. Abrahams, of Liverpool, which very much resembles that of Mr. Ross, in fig. 45. The stage employed in this microscope has either a rack movement or is one after the plan of the author, in which two levers, capable of being removed, are used to give motion in two opposite directions. Mr. Abrahams also supplies a lenticular achromatic prism, as a substitute for the mirror and condenser. FOREIGN MICROSCOPES. For the Information of such of his readers as may be desirous of knowing what difference of construction exists be- tween English microscopes and those employed on the con- tinent, the author has thought it advisable to describe the forms of stand manufactured by some of the most approved foreign opticians. Amongst these Plossel and Schick, of Vienna ; Pistor, of Berlin ; Chevalier, Oberhauser, and Nachet, of Paris, deserve especial mention. 102 PRACTICAL TREATISE ON THE MICROSCOPE. THE MICROSCOPE OF SCHIEK. The microscope of Schick (for the loan of which the author is indebted to Mr. W. Francis) is represented in fig. 50; it Fig. 50. THE COMPOUND MICROSCOPE. 103 consists of a stout piUar of brass, A, supported on three feet, BCD, and having at its upper part a cradle joint, E, to which is attached a triangular bar of steel, F, upon which shdes the support, G I, of the compound body, K, and that of the stage, IS". The coarse adjustment is made by the milled head, H, by which the compound body is raised or depressed on the triangular bar; but the fine, by a long screw, L, having a nut, M, attached to the support of the stage, N, the adjust- ment being effected by raising or lowering the stage. The mu:ror, O, is of the usual construction. MICROSCOPE OF PISTOR. The microscope of Pistor, as seen in fig. 51, stands on three feet of brass, ABC, capable of being folded together; these support a long steel bar, D E, upon which the tube, F, carrying the curved arm, Gr, supporting the compound body, H, is made to slide by raising or depressing the handle, I ; this forms the coarse adjustment. The fine adjustment is effected by the milled head, L, acting on a screw at the upper part of a steel rod, K, which passes through a block of brass, M, attached to the back part of the triangular tube, F ; to the lower part of this rod is fixed a nut with a spiral spring, P, and above it is another block of brass, Q, attached to the back of the triangular bar ; over this is seen another piece of brass, E, capable of being moved up and down the steel rod by the handle, S. The two milled heads, N O, serve to keep secure the blocks, M K, to the rod, K ; when that at N is un- screwed, the tube, F, and with it the compound body, are capable of being moved up and down the bar, so as to form the coarse adjustment; but when the steel rod, K, is fixed to the block, M, by the screw, N, and the spring, P, is kept stretched by the block, E., and screw, O, the compound body may be slowly raised or depressed by the nut, L, which forms the fine adjustment. The stage, T, is fixed to the triangular bar, it is of small size, and has two diagonal movements by means of screws, the milled heads of which, V W, are gra- duated into one hundred parts. The mirror, X, of the usual form, is attached to the lower end of the triangular bar. 104 PEACTICAL TREATISE ON THE MICROSCOPE. With this instrument and the preceding are supplied six object-glasses, capable of being employed singly or three at once ; the three smallest constituting the highest power. Fig. 51. THE COMPOUND MICROSCOPE. 105 THE MICROSCOPE OF CHEVALIER. This instrument, called the universal microscope, is repre- sented by fig. 52. The foot, or base, is formed by the box Fig. 52. in which the microscope is packed; into this is screwed a stout pillar, A A, supporting a square piece of brass, B, having a cradle joint, C C, at each extremity. With the upper surface of this piece of brass, B B, is connected the compound body, D, having at one end a piece of tube, M, containing a smaU prism, m o, and at right angles to it a smaller tube, carrying the object-glasses, n. To the lower 106 PKACTICAL TREATISE ON THE MICEOSCOPE. part of the piece of brass, B B, is fixed a square stem, E E, the posterior surface of which, r r r, is provided with a rack, by means of which the supports, G, of the stage, P, and of the mirror, H, can be raised or depressed. By this means the coarse adjustment is formed, the fine being effected by the screw, L L, which moves the stage up or down without affecting the rack-work. The compound body, D, has a draw-tube, Q, capable of being moved out or in by the rack, R, and a pinion connected with the milled head, S. This microscope is generally used in the position represented by ■fig. 52, but when the tube, M, is removed, and a straight piece to carry the object-glass is substituted, it may be con- verted into a vertical microscope, by means of the joint C B, or again into a horizontal one by the joint B C ; the prism, m o, being for the purpose of bending the rays, so that they may pass through the compound body. In order to know when the stage, P, is perfectly horizontal, a stop, F, is fixed to the bottom of the square stem, E E. The mirror, H, like the stage, can be raised or depressed on the stem by rack and pinion. THE MICEOSCOPE OF OBEEHAUSEE. M. Oberhauser, of Paris, constructs two kinds of micro- scopes, one for dissection, the other for general purposes. The former was described in the first edition of this work, the latter is represented by fig. 53. It consists of a circular foot or base, four inches in diameter, loaded with lead ; upon this is fitted a stout tube, two inches high, on which the stage rests. This tube has^an oblong opening in front for the light to fall on the mirror, and the tube itself is capable of being turned upon the foot, and the stage upon it, so that not only can the light falling upon the mirror be put in any situation, but the stage, and with it the object, can be revolved, so that rays, however oblique, may be thrown upon all sides of any object. To the stage is fixed the support of the compound body, in which are contained the adjustments; the coarse effected by rack and pinion, and the fine by a screw. A very coarse adjustment is made by sliding the compound body up and THE COMPOUND MICROSCOPE. lO'i Fig. 53. down the tube into which it fits.* The centre of the stage is made of black glass, ground very smooth, which looks neat, and is not easily soiled or scratched. nachet's microscope. The form of stand adopted by M. Nachet, to whom micro- scopists are indebted for several ingenious pieces of apparatus presently to be described, is, in many respects, similar to that of Oberhauser, and is represented by fig. 54 ; the chief diflfe- rence in the base of the stand being the length of the tube, F, for the purpose of adapting the sliding frame, T, and lever, L. The coarse adjustment is made by rack and pinion, by which the tube, B, into which the compound body. A, shdes can be moved up and down ; whilst the fine is effected by the screw, * M. Oberhauser, in his later instruments, has done away with the rack movement, and has placed the milled head for the fine adjustment at the bottom of the support of the compound body, instead of at the top. He has also increased the length of the compound body. 108 PKACTICAL TREATISE ON THE MICROSCOPE. G, by which the support and the compound body also are raised or depressed. The compound body, A, can not only Fig. 54. be moved up and down in the tube, B, but can be taken away, and another body employed for the purposes of dissec- THE COMPOUND MICROSCOPE. 109 T^ rr^i^^ substituted, which wiU be subsequently described, xne black glass for the stage, and the mirror, together with all the motions of the tube, F, upon the foot, and that of the stage, and with It the compound body upon the tube, are all simUar to those of M. Oberhauser, but the mode of applying the polarizing apparatus, achromatic condenser, &c,, beneath the stage, are so very ingenious, as to require a separate description. In fig. 55 are shown the stage and a portion of the tube sup- Fig. 55. porting the same, but in order to render the use of the slide, T, more plain, it has been represented as drawn out to its fullest extent. In the centre of the slide is seen the tube, V, which is capable of being raised or depressed by the lever, L ; into this tube, the polarizing apparatus, the achromatic con- denser, the oblique prism, &c., are placed; the slide, T, being pushed in as far as it will go, the tube, V, is then im- mediately under the hole in the stage, O ; in this position the tube can be raised or depressed as accurately as by a screw or rack-work. The knobs, E, are for the purpose of drawing out the slide, T, the hole in the tube, O, is to allow of the movements of the lever. By this arrangement, any kind of condenser or the polarizing apparatus may be placed under an object on the stage, without its being in the least disturbed. To this microscope M. Nachet adds a moveable stage, composed of a sliding-plate, which is made to move by two screws, placed 110 PRACTICAL TREATISE ON THE MICROSCOPE. diagonally, a ciirved spring keeping the plate in contact with the screws. This stage has three pieces of brass projecting from its circumference to fit over the edge of the stage-plate of the microscope, and by these it can be so elevated above the stage- plate, as to allow of light being thrown very obliquely under any object, by means of a prism invented by Amici, which will be described in another part of this work. The workman- ship of this instrument is exceedingly well executed, and of the continental microscopes, it is certainly one of the most perfect and complete in aU its parts. The author is indebted to Mr. Warren De La Rue for the loan of the microscope of which fig. 54 is a representation. The object-glasses supphed with all the above-described microscopes, except those of Messrs. Powell, Eoss, and Smith, and the lowest, viz., two-inch, one-inch, and half-inch of Mr. Dancer, are all constructed nearly on the same plan, and will be described in the chapter devoted to the "Magnifying Powers." Having noticed all the important points in the con- struction of the principal English and foreign microscope stands, whereby great steadiness, accuracy of adjustment, portability, and other valuable requisites have been so suc- cessfully carried out, our attention must now be directed to the apparatus that may be added to any instrument to render it complete for all the purposes of scientific investigation. CHAPTER in. ACCESSORY INSTRUMENTS. Besides the object-glasses, the eye-pieces, and the mirror, together with the parts constituting the stand of a microscope, such as the compound body and the stage, with the supports and adjustments for each, it has been found in practice highly essential that certain other instruments should be supplied. These may be divided into two classes ; first, into those which ACCESSORY INSTRUMENTS. Ill are subservient to the iUumination of objects, and, secondly, into those for the purpose of keeping objects in, whilst they are^ being examined, or for preparing them for exami- nation. Amongst the former may be mentioned all the various kinds of diaphragms, condensers, illuminators, po- larizing apparatus, dark wells, &c. ; and amongst the latter, the live boxes, animalcule cages, fishing tubes, &c., all of which require special notice. The Diaphraffm. — A very useful piece of apparatus applied to the under surface of the stage in most microscopes is the diaphragm, represented by fig. 56 ; it consists of two or more plates of brass, one of which is perfo- rated with four or five holes of different sizes, this plate is of a circular figure, and is made to revolve upon another plate by a central pin or axis ; this last plate is also provided with a hole as large as the largest in the diaphragm- plate, and corresponds in situation to -. ; ^^ the axis of the compound body. To f I «Ly wK ascertain when either of the holes in i I ^ py the diaphragm-plate is in the centre, a ' bent spring is fitted into the second plate, and rubs against the edge of the diaphragm-plate, which is provided with notches, so that when either of the holes is brought into its proper position, the end of the spring drops into the notch. The space between the largest and smallest hole is greater than that between any other two ; this answers the purpose of stopping off all the light if necessary. The diaphragm is attached to the under surface of the stage, either by a sliding- plate, as seen in the figure, or by a short piece of tube fitting into the hole of the stage, and securely fixed in the proper position by a bayonet-joint. The former method is adopted by Mr. Eoss and Mr. Smith, and the latter by Mr. Powell ; every part of this instrument through which the ^' C Fig. 56. 112 PEACTICAL TREATISE ON THE MICROSCOPE. light passes is blackened, so that no other rays than those from the mirror should interfere with the illumination. The use of the diaphragm is to modify the rays reflected from the mirror, and to limit the angle of the pencil of light allowed to fall on the object under examination. When a very bright light is employed for some time, the eye will often sufier greatly from fatigue, and when taken away from the instrument, a dark spot wiU be seen upon any object that is white ; to remedy this inconvenience, a piece of grey or neutral tint glass may be placed over the hole in the fixed plate, and when the light is passed through either of these, it is so very much softened, that the relief afforded to the eye is truly astonishing. Dark Chamber. — This instrument, like the diaphragm, is fitted to the under surface of the stage, and is represented by fig. 57 ; it consists of a plate of brass, c, into which is soldered a short piece of tube, having a dia- phragm or stop, a, in which is an aperture equal in area to the field of view of the lens, and no larger; below this is a sliding tube, b, with an aperture rather larger than that at a ; this last can be moved up and down until the light at a is of the greatest intensity, the aper- ture at a being always in proportion to the size of the lens employed ; this instrument is the contrivance of Mr. Varley, and is described by him in the forty-eighth vol. of the Trans- acUons of the Society of Arts. He applies it always to his instruments, and on account of there being no lens in its con- struction, the light is not decomposed ; he, therefore, prefers it to the Wollaston light for a condenser. It is always em- ployed with his phial-holder, and will be again alluded to. Wollaston Condenser, — This instrument, like the preceding, is also fitted to the under surface of the stage ; it consists of a short tube, in which a planoconvex lens, of about three-quar- ters of an inch focal length, is made to slide up and down ; this apparatus is represented in section by fig. 19, or as applied to a microscope in fig. 38, where the lens, set in a ACCESSORY INSTRUMENTS, 113 frame, is moved up or down by two small handles. For cor- rect definition, Dr. "WoUaston employed a stop immediately above the mirror, between the mirror and the lens, but it has been found much better in practice to apply the stop between the lens and the object ; this improvement was made by Dr. Goring, and by it the length of tube employed is not only much shorter than that suggested by WoUaston, but the definition is greatly improved by the arrangement. Dr. WoUaston states that " the intensity of illumination wiU de- pend upon the diameter of the Uluminating lens and the pro- portion of the image to the perforation, and may be regulated according to the wish of the observer." Achromatic Condenser.— The condenser of WoUaston, just described, although a very great improvement over the ordi- nary methods of illuminating, is, nevertheless, to a certain extent, faulty, in consequence of not being suppUed with an achromatic lens ; to remedy this inconvenience, M. Dujardin, in 1840, contrived an instrument which he termed an eclair age, for the purpose of illuminating objects with achromatic light ; a modification of this apparatus is now supplied with aU the best microscopes, and is known as the achromatic condenser, and although it is applied in different ways to the micro- scopes of our three eminent makers, it, nevertheless, consists of three essential parts ; viz., an ^ achromatic combination, an _i adjustment of focus for the same, and a means of making the axes of the object-glass and of the condenser coincide exactly. When the com- pound body is made to turn away from the stage, the apparatus for adjusting the axes is very simple, and the plan adopted by Mr. Ross and Mr. PoweU is represented by fig. 58 ; it consists of two tubes, sliding one within the Fig- 58. 114 PRACTICAL TREATISE ON THE MICROSCOPE. other, to the outer one, b, is attached a flat plate, a, which slides underneath the stage, and is adjusted for distance by the screw, f; at c is seen a milled head, which is connected to a pinion, and by means of a rack attached, the inner tube, carrying the achromatic combination, d, is raised or depressed ; the upper part of the outer tube, b, is larger than that at c, this is for the purpose of allowing the miUed ridge of the achromatic combination to pass up and down freely. For the low powers, such as the half and quarter of an inch, the com- bination, d, only is used ; but with the higher powers, the second part, e, may be slipped over d, whereby the focal point of the illuminating rays will be materially lessened in dia- meter, although increased in brilliancy. The flat mirror is generally used as the reflector or the prism described in page 118. Wlien the compound body can be turned away from the stage, the adjustment of the axes of the illuminator and ob- ject-glass is a very simple matter, the only movement required in the condenser is that of either increasing or di- minishing the distance the flat plate, a, has to slide through ; this is done either by screwing or unscrewing the screw, f, until the spot of light formed on the ob- ject by the illuminator is in the centre of the field of the object-glass. But when the compound body is a fixture, then it is neces- sary that the condenser should have two adjust- ments ; a section of such a condenser is represented by fig. 59, as constructed by Mr. Ross, a a exhibit Fig. 59. the plate by which it is ACCESSORY INSTRUMENTS. 115 attached to the stage, b a portion of large tube, having affixed to it a ring of brass, into which is soldered a smaller tube carrying the pinion with a milled head, f; within this tube a still smaller one, d, with a screw at the top to carry the illuminator, g, and a diaphragm at the bottom, to cut off all ex- traneous light, is moved up and down by a rack, in which works the pinion, e. The vertical adjustment of this instrument is made by the small screw attached to the plate, a a, whilst all the other movements are effected by turning three or more screws in the ring of brass, by which the inner tube carrying the Ulumi- nator, can be moved in various directions, so as to bring its axis to coincide with that of the object-glass. Two of these screws are seen at c and d. This plan was first suggested by Mr. Koss, and is adapted to all his instruments in which the arm carrying the compound body is a fixture. The several parts of the illuminator, g, unscrew, so that they may be used either combined or separate. The achromatic condenser supphed with the largest micro- scopes of Messrs. Smith and Beck, is represented by figs. 60 and 61 ; and for the better exhibition of its several parts, Fig. 60. 116 PRACTICAL TREATISE ON THE MICROSCOPE. the drawings have been made of the actual size, but in an inverted position. In fig. 60, c represents a tube of brass, within which a smaller tube, b, carrying the illuminator, d, is moved up and down by turning the milled heads, a a. The tube, c, is screwed into a plate of brass, which turns upon another larger plate ; by this last, the entire condenser is adapted to the under surface of the stage, it being provided with a screw, /, at its front part, to regu- ' iiiijlllill late the distance that it should be slid in under the stage, so as to bring the illuminator, d, into the axis of the object-glass ; but as the arm sup- porting the com- '^' ■ pound body does not move from side to side, the adjustment, to remedy this, is rather more complicated. The brass plate into which the tube, c, screws, is made to turn upon a large pin, fixed to the bottom plate, and by means of a spring and a small raised block of brass, the former is always firmly pressed against the screw, e, as seen in fig. 61 ; when, therefore, this screw is turned, the plate, and with it the tube, c, together with the illuminator, are carried slowly from side to side, and when the exact position is found, the plate may be fixed by the screw, g. Mr. WenharrCs Illuminator. — The principle of this instrument consists in placing a dark well or stop behind the object, and causing an intense achromatic light to^pass over and around it at such an angle that no rays can enter the obj ect-glass, consequently the field of view appears quite dark. When a transparent object is placed above this dark well, it wiU be rendered luminous, as it intercepts a portion of the light which passes over the circumference of the dark well, and as we see the object with its own radiant light only, it will appear beautifully illumi- nated in all its natural colours, on a jet black ground. The ACCESSORY INSTRUMENTS. 117 Fig. 62. %ht reflected from a metallic surface is preferred for this method of illumination/on account of its purity. Fig. 62 re- presents a section of the apparatus, drawn half the size of the original, a a is a trun- cated parabolic reflec- tor, with a polished silver surface; at the apex of the reflector is placed a meniscus, b, of a focus and curva- ture suitable for cor- recting the aberrations caused by the plate of glass under the object. At the base of the parabola is a disc of glass, "c c, in the centre of which is cemented a dark well, d, with a flange equal in diameter to the aperture at the top of the reflector. The dark well is less in diameter than the flange, and has a eliding adjustment, by which it is raised till the fleld appears dark under the highest powers; therefore the aperture of the illmninator must exceed that of any of the object-glasses. The reflector is moved to or from the object by means of the rack and pinion, e, and has similar adjust- ments for centring, and is fixed under the stage of the micro- scope in the same way as the ordinary achromatic condenser. In addition there is a revolving diaphragm, _^ made to shde on the bottom tube of the apparatus ; it has two apertures, ff g, placed diametrically opposite, for the purpose of obtaining two pencils of oblique light in opposite directions, which is useful for viewing some test objects. Before use, the axis of the illuminator must be made to coincide with that of the object- glass ; to effect which, fix the apparatus under the stage, and move the lateral or longitudinal adjusting screws, till the hole in the centre of the cap, which screws on the top of the re- flector, is in the centre of the field of view, using the inch 118 PRACTICAL TREATISE ON THE MICROSCOPE. object-glass ; the cap is then removed and the object placed on the stage, and the light obtained from a white cloud or bright sky, using the plane mirror; the reflector is then moved to or fro till the object is best illuminated. The rays of lamp or candlelight must be rendered parallel, by means of the large planoconvex lens or condenser, placed with its flat side near to the lamp ; the light is then reflected through the illuminator by means of the plane mirror, as before. The readiest way of ascertaining if the rays of light be parallel and thrown in a proper direction, is to hold a card or sheet of paper on the mirror, and adjust the distance of the con- densing lens from the lamp till the circle of light is of the same diameter as the lens employed, and occupies the centre of the mirror. The apparatus just described is made of various sizes ; but as a very intense light is required for this principle of illumination, it is advisable that the reflector should be of as large a size as the stage fittings will admit ; for if we double the diameter of the reflector, we obtain four times the quantity of light, the areas of circles being to each other as the square of their diameters. Prism. — M. Dujardin, to whom we are indebted for the achromatic condenser, found that to produce the best efiects, a prism of glass, of the form represented by fig. 63, should be used with it, instead of a mirror, a re- presents a short piece of brass tube, b a glass prism, connected by screws to the tube, a, by two supports, c c. The tube is made to slide upon the end of the con- denser, and to turn upon it in such a man- ner, that, in whatever position the lamp or white cloud may be, the prism may be adjusted to it ; the revolution of the prism being performed upon the screws, the extre- pio-. 63. mities of which are conical and fit into cor- responding depressions in the side of the prism. This instrument has some few advantages over the plane mirror : the quantity of light is greater, and all test objects in which delicate markings exist, may be shown to ACCESSOEY INSTRUMENTS. 119 the best advantage, in consequence of all the rays being re- flected from the same surface, which is not the case with a silvered glass mirror. Achromatic Prism and Condenser. — This very important in- strument, answering the purpose both of mirror and achromatic condenser, was presented to the author by Mr. Abrahams, optician, of Liverpool, and is shewn of the natural size in fig. 64. The prism is made up of two kinds of glass, set in a Fig. 64. frame of brass ; the part employed as the reflector. A, is of flint glass, hollowed out at its upper surface, and into this is accu- rately fitted a double convex lens of crown glass, B, so contrived as to have a focus of about four inches. By the tube, Gr, the prism can be applied to the ordinary support of the mirror, and by means of the flat semicircle, D, and a joint in the con- necting piece, F, it can be turned in every possible direction, the semicircle sliding through a spring clip at E. By this instnmient achromatic condensed light may be thrown upon any object on the stage. The prism has the usual swinging motion, accomplished by the frame turning on two screws, one of which is seen at C, at the end of the semi- circle. Oblique Prism. — This instrument, invented by M. Nachet, of Paris, is represented in section by fig. 65 ; it consists of a prism of glass, having both its surfaces, a b and c d, convex, by which means the rays of light, I, reflected from the mirror. 120 PRACTICAL TREATISE ON THE MICROSCOPE. m, instead of passing on in a straight line to the object, i i, are converged by the first surface, b a, upon the oblique plane, r ; from this they are reflected to v, where they receive a second reflection, and are finally converged by the convex surface, c d, upon an object, i i. This prism is set obliquely in a tube of brass, and should be so contrived that it may be revolved, in order that the effect of oblique light may be shewn upon all parts of an object. In the microscope of M. Nachet, the stage can be re- volved, but in all our English instru- ments, except those provided with a stage, such as that proposed by Mr. Legg, the prism itself must be turned. Mr. Shadbolt has given the curves which answer best for the prism, in a paper in the third volume of the Trans- actions of the Microscopical Society. In the prisms first supplied by M. Nachet, the angle was 30°, and both upper and under surfaces were convex ; he now makes the lower surface plane, and, as it turns out, the plan for some time adopted by M. Nachet is precisely that determined mathe- matically by Mr. Shadbolt. The condensers of M. Nobert, Mr. Shadbolt, and Mr. Gillett, together with a prism of Amici, and some other equally useful pieces of apparatus concerned in the illumi- nation of objects, will be described in Part II., relating to the " Use of the Microscope." Polarizing Apparatus. — This consists of two prisms of cal- careous spar, constructed after the plan of Mr. Nicol, of Edin- bm-gh, and composed each of two pieces of the same spar, cemented together so as to transmit a single image only. One of these is mounted in a tube, and adapted to a flat plate of brass, as represented by fig. 66, by which it can be applied to the under surface of the stage-plate, like the achromatic con- Fig. 65. ACCESSORY INSTRUMENTS. 121 denser; upon this plate the tube carrying the prism is made to revolve, by turning the large circular plate at the bottom with Fig. 66. a milled edge; this lower prism is termed the polarizer, in contradistinction to another fitted to the top of one of the eye-pieces, and termed the analyzer. An end view of one of the prisms is seen at fig. 67, and a vertical section at fig. 68. When applied to the mi- croscope, it is necessary that the axes of both crys- tals should coincide with each other and with the optical parts of the mi- croscope, as in the case of the achromatic conden- ser; this may be known by revolving either of the prisms after the light has been sent through them by the mirror. If they are properly ad- justed, it will be found that there are two positions in which no light will pass through the prisms at aU ; if this does not take place, and only part of the field of view is darkened, then, either ^by turning the arm carrying the compound body or the screw in the plate bearing the polarizer, the two can be made to obscure each other ; they are then in a condition to be used. If now a crystalline plate of sulphate of lime be placed in the focus of the object-glass, it will be Fig. 67. Fig. 68. 122 PRACTICAL TREATISE ON THE MICROSCOPE. seen that this crystal, in common with many others, has the property of bending the rays of light that have traversed the polarizer, and of causing them to pass through the analyzer ; according to the thickness of the crystalline plate, so wUl either a green or red colour prevail. The cause of these appearances, and the various applications of the polarizing apparatus, will be further aUuded to in the chapter devoted to this subject. Some microscopists employ a bundle of thin glass plates for a polarizer, and a tourmaline for an analyzer ; but the colour of the latter renders its use objectionable. Condensing Lens. — An indispensable instrument for the illumination of opaque ob- jects, or of the mirror when a great quantity of light is re- quired, is the condensing lens or bull's-eye. This is gene- rally a planoconvex lens of great thickness, from two to three or more inches in di- ameter, mounted in the man- ner represented by fig. 69, on a stem of brass attached to a heavy circular foot. Upon this stem a short tube, hav- ing another piece of simi- lar tube fastened into it at right angles, is made to slide ; into this last fits a short rod or tube, to support the lens and allow of its being in- clined at any angle. This method of mounting the lens is adopted by Messrs. Koss and Powell; but Mr. Smith, following Mr. Tulley, em- ploys the same land of stem and foot, and, in addition to being inchned at any angle, the lens is provided with Fig. 69. Fig. 70. ACCESSORY INSTRUMENTS. 123 a swivel-joint, as seen in fig. 70, so' that it can be brought near to the lamp or candle used as the illuminating body, with- out moving the other parts of the stand. Another very convenient way of mounting the condensing lens is represented by fig. 71, as adopted by Messrs. Smith and Beck ; the foot, a, is the same as in the other instrument, but instead of a solid stem, it is provided with a short tube, b, into this slides a smaller one, c, having at its upper extremity a cradle- joint, d, connected with a smaU tube, e, through which slides a wire arm, f, sup- porting a small con- denser, g. This plan of mounting a con- densing lens is very convenient, it has aU the motions of the pre- ceding instruments, with the great advan- tage that they can be effected with one hand applied to the arm,^ A smaller lens is supplied with some microscopes for the purpose of further condensing the rays of the larger condenser, or of rendering the converging rays of the larger one parallel, whereby a greater field of view is illuminated, a plan very useful where dissections are being carried on under a lens. One of these instruments is represented by fig. 72. The method of mounting the smaU lens is somewhat similar to that last described ; it may be fixed into some part of the micro- Fig. 71. 124 PRACTICAL TEEATISE ON THE MICROSCOPE. scope Stand, as seen in Plate 3 at d d, or may be provided with a support of its own, as adopted by Messrs. Powell and Lealand. If necessary, both the large and small condenser Fig. 72. may be mounted on the same stem and foot, as represented in fig. 73, a plan adopted by Mr. Leonard; by this means the two may be used either separately or combined. All the Fig. 73. different methods of employing the two forms of condensers will be fully explained in the chapters devoted to the illumi- nation of opaque and transparent objects. ACCESSORY INSTRUMENTS. 125 Fig, 74. Messrs. Powell and Lealand supply, with some of their microscopes, a diaphragm of the form represented by fig. 74, when used, it is adapted to the stand of the large condensing lens, and placed in front of the lamp, at about eight inches distant from the mirror ; it consists of two plates of thin sheet iron, blackened ; one of these is of a circu- lar figure, being provided with five holes of different sizes, and capable of being revolved upon a larger plate in the same way as the diaphragm before described, as adapted to the under side of the stage. When this diaphragm is used, an image of the size of the aperture employed, should be shown on the mirror ; by this, only a part of the field of view will be illumi- nated, the centre will be light, but around the margin there will be darkness ; this oftentimes is very useftd in rendering very deli- cate markings more distinct. The size of the illuminated spot will de- pend upon that of the aperture employed, and also upon the relative distances of the mirror from the ob- ject, and of the diaphragm from the mirror. Erector. — Those microscopes fur- nished with a draw-tube are capable of having adapted to them the erector or erecting eye-piece ; this is repre- sented by fig. 75, as being screwed into the lower end of the draw-tube ; it consists of a piece of brass tube. Fiff. 75. 126 PRACTICAL TREATISE ON THE MICROSCOPE. three inches in length, and five-eighths of an inch in diameter, as seen in fig. 76, into the opposite ends of which are screwed two lenses, a, c; a being a meniscus, and c a planoconvex, and both having their convex surfaces towards the eye-piece which is situated in the upper part of the draw- tube ; between them is placed a stop, b, with a small hole in it. The use of this instru- ment is similar to that of the same arrange- ment of lenses in the eye-piece of a tele- scope, viz., to cause the image of any object to be seen in the erect or natural position. The field of view is also greatly increased, and an object as long as the three- fourths of an inch, can be taken in at once with the erector and a two-inch object-glass.. By pulling out the draw-tube, and therefore increasing the distance between the erector itself and the object-glass, the magnifying power of the instrument is increased, and by pushing it in again the power is diminished ; so that a microscope with a two-inch object- glass and the erector can be made to take in as much of a rule as three-fourths of an inch in length when the draw-tube is only slightly pulled out; and when the tube is drawn out to its fullest extent, it will magnify the divisions on the rule so much, that one-sixth of the same object alone, will fiU the whole field of view. The erector was first applied to the compound microscope, represented by fig. 21, by Mr. Lister ; it is extremely useful for taking in large objects, but more particularly for dissect- ing, as heretofore the inversion of the object by the compound microscope, entirely prevented any dissection being carried on under any of the low magnifying powers; but, with the erector, it can be done very readily. Lieberkuhns. — These are concave silvered specula, so named from their iUustrious inventor; they are attached to all the object-glasses, from the two-inch to the one-fourth; that for the ACCESSORY INSTRUMENTS. 127 Fig. 77. half-inch is represented by fig. 77. The rays of light re- flected from the mirror, either in parallel or converging lines, are brought into a focus upon an object, placed between it and the mirror, but not too large to inter- cept all the light. The object may either be mounted on glass in the usual manner, or held in the for- ceps, represented in fig. 80; and when too small to fill up the entire field of view, or when transparent, it is necessary to place behind it one of the dark wells represented by fig. 79. Each Lieberkuhn being mounted on a short piece of tube, can be shd up and down on the outside of the object-glass, so that the maximum of illu- mination may be readily obtained. In all the higher powers, the end of the object-glass is turned small, and passes through the aperture in the centre of the Lieberkuhn, but in the lower powers, the distance of the object-glass from the object will allow a Lieberkuhn of sufficient size to be used without the above arrangement. Side Reflector. — As a substitute for the Lieberkuhn, Mr. Ross supplies with his microscopes what he terms a side-illu- minator or reflector ; it consists of a concave speculum of a rectangular figure, highly polished and mounted on a jointed arm, as represented by fig. 78; like the small condensing lens, it is attached to some immovable part, or, still better, to the body of the instrument, and parallel rays of light from the lamp are thrown upon it by the bull's-eye Fig. 78. 128 PEACTICAL TREATISE ON THE MICROSCOPE. placed close to the lamp; by means of the jointed arm, the light may be reflected from it upon any object, how- ever large, on the stage. This is much better than a Lieberkuhn for most purposes ; for, with the latter, the ob- jects cannot exceed a certain size, otherwise the greater portion of light from the mirror will be intercepted in its passage ; it has also this advantage over the Lieberkuhn, that not only is a greater amount of light condensed upon any object, but being thrown obliquely, many minute markings can be seen, which the vertically reflected light is imable to bring out. Dark Stops or Wells. — These consist of small cup-like pieces of brass, mounted on wire stems or supports ; the shapes ^^^ _^^ generally employed are represented by fig. 79. ^^P p™«il They are used with the Lieberkuhns, and three diflrerent sizes are usually supplied with the best microscopes, the largest being always employed with the lowest power object-glasses. Their use is to cut off all the rays of light that would otherwise pass into the object-glass, hence they are required in all cases where the object to be viewed is transparent. The long stem fits into a small arm attached to the under surface of the stage, and capable of being moved into the centre of the aperture therein, and by it the well at the top can be I raised up so high, as nearly to touch the object itself; the cup-shaped form is used, in order Fig. 79. that the bottom may not be sufiiciently illumi- nated to form a light ground to the object, which might happen if a disc were employed. Forceps. — For the purpose of holding minute objects, such as parts of plants, or insects, to be examined either as trans- parent or opaque objects, various forms of forceps have been contrived. The most useful of these is represented by fig. 80. It consists of a piece of steel wire, about three inches long, which slides through a small tube, connected to a stout pin by means of a cradle-joint ; to one end of the wire is attached a ACCESSORY INSTRUMENTS. 129 pair of blades, fitting closely together by their own elasticity, but which, for the reception of any object, may be separated Fig. 80. by pressing the two projecting studs ; to the opposite end of the wire is adapted a small brass cup, filled with cork, into which, pins passed through discs of cork, cardboard, or other material having objects mounted on them, may be stuck ; or, if preferred, instead of the cork, a pair of blades, fitting accu- rately together, may be employed, with small notches in each, to receive the pins. With aU the old microscopes, one end of the wire carrying the forceps was made pointed, and to it was adapted a small cylindrical piece of ivory, having one of its ends white and the other black, on these surfaces the objects for examination were laid. Mr. Ross and Mr. Smith some- times supply a pair of three-pronged forceps ; the prongs are made of steel wire, curved and pointed at one end, and by means of a sliding ring, capable of being opened or closed. An instrument of this kind was in use as long ago as 1787, and is - figured in the work of the younger Adams, published in that year. The method of using these different forms of forceps is extremely simple : the object-plate of the stage of the micro- scope has one or more holes, into which the pin of the forceps may fit; on this pin they may be turned in a hori- zontal direction, and by the joint above the pin they may also be inclined at any angle; when once adjusted, the stage movements will suflSce to bring all the parts of the object which they hold into the field of view in succession. With some of the foreign microscopes are supplied other forms of forceps, constructed after the plan of our spring pliers or scissors; one of these, with flat lips for holding objects, se- cured to the object- plate of the stage, and another, either held 130 PRACTICAL TREATISE ON THE MICROSCOPE. in the hand, or similarly attached to the opposite side of the same plate, but provided with cutting edges like the pair of scissors said to have been invented by Swammerdam, are employed together; the former of these retains the subject firmly whilst it is being cut by the latter. These forceps will be again alluded to in the chapter devoted to dissecting in- struments. Animalcule Cages. — Instruments known by the name of " live boxes " have been in use for many years, and all the old microscopes were furnished with them ; they consisted of a brass cell, from three-quarters to one inch-and-a-quarter in diameter, into which a planoconcave glass was made to drop ; upon the concave side the insect was placed for examination, and a flat piece of glass of the same size, but fastened into the bottom of another cell, could be screwed down upon the insect, so as to prevent its movement ; this instrument has now been entirely superseded by more convenient forms, and amongst them may be mentioned the animalcule cage of Mr. Tulley, and the capillary tablets of Mr. Varley. The animalcule cage sup- plied with the compound achromatic microscope of the late Mr. Tulley is represented by fig. 81 ; it consists of a plate of brass, from three to four inches in length, to the middle of this was attached a piece of brass tube, about three-quarters of an inch in diameter, into the Fig. 81. top of which was fastened a plate of thick glass ; over this tube an- other short one, having a cover of thin glass cemented to a rim at its top, is made to slide ; this last tube is sufficiently short to allow the thin glass cover and the plate in the fixed tube to be brought into contact. The drop of water contain- ing the animalcules to be examined, is put upon the piece of plate-glass, which may be termed the object-plate, and the tube containing the thin glass cover is then to be slid down carefully, so that the drop may be flattened out ; in order to allow the contained air to escape in the sliding down of the cover, a small hole is drilled in the top ; this may be subse- ACCESSORY INSTRUMENTS. 131 quently closed with sealing-wax, if it be required to preserve the fluid for future examination. Mr. Varley, in the year 1831, greatly improved this form of instrument, and gave to it the name of capillary tablet or cage, in a paper published in the forty-eighth volume of the Transactions of the Society of Arts. This great improvement consists in making a channel aU round the object-plate, so that the fluid and the animalcules in it are retained at the top of the object-plate only, by capillary attraction, and will bear turn- ing about in all directions without leaving the top, provided it be not suddenly shaken. The cover also is made to screw down upon the object-plate, and not to slide as in the pre- viously described instrument; but in practice it has been found most convenient to adopt the sliding tube, as the act of screwing sometimes deranges the objects. The ^ late of brass to which the tube supporting the tablet and cover is attached, is of a circular figure, slightly flattened on two opposite sides, for convenience of package, as several of them can be contained in a small cylindrical case. The improvement made by Mr. Varley, in the object-plate or tablet, is now adopted by all our first-rate microscope makers, but with some few sight modifications ; one of these instruments, as constructed at the present time, is represented by fig. 82 in elevation, and in section by fig. 83. Fig. 82. Fig. 83. glass, a, is cemented. A B in both figures ex- hibit the flat plate of brass to which the short tube, carrying the object- plate, or tablet, is fixed; d, fig. 83, exhibits the piece of brass into which the tablet, c, is fastened, b the tubular part of the cover, into the rim of which the thin plate of This thin glass cover is often either broken or becomes uncemented ; to remedy the inconvenience of re-cementing, Mr. Powell adopts a very excellent plan, by 9* 132 PRACTICAL TREATISE ON THE MICROSCOPE. which a new cover can be adapted with little trouble; the tubular top is provided with a screw, upon the edge of which the cover of thin glass or mica is laid, over this a cap is screwed to keep the cover firm. Fig. 84 represents the tubular top, with its screw cap, and fig. 85 a section of the entire Fig. 85. Fig. 84. instrument, A B being the flat support, c the object-plate or tablet, d the channel around the same, b the tubular top with its screw-cap, e, holding down the thin glass cover, a. When the glass cover is of tolerably stout glass, these cages, besides being only employed for animalcules, may be used for com- pressing such objects as are soft, but still too opaque to be seen through. When these are moderately compressed, their structure is readily made out ; but an instrument constructed for this purpose especially, and known as the Compressorium, will be presently described. To use these animalcule cages, all that is necessary is to place a small quantity of the fluid containing the animalcules upon the object-plate or tablet, and to slide the cover carefully until the drop is flattened out to the required degree of thin- ness; this should never exceed the size of the tablet itself. When the drop of fluid is made flat, the objects it contains may not only be viewed with great ease and convenience, but they may be carried about and kept for some considerable time under observation ; the capillary attraction will preserve the fluid between the two glasses, and no shaking or turning that is not sudden will injure them in the least. When more fluid than is necessary is placed upon the bottom glass, the excess will escape into the channel, and, in all probability. ACCESSORY INSTRUMENTS. 133 most of the animalcules with it ; in this case, it is by far the best plan to wipe away all the fluid from the bottom-plate and the channel, and make the latter and the under surface of the thin glass cover perfectly dry before another drop is put upon the bottom glass, otherwise the channel, when once made wet, will attract the fluid again. In the animalcule cages, or live boxes, manufactured by Mr. Pritchard, the bottom plate of glass is ruled with fine lines, the one-hun- dredth part of an inch or less apart, to serve as a micrometer. When used dry, the lines are visible, but when fluid is inter- posed, they can not only hardly be seen, but all measurements made by such micrometers are manifestly incorrect with objects of any degree of thickness, as their true outline is not in focus at the same time as the lines of the micrometer ; this point will be particularly dwelt upon in the chapter devoted to the measurement of objects, but in this place it merely requires to be noticed in connection with the instrument to which it is applied. Fishing Tubes for Animalcules. — These consist of tubes of glass, about nine inches in length, open at both ends, and from one eighth to one-fourth of an inch in diameter ; the ends should be nicely rounded off in the flame of the blow-pipe; some of them may be straight, as shown by A, fig. 86, whilst others should be drawn out to a fine point, as C, or curved as B, D ; in short, they may be made of either of the shapes represented in fig. 86, all of which have been found exceedingly useful. Mr. Varley, to whom we are indebted for this valuable inven- tion, describes the method of using them in vol. forty-eight of the Transactions of the Society of Arts. Supposing the ani- malcules about to be examined to be contained in a phial or glass jar, as in fig. 87 ; having observed where they are most numerous, either with the naked eye if they are large, or with a pocket magnifier or the watchmaker's lens described at page 50 if they are small ; either of the glass tubes, having one end previously closed by the thumb or fore- finger wetted for the purpose, is introduced into the phial in the manner represented by the figure ; this prevents the water from entering the tube, and when the end is near to the 134 PEACTICAL TREATISE ON THE MICEOSCOPE. object which it is wished to obtain, the finger is to be quickly removed and as quickly replaced ; the moment the finger is Fig. 87. Fis. 86. taken off, the atmospheric pressure will force the water, and with it, in all probability, the desired objects up the tube; when the finger has been .replaced, the tube containing the fluid may be withdrawn from the phial, and as the tube is almost certain to contain much more fluid than is requisite, Mr. Varley adopts the following plan for getting rid of the excess. Being provided with some watch glasses and some pieces of plane glass, if the tube should contain more fluid than is neces- ACCESSORY INSTRUMENTS. 135 sary, the entire quantity must be dropped into a watch glass, which spreads it, and the insect may be again caught by putting the tube over it, when a small quantity of fluid is sure to run in by capillary attraction ; this small quantity is to be placed upon the tablet ; but should there be still foo much for the tablet, if it be touched with the tube again, it wUl be diminished ; and should the object be wanting, the fluid must be wiped off, and the opera- tion repeated until we are satisfied of its presence. If we wish to place several individuals together on the tablet, it is necessary that each should be taken up with the smallest amount of water; to effect this, Mr. Varley suggests that the tube should be emptied on a slip of glass, in separate drops, as in fig. 8 8, and with one of the capillary tubes, but little larger than enough to catch them, they may be lifted out one by one, and be placed on the tablet. Generally speaking, it is necessary to add a small quantity of vegetable matter to animalcules to keep them alive ; and as many species of them are found on confervas and duck-weed, some instrument is required to take small portions of these plants out of the jar in which they are growing ; for this purpose Mr. Varley has contrived the forceps represented by fig. 89 ; Fig. 88. Fig. 89. they are made of brass or German silver, with points a little curved ; to keep them accurately together, they are provided with a hole and steady pin. Being thin and easily closed, they answer very well to put into a phial and take out small portions of vegetable matter; but when jars, such as those in which chara or vallisneria are kept, are deep, then the long forceps. 136 PRACTICAL TREATISE ON THE MICROSCOPE. the invention of the author's late brother, Mr. Edwin Quekett, and represented by fig. 90, will be found extremely useful. They should be made either of brass or German silver, and may be of any length, from nine inches upwards. The central part is a piece of wire about one-eighth of an inch in diameter; its upper end is fas- tened to a flat piece of metal, bent round into two loops, as repre- sented by fig. 90, for the first and second finger of the right hand to be placed in. The lower part of the wire is split, and having been well hammered to make it springy, is bent into the form of a pair of forceps. On the outside of the wire is a piece of tube about one- fourth of an inch in diameter, and shorter than the wire ; to its upper part is soldered a piece of smaller wire, bent into the form of a ring. The use of this instrument must be obvious from the figure; the first and second finger of the right hand being placed in the two loops, the thumb is put into the ring at the top, the wire by the fingers is kept steady, and by the motion of the thumb the tube is raised or depressed ; when the tube is raised, the blades of the forceps being springy, open readily, and when the thumb is depressed, the blades are as easily closed. This pair of forceps will be found very useful for taking hold of small pieces of valisneria and chara, and * ACCKSSOEY INSTRUMENTS. 137 a pair of blades may be applied to them for the purpose of cutting off portions of these plants close down to the roots, even in tall jars that are too small to admit of the intro- duction of the hand. COMPKESSOEIUM. The compressorium is an instrument by which objects may be gradually compressed between two parallel plates of glass. The pressure may be applied whilst the object is being ex- amined with the microscope, and may be kept up at will, so that the alteration which it assumes, as the pressure is being applied, can be observed with facility ; it is extremely useful for crushing or compressing such objects as are so thick that the light cannot readily be transmitted through them, or for making flat others the elasticity of which is sufficient to raise up the thin cover when they are placed between glasses to be viewed in the ordinary way. There are many kinds in use, some of foreign, others of home invention. The most simple, and the one in which the power employed cannot exceed the force of two spiral springs, is made by Mr. Smith, after a plan of Mr. Lister's, and is represented by fig. 91. It consists of a bottom Fig. 91. plate of brass, to the centre of which is attached a piece of tube having on its outside a short screw, on which works a large circular nut, with a milled head ; to the inside of the tube a circular piece of plate-glass is fixed, projecting slightly above its edges ; this may be called the object-plate ; two small up- right rods, fastened into the bottom plate, are provided with spiral springs, their tops being surmounted by small nuts, which keep the springs in place. A plate of brass, with a hole in it larger than the object-plate, is made to shde up and down the rods in a state of parallelism, by means of the large 138 PEACTICAL TREATISE ON THE MICEOSCOPE. circular nut ; and two wedge-shaped tongues of watch spring are placed between the spiral springs and this plate. These tongue-shaped springs are capable of being moved round upon the rods, and are for the purpose of communicating pressure to a thin plate of glass resting upon the plate, which is pre- vented from sliding off by a raised edge. The plate carrying the thin glass cover is capable of being raised or depressed at wiU, by means of the circular nut. It wiU be seen, that when the plate carrying the thin glass cover is raised up as high as it will go by the mUled nut, the cover will not touch the lower plate of glass ; when this is the case, the instrument is ready for the reception of an object. The ends of the little steel springs must be lifted up by the finger-naU or some thin instrument, and then rotated so far outwards, as to get them clear of the cover. The cover being lifted off, the object is to be placed upon the bottom plate with as much fluid as necessary, and the cover being replaced, the springs may be lifted up and turned back to their original position. If now the nut be screwed down, the spiral springs will cause the plate to follow the nut, and when the nut has been turned far enough to allow the cover to come In contact either with the object or the fluid, it wiU be noticed that as the screwing is being proceeded with, both the fluid and the object wiU be more and more flattened, until it arrives at a maximum. If the screwing be continued further, the nut will leave the plate carrying the thin glass cover, and the cover itself wiU remain pressed down upon the object-plate with all the force exerted by the spiral and by the tongue-shaped springs. Mr. E.OSS has improved upon the compressorium of Mr. Lister, by making the plate carrying the thin glass cover, square, and by adding to it two other pillars, making four in all; upon two of these, situated at opposite corners, strong spiral steel springs are wound, and to the two others are applied finger-shaped pieces of German silver, to keep down the thin glass cover. The action of the large nut is the same as in Mr. Lister's instrument, but the pressure exerted by the springs is more powerful than in it. The finger-shaped pieces ACCESSORY INSTRUMENTS. 139 of German silver yielding but slightly, and the steel springs being much stronger than the brass ones, the power of com- pression is greatly increased. When a more powerful compressorium is required, the form represented by fig. 92 is highly useful. It consists of a Fig. 92. plate of brass, three or more inches long and one-and-a-half broad, having in its middle a circular piece of plate-glass for an object-holder ; this is slightly raised above the metal plate ; at one end of the latter is a circular piece of brass, having attached to it another piece of brass, carrying an arm capable of being moved up and down by means of a screw at one end, whilst at the other is a semicircle supporting by screws a ring of metal, to the under side of which a piece of thin glass is cemented ; the semicircle is made to turn upon the arm, and the arm and aU that is attached to it is capable of being turned upon the bottom plate. The use of this instrument is obvious ; if we wish to com- press any substance, we must first, by means of the screw, elevate the opposite end of the arm from the object-plate ; the arm, with all its appurtenances, is then to be turned away from the object-plate, and the object being placed on the plate with a requisite quantity of fluid, the arm is then to be brought into its proper place again, and by means of the screw, the metal ring with the thin glass cover can be made to exert as much pressure as the thin glass cover will stand without breaking. Messrs. PoweU and Lealand have lately constructed a much stronger instrument than that represented by fig. 92, and have made their object-plate of a thick piece of parallel glass, raised as much as the one-eighth of an inch above the bottom plate, so that it can be cleaned without much trouble ; the ring containing the glass cover is also made 140 PRACTICAL TREATISE ON THE MICROSCOPE. much stouter, and fits accurately upon the raised object- plate. Troughs for Chara and Polyps. — These consist of two plates of glass cemented together, with strips of the same material, or of metal between them, to form the sides of the trough; one of these, as described by Mr. Varley, in the forty-eighth vol. of the Transactions of the Society of Arts, is represented by fig. 93. c is a bottom-plate of stout glass, upon which is cemented with pitch and bees'-wax a thin cover, d, with slips of glass between it and the bottom-plate, to form the sides. The cover, d, is not so broad as the plate, c, in order Fig. 93. that a slip of chara may be more readily placed in the trotigh, as it can be first laid upon c, and then gradually slid down between it and the cover, d. In order to render the trough more manageable, it may be cemented to a larger bottom-plate, a b, by Canada balsam ; but it will be found far more advantageous if the bottom-plate itself be as large and as broad as a h, and if the cover, d, be cemented to it and not to another plate, as then two extra surfaces will be dispensed with. Mr. Varley informs us that a piece of wire bent into the shape of the slips of glass represented in the figure, and covered thickly with a cement composed of bees'-wax and pitch, will form an excellent substitute for the slips, and look very neat ; the cements of Canada balsam or sealing wax are much too brittle to last long, as a sudden jar will cause them to give way. Messrs. Smith and Beck supply with their microscopes a larger and much thicker trough for chara and polyps, as represented by fig. 94 ; the front is composed of much thinner glass than the back, and the method adopted of confining objects near to the front varies according to circum- stances. One of the most convenient plans, is to place in the trough a piece of glass that will stand across it diagonally, as represented by fig. 94, and if the object be heavier than water, it will sink, until it is stopped by the diagonal plate. ACCESSORY INSTRUMENTS. 141 At other times, when chara is being observed, the diagonal plate may be made to press it close to the front by means of thin strips of glass, or a wedge of cork, or even a folded spring of thin whalebone. When either of these instruments is used, it may perhaps be necessary to remind the reader that the microscope must be so far inclined as to be nearly horizontal. Messrs. Smith and Beck adapt to the object- plate of their large microscopes a strong steel pin, upon which a spring-holder is made to fit; this serves to keep the trough firm and to prevent its falling off, even when the microscope is per- fectly horizontal. This form of trough proved very serviceable to Mr. Lister, in 1834, during Fii'. 94. ^is investigations into the structure of some of the higher orders of polyps, and will be found of very great value to those who devote their attention to this most interesting branch of scientific inquiry. Frog-Plate. — This consists of a plate of brass, a a, about six inches in length, and two-and-a-half in breadth, and either of the shape represented by fig. 95, or of the same breadth say from the -^l^ to the TsViT of an inch, as was done by the late Mr. Coventry and Sir John Barton. The most convenient form of micrometer, for all purposes, wUl be one divided into hundreds, and one of these divisions into ten parts, or thousands. Those of glass are the best for transparent objects, and the mother-of-pearl, or ivory, for opaques; and, to make the lines more evident, finely levigated blacklead should be rubbed in to fill them up. The object to be measured is to be laid on the glass, or mother-of-pearl, and both it and the lines must be viewed at one and the same time, with the lowest power that can con- veniently be used ; the number of the divisions occupied by the object will give the measurement required. Example : — Thus, suppose an object occupied ten of the large divisions, its linear measurement would then be -^^, which if reduced to its lowest terms, would be the -^^ of an inch ; if fifty, then it would measure half an inch, if fifteen divisions, then it would be -jJ/g, or nearly a of an inch. It follows, therefore, that an object to be measured in this way must be very thin and the power low, in order that it and the lines may be in focus at the same time. This micrometer answers better for the simple microscope than for the compound achromatic instrument, as in the latter the focus even of a low power object-glass is so exact, that but very few objects are thin enough to be seen at the same time as the lines. Most of the objects requiring measurement are those which must of necessity be examined in fluid; this will render the lines on the glass micrometer all but invisible, unless they are very boldly ruled and filled up with some opaque matter. . Mr. Pritchard supplies with his microscopes animalcule cages, on the upper surface of the bottom glass of which, lines arc 218 USE OF THE MICROSCOPE. ruled from the ^J^ to the ^|^ of an inch apart; these can hardly be seen when the fluid containing animalcules is present, although they are very coarsely ruled, and, with objects of any thickness, all measurements are incorrect, in consquence of the rays of light from the object and the micrometer not being given off at the same angle ; so that the object is referred to a point of the micrometer larger than it really is. Second. — By the Micrometer Eye-piece. — In the description of this instrument, given previously in pp. 212-3, it was stated to consist of an eye-piece in the focus of which a piece of glass, having short bold lines ruled upon its upper surface, was placed generally -^^ of an inch apart, with every fifth longer, and every tenth longer still, to facilitate coimting; but it is not necessary that the number of lines in an inch be known, as long as they are equidistant ; and let us take, in the first place, the negative eye-piece, as supplied with Mr. Powell or Mr. Smith's microscopes. To find the Value of the Lines in the Negative Eye-piece Micrometer. — The micrometer set in its brass frame, as seen in fig. 140, is passed so far through the slits in the eye-piece, as that the lines may be seen to occupy the centre of the field of view, and to be in the focus of the eye-glass. The eye- piece having been placed in the end of the compound body, as far as it will go, the next step is to determine the value of the divisions of this eye-piece micrometer with each of the object-glasses. This is done by laying on the stage of the microscope a glass micrometer, divided, say into y-J-g- and Tinrir of an inch, and the lowest object-glass being screwed to the compound body, the lines on the stage micrometer are to be brought into focus; and either the eye-piece or the stage micrometer having been so turned as to bring the lines in both micrometers parallel, we must then observe how far the lines in each coincide, whether every third, fourth, fifth, and so on ; and, for the sake of simplicity, let us suppose the one- inch object-glass to be employed, and it having been noticed that every division of a hundredth of an inch in the stage micrometer coincides with ten in that of the eye-piece, it MICROMETER. 219 follows that the divisions or lines, in the eye-piece micro- meter, were joVt of an inch apart; the stage micrometer may now be removed, and every object viewed with the micro- meter eye-piece and the one-inch object-glass can be measured to the y^ij^-g^ of an inch. Should, therefore, one of the ^^^^ of the eye-piece micrometer be divided into four parts, then every one of these divisions would be the -jTyVo- Again, if with an- other power it was found that ^^^,^0 of an inch on the stage micrometer coincided with ten of the spaces of the eye-piece micrometer, then the value of each of the eye-piece micro- meter divisions would be the 1-7^,^55 of an inch. It wUl be found in practice, when high numbers are being observed, that the stage movements are rather too coarse to bring the lines in the two micrometers accurately over each other; the small screw at the end of the brass, as shown in fig. 140, must then be employed. We have hitherto spoken only of the numbers in the eye-piece micrometer coming out even, but such is rarely the case ; the chances are, that there will be a very minute variation between any two or more of both sets of lines. If it be not a matter of much moment to determine the value of the divisons accurately, and if no two lines coincide, the mean of a number of observations may be taken as an approximation to the truth ; but by far the most desirable way to remedy the evil is to employ the draw-tube, before described at page 69 ; this, which must be graduated on one side, as shown in fig. 42, into inches and tenths, may be pulled out so as to make some of the lines in each micrometer agree. The value of the divisions in the eye-piece micrometer should be found with all the object-glasses, and be put down in a tabular form, as follows; and, if the instrument be pro- vided with a draw-tube, the table should include the extent to which it ought to be drawn out, in order to make the value of the micrometer divisions even mmibers : — 220 USB OF THE MICEOSCOrE. Object-glass. Draw-tube. Value of small Divi- sions in Parts of an Inch. 1 Inch. 1 Inch, close. 1 tV Inch. 1,000 of an Inch. 2,500 5,000 10,000 „ It should be borne in mind that, when it is required to ascertain the value of the divisions in the eye-piece micro- meter with the highest powers, the division into hundreds of the stage micrometer wiU occupy too much of the field of view : some smaller parts of an inch, such as the two-hundredth or five-hundredth, should then be used, and the number of the divisions corresponding to that quantity be multiplied by two hundred or five hundred, as the case may be. To find the Value of the Divisions in the Positive Eye-piece Micrometer. — This instrument, before described at page 212, is used in the same manner as the above-mentioned. Mr. Eoss^ who always adopts this form in preference to the negative, does not employ the draw-tube with his microscopes. The micrometers, as shown in fig. 139, are ruled in squares, and one or more of them, of different degrees of minuteness in their ruling, may be employed; but it has been found in practice that divisions ruled about ■^\-^ of an inch apart wiU suit nearly all the powers. The method of finding the value of the divisions, with each of the object-glasses, is performed by means of a stage micrometer, in the manner previously described. The positive eye-piece gives a much better view of the micrometer than the negative one, but the definition of an object to be measured is not quite so good. Mr. Ross generally rules his micrometers in squares, but Mr. Jackson prefers lines, on account of the greater facility afforded for counting. As with the negative eye-piece micrometer, so with t'lis, it becomes necessary to put down in a tabular form the MICROMETER. 221 value of the sides of the squares, with the different object- glasses. The plan adopted by Mr. Eoss is here shown. The upper line indicating the value of the divisions in frac- tions of an inch, the lower line the same value in a decimal notation. Value of each space in the Micrometer Eye- glass, with the various Object- glasses. 2 In. 1 In. |In. iln. iln. sis ■0025 5TS •001031 TSoS •000526 i'JOO •0002325 •0001111 To find the Value of each Revolution of the Screw, or parts of a Revolution of the same, in the Cobweb Micrometer. — y.^hen about to be used, the cylindrical portion containing the eye and field lens is to be placed in the end of the compound body, in the same manner as an ordinary eye-piece, and a micrometer divided into hundreds and thousands, as employed with the other instruments, placed on the stage, and its lines brought into the focus of the object-glass. The graduated head of the micrometer being set at zero, and the cobwebs exactly coinciding with each other, the milled head is now to be turned, and notice taken, how many revolutions or parts of a revolution have been made when the cobwebs are opened sufficiently wide, to cover a certain number of the divisions of the image of the stage micrometer. It having been previously stated, that the screw employed to separate the cobwebs is provided with a graduated circle divided into a hundred parts ; should it be found that five revolutions of the screw cause the cobwebs to open, so as to cover ten divisions in that part of the stage micrometer divided into thousands, it follows that one revolution of the screw wotdd be equal to the -^^-^ of an inch, and one division of the graduated head would be ^^^ of ^-^ q or s-^.^TyTj- of an inch. The comb at the bottom of the field of view, as shown by fig. 144, wiU indicate the number of the 222 USE OF THE MICROSCOPE. revolutions, as the teeth commence from the fixed cobweb. In those microscopes provided with a draw-tube, the divisions may always be brought out even numbers, but such wiU not be the case with aU the object-glasses in a microscope not so provided, and the determination of the value of each revolution will then become a rather more complicated matter, as will presently be shown. The value of each revolution of the screw, with the different object-glasses, should be set down in a tabular form, as was shown in the case of the eye-piece micrometers. Directions for the Use of the Eye-piece Micrometer. — If there be a draw-tube to the compound body, it should be adjusted according to the table shown at page 220, the object having been brought into the centre of the field, and the micrometer properly adjusted, so that the horizontal fine be in the direc- tion of the diameter to be measured. Read the measurement in the small divisions, and suppose that, ,with the half-inch object-glass, an object occupies seventeen of these ; and it having been shown by the table at page 200, that the value of each division of the eye-piece micrometer, with the half-inch object-glass, was the jjVo ^^ ^^ inch, this number, or rather, the denominator, must therefore be divided by seventeen, and the result will be the ^-i^ oF an inch, or the diameter required to be found; or, should it be preferred to set down the diameter in decimals, then, by adding ciphers to the seventeen, and making it the dividend, and 2,500 the divisor, it may be shown that the diameter is .0068. The positive eye-piece micrometer supplied by Mr. Ross is used precisely in the same way as the above-mentioned instrument ; but, there being no draw-tube, the value of the numbers of the glass micrometer cannot be altered in any way from those mentioned in the table. The squares must be counted as the straight lines are in Mr. Jackson's form, and the dimensions of any object ascertained precisely in the same manner as before described, viz., by dividing the value of each square, as given in the table, by the number of squares occupied by the object ; or, if the decimal notation be preferred, by adding ciphers to the MICROMETER. 223 number of the squares, and dividing it by the value of one square, and the quotient will be the dimensions required. To Use the Cobweb Micrometer. — Before an object is mea- sured, it must be brought to the middle of the field, and, after the table has been consulted, which shows the value of each revolution of the screw, and of each division of the wheel aflixed to it, the cobwebs must be examined, in order to see whether they both accurately coincide when the zero point of the graduated wheel is opposite the index. The screw being now turned, until the image of the object is, as it were, enclosed between the cobwebs, the number of turns and parts of a turn are both shown by the indices ; the former by the comb at the lower part of the field ofview, the latter by the division opposite to which the index points. It must, how- ever, be borne in mind, that both with this micrometer and with those of ruled glass several measurements of the same object ought to be made ; and if there should be any difference between them, the mean of the two extremes should be taken as the correct one. The measurements made with the cobweb micrometer are said to excel those of all the other forms of instruments ; and that, with an object-glass of one-eighth of an inch focus, even a quantity as small as the eight-hundred- thousandth of an inch can be appreciated. This is, at least, ten times as dehcate as is required ; for Mr. Ross, in his experi- ments, preliminary to the constructing of his beautiful divid- ing-engine, found that, with the highest magnifying powers, it was impossible to ascertain the position of a line nearer than to the eighty-thousandth of an inch. Measurements of an Object made by means of a Stage Micro- meter and a Camera Lucida. — For this very valuable plan, we are indebted to Mr. Lister. By means of the camera lucida, a sketch of the object is first made ; the microscope being fixed in the horizontal position, the object is then removed and a stage micrometer placed in the focus of the object-glass instead ; a sketch of its divisions is also to be made on the same paper as the object itself was sketched on, with aU the optical arrangements of the microscope unaltered. The object, therefore, and the micrometer being both magnified to the 224 USE OF THE MICROSCOPE. same extent, their images will consequently bear the same relation to each other as the bodies themselves. The method of effecting this operation is exhibited by the following figure. In fig. 145 is shown how the rays of light coming from the eye-piece, or from any distant object, are reflected by the prism, in such a manner as to enter the eye, at right angles to their original direction; and as the image of an object is always re- ferred by the eye to a situation in the same direction as that from which the rays entering the eye proceed, the magnified image of the object will be seen on a paper laid on a table beneath the camera, and can there be readily sketched. Supposing, in the pre- ^'S- 145. sent case, that the objects under examination be granules of starch, or even blood discs, these can easily be sketched in out- line ; when, therefore, the micrometer is substituted for the starch, its divisions or squares can be drawn over the starch granules, as shown by figs. 146 and 147, the former being squares of 3-J^ of an inch, the latter, ^^V'ff' ^^^> ^^ t^® value of the squares, or divisions of the micrometer, is known, the objects over which the lines are not drawn can also be readily A - i k S^' (( tt \ Fig. 146. Fig. 147. DETERMINATION OF THE MAGNIFYING POWER. 225 ascertained by the application of a pair of compasses ; and, if necessary, the squares, as shown in figs. 146 and 147, can be further subdivided into four, so that the diameter of an object can be measured even to the fourth part of the quantity given by the lines on the micrometer. The whole field of view need not be covered with lines, .as shown by the figures; but a single square, or even the exact distance of any twO' or more of the lines of the micrometer, together with a pair of com- passes, will be all that is required. It must, however, be borne in mind, that when the size of an object is ascertained by the above method, the distance between the camera and the table must be always the same ; if the end of the compound body carrying the camera were ever so slightly raised or depressed, there would be either an increase or diminution of the value of the squares of the micrometer. CHAPTER IV. ON THE METHODS OP OBTAINING THE MAGNIFYING POWER OF SINGLE AND COMPOUND MICROSCOPES. The method of estimating the magnifying power, either of simple or compound lenses, is to compare an object of known size, such as a micrometer, seen through them with the nearest distance that another object, also of known size, can be dis- tinctly seen at : this latter distance is termed the standard of distinct vision, and with it all the magnifying powers are com- pared. All opticians of the present day adopt ten inches as a standard ; Sir David Brewster adopts five inches. In the old optical works, eight inches were generally employed as the standard; but the number ten, being a decimal, will be found most suitable for all purposes. With this decimal standard, the magnifying power of lenses, of any focal length, can be readily determined. Thus, for instance, if the lens uuder examination be of one inch focus, we have merely to 15 226 USE OF THE MICROSCOPE. add a cipher to the denominator of the fraction which expresses the focal length of the lens, and the result will be the magnifying power. Thus, if the lens be half an inch in focal length, the magnifying power will be twenty diameters ; if one quarter, than forty diameters ; and if one inch, then ten diameters, and so on. When, however, the focal length of a lens is very small, it becomes a difficult task to measure accurately its distance of focus. In such cases, says Mr. Eoss,* " the best plan to obtain the focal length for parallel or nearly parallel rays is to view the image of some distant object, formed by the lens in question, through another lens of one inch solar focal length, keeping both eyes open, and comparing the image presented through the two lenses with that of the naked eye. The proportion between the two images so seen will be the focal length required. Thus, if the image seen by the naked eye is ten times aa large as that shown by the lenses, the focal length of the lens in question is one-tenth of an inch. The panes of glass in a window, or courses of bricks in a wall, are convenient objects for this purpose. In which ever way the focal length of the lens is ascertained, the rules given for deducing its magnifying power are not rigorously correct, though they are sufficiently so for all practical purposes, particularly as the whole rests on an assumption, in regard to the focal length of the eye, and as it does not in any way affect the actual measurement of the object." In the preceding account, we have estimated the magni- fying power in diameters, or according to the measure termed linear; but as every object is magnified in breadth as well as in length, it follows that, if it were drawn as broad as it is long, a very different idea of its measure would result : — thus, suppose a, in fig. 148, to represent an object of its natural size, and that it be required to represent the same when magnified four times in length and four times in breadth, the square h c d e will give such view; and if this be compared with the original object a, it wiU be seen that there are sixteen Article "Microscope," Penny Ci/clopcedia. DETERMINATION OF THE MAGNIFYING POWEK. 227 H a such squares contained in it. This mea- l surement, which is rarely used, except to astonish the pubhc, is called the super- ficial magnifying power, and is found by squaring the linear, or, in other Tvords, by multiplying the linear by itself. The magnifying power of single lenses, like the value of the divisions in the different kinds of micrometer, may Fig. 148. be set down in a tabular form, thus : — Focal lengths Linear Superficial in inches. Magnifying Power. Magnifying Power. 2 5 25 IJ 6.6 43.5 1 10 100 1 1.3.3 176.8 i 20 400 i 40 1600 1 80 6400 tV 100 10000 To ascertain the magnifying power of the compound micro- scope, the method employed is the same as that proposed by Hooke, in his Micrographia, as long ago as 1667, and before described at page 4 ; it is thus accomplished : — Place on the stage of the microscope a micrometer either of glass, ivory, or mother-of-pearl, divided to some fraction of an inch ( j^ ^ will answer the purpose for' all the lower powers), and, at ten inches distant from the eye, hold a rule, divided into tenths of an inch, in a line parallel with the micrometer ; a magnifier and eye- piece being adapted to the compound body, the lines on the stage micrometer are to be brought into focus; if now the eye not employed in looking through the eye-piece be cast as it were down on the rule, the micrometer divisions, and 15* 228 USE OF THE MICKOSCOl'E. those on the rule, will be seen at one and the same time; and, after a little practice, it will be found a very easy matter to count how many divisions of the rule are covered by two or more of the micrometer. Thus, for example, suppose that each division of -j^-g of an inch occupies just one inch of the rule, the magnifying power will then be one hundred times, as j^^ of an inch is made as large as one inch ; if the same divisions, either with another object-glass or eye-piece, correspond to one-and-a-half inch of the rule, then the power will be one hundred and fifty diameters. The magnifying powers of the different object-glasses, with each of the eye- pieces, should, also, for convenience of reference, be set down in a tabular form. Mr. Ross supplies with his microscopes a table of the following construction, having appended to it, that described at page 221, with the value of the divisions of the eye-piece micrometer with each of the object-glasses. For the use of those who have draw-tubes to their micro- scopes, another column might be added to denote the distances it must be drawn out, in order that the value of the micrometer divisions and the magnifying power may be set down in even numbers. Magnify! ng Power of the Object-glasses with the various Kye-glasses. Eye-glasses. OBJECT-GI/ASSES. 2 In. lln. iln. iln. iln. tV In. A 20 60 100 220 420 600 B 30 80 130 350 670 870 C 40 100 180 500 900 1400 Several other methods, besides those now described, have been from time to time adopted, to ascertain the magnifying power of the compound microscope; but as none are so accurate as that of Hooke, it has been thought vumecessary to describe DETERMINATION OF THE MAGNIFYING POWER. 229 them. The value of the divisions in the micrometer scale and the magnifying powers of the different object-glasses are, in most cases, supplied by the maker of the instrument — they should be always so ; but for those who are anxious to work the subjects out for themselves, it is hoped that the preceding directions will not be deemed out of place. The late Mr. Coventry and Sir John Barton were cele- brated for ruling micrometers of extreme delicacy ; some of these, stiU extant, have as many as ten thousand lines in an inch ; Mr. George Jackson, a gentleman whose name has been so frequently mentioned in connection with the improvements in the mechanical arrangements of the microscope, has also paid considerable attention to this subject, and has succeeded in ruling ten thousand lines in an inch, and in crossing these at right angles with others precisely the same distance apart, so that a series of squares are formed, each having a super- ficial area of the one hundred millionth of an inch. These are very remarkable as specimens of skill, but are far too minute, either for measuring the magnifying power of an instrument or the value of an eye -piece micrometer. In England, microscopists are in the habit of setting down the measurements made by micrometers either in inches, or in fractional or decimal parts of an inch ; but in France, and some other parts of the continent, either a line or millimetre, and fractional divisions of the same, are employed for a like purpose. For the convenience of those who may wish to compare foreign measures of length with our own, the follow- ing tables have been drawn up, together with directions for converting either English inches, or parts of an inch, into lines or millimetres, or these last into English measures. Parts of an English Inch. A Paris line = .088815 or -^ of an English inch. A metre = 39.37100 inches English, or 3.281 feet. A centimetre ^ .39371 „ or a little less than f of an inch. A millimetre = .039371 „ or a little less than ^'s of an inch, 230 USE OF THE MICBOSCOrE. To convert Paris Lines into English Measure. Multiply the numerator of the fraction -^-^ by the number of Paris lines stated, or divide the denominator of the fraction by the same nmnber, or multiply the number .088815 by the number of lines and parts of the same. To convert Millimetres into English Measure. Multiply the number .039371 by the number of millimetres and parts of the same, the quotient will be the equivalent measure in decimal parts of the English inch. The line is often made use of in scientific works in this country ; but as no two persons are agreed as to whether its ^•alue is the one-tenth or one-twelfth of an inch, it follows that, in all measures in which it is employed, the value attached to it should be stated ; if the Paris line be the one adopted, neither the one-tenth or one-twelfth of an English inch is its correct value, although the latter number comes nearest the truth. CHAPTER V. CAMERA LUCIDA. The camera lucida, before described at page 144, was invented by Wollaston in 1807. It consists of a four-sided prism of glass, as represented in sec- tion by fig. 149, the sides and angles being similar to those shown by A B C D. The rays of light proceeding from an ob- ject, MN, after being reflected by the faces, D C, C B to the eye will be referred by an ob- server to m n, and an image of the object will be there CAMERA LUCIDA. 231 seen, and if a piece of white paper be placed there the image will appear to be on it. The prism is generally set in a frame of brass, in the manner exhibited by fig, 99, all parts of it being covered over except the side next the eye- piece and a small portion of the edge to which the eye is to be applied ; the frame is capable of being adapted to either of the eye-pieces by a short tube. The prism itself has two sUght adjustments, one to bring its upper face horizontal, and the other to make the image of the object fall flat on the paper on which it is to be drawn. A lens is generally placed under the camera, in order that the rays of light from the pencil employed in sketching and the object itself may be seen under the same angle. Several contrivances have been had recourse to, in order to simplify certain difficulties that arise in the use of this instrument ; one of these plans is shown in section by fig. 150, and con- sists of a mirror, M, composed of a thin piece of dark coloured glass cemented to a piece of plate glass, inclined at an angle of 45"^, in front of the first lens of the eye-piece, E. The light escaping from the object, through the lens, E, is assisted in its reflection upwards to the eye by the dark glass ; and this, says Mr. Koss,* " effects a fui'ther useful purpose of rendering the paper less briUiant ; and thus enabling the eye better to see the reflected image." If required, a double convex lens, L, may be placed beneath the mirror, as in the case of the prism. The polished steel disc, the invention of Soemmering, before described at page 145, may be employed as a substitute for the camera, over which it is said to have some few advantages. Mr. Powell supplies with some of his microscopes an apparatus represented in fig. 151 ; it consists of a plate of neutral tint glass fitted in a frame, and so mounted as to be capable of being applied to the eye-piece; Article " Microscope,'' Pennj Cyclopedia. Fig. 150. 232 USE OF THE MICROSCOPE. with this instrument there is no difficulty in seeing the object and pencil, but the paper must be shielded from the light. M. Nachet has contrived an excellent camera lucida, in which the rays undergo one reflection only, and as his micro- scopes do not incline, the paper on which the drawing is to be made is placed on a small desk in front of the observer. The camera, and mode of attaching it to the eye-piece, are repre- sented by V in fig. 152,* and a section of the camera by p v, and of the desk c, together with the course of one ray of light, by X r a. Where this camera is employed, it is not necessary to keep the compound body in a vertical position; the camera allowing of its being inclined at an angle of 45°, the paper can be laid on the table, and the drawing made in the usual way ; by this arrangement the field of view will be of much laro-er dimensions than where the body is placed horizontally. Method of using the Camera Lucida with the Microscope. The first step to be taken after the object about to be drawn has been properly iUuminated, adjusted,. and brought into the centre of the field of view, is to place the compound body of the microscope in a horizontal position, and to fix it there. * Robin — Sur le Microscope. CA^IERA LUCIDA. 233 The cap of the eye-piece having, been removed, the camera is to be slid on in its stead ; if the prism be properly adjusted, a circle of white hght, with the object within it, will be seen on a piece of white paper placed on the table immediately under the camera, when the eye of the observer is placed over the uncovered edge of the prism, and its axis directed towards the paper on the table. Should, however, the field of view be only in part illuminated, the prism must either be turned round on the eye-piece, or be revolved on its axis, by the screws affixed to its frame-work, until the entire field is illu- minated. The next step is to procure a hard, sharp^pointed pencil, which, in order to be well seen, may be blackened with ink round the point, the observer is then to bring his eye so near the edge of the prism, that he may be able to see on the paper, at one and the same time, the pencil point and the image of the object; when he has accomplished this, the pencil may be moved along the outhne of the image so as to trace it on the paper; however easy this may appear in description, it will be found very difficult in practice, and the observer must not be foiled in his first attempts, but must persevere until he accomplishes his purpose. Sometimes he will find that he can see the pencil point, and all at once it disappears; this happens from the movement of the axis of the eye ; the plan then is to keep the pencil upon the paper, and to move about the eye until the pencil is again seen, when the eye is to be kept steadfastly fixed in the same position until the entire outline is traced. It will be found the best plan for the beginner to employ, at first, an inch object-glass, and some object, such as a piece of moss, that has a well-defined outline, and to make many tracings, and examine how nearly they agree with each other, and, when he has succeeded to his liking, he may then take a more complicated subject. If the operation be conducted by lamp- light, it will be found very advantageous not to illuminate the object too much, but rather to illuminate the paper on which the sketch is to be made, either by means of the lamp with the condensing lens, or a small taper placed near it. When the object is so complicated that too much time would 234 USE OF THE MICROSCOPE. be required for it to be completed at one sitting, the paper should be fixed to the table by a weight or on a board by drawing-pins. An excellent plan to adopt is to fix the microscope on a piece of deal about two feet in length and one foot in breadth, and to pin the paper to the same ; there will then be no risk of the shifting of the paper, as when the wood is moved, both microscope and paper will move with it. In all sketches made by the camera, certain things must be borne in mind ; the eye, when once applied to it, should be kept steadily fixed in one position, and if the sketches are to be reserved for comparison with others, the distance between the paper and the camera should be always the same. A short rule or a piece of wood may be placed between the paper and the under surface either of the compound body or the arm supporting it, in order to regulate the distance, unless the joint be furnished with a stop, as the size of the drawing made by the camera will depend upon the distance between it and the paper. It is also very desirable, before the camera is removed, to make a tracing in some part of the paper of two or more of the divisions of the stage micrometer, in order that they may form a guide to the measurement of all parts of the object. Some persons cover the whole of the drawing over with squares, to facilitate not only the measurement, but in order that a larger or smaller drawing may be made from it than that given by the camera. It must be recollected that an accurate outline is the only thing the camera will give, the finishing of the picttire will depend entirely upon the skill of the artist himself. Uses to which the Camera Lucida may be applied. — Besides the valuable assistance rendered to the draughtsman by this instrument, its application to micrometry is also of no small utility ; the method before described at page 223, will answer for all purposes. If it be required to make a very large but accurate diagram of any microscopic object, a true outline of it may be drawn by following these directions. A large sheet of paper, attached by pins to a drawing-board, having been laid on the floor, and the microscope placed hori- zontally, with its compound body projecting as far over the CAMERA LUCIDA. 235 table as possible, and the object and camera having been pro- perly adjusted, a pencil fastened into a long piece of light hollow cane must then be provided, and the artist, either stand- ing or sitting, and looking down through the camera, will see the image on the paper, and after a little practice will be able to trace its outline as easily as when the paper was placed on the table only a few inches below it. Another way of effecting the same end is shown in fig. 153, which is that of placing the tracing, M N, made from the miscroscope by the camera as an object for another camera, c, of the kind employed by artists Fig. 153. 236 USE OF 'J'HE MICROSCOPE. for making sketches of landscapes : this being fastened to the table, D, by the screw, b, and the object, M N, set up in front of it, an accurate outline on a larger scale, M' N', can then be made on the floor, as in the preceding method ; the pencil, P, with a long handle, F, being held by the hand, H, the artist standing either in front or on one side of the camera, and applying his eye to it as at E, The size of the picture will, like all others made by the camera, depend on the relative dis- tance between the object and the paper. It will, however, be found in practice, that about four feet wiU be the utmost limit of the space between c and P to allow of the pencil being used with any advantage. In this case a lens must be placed at c, before the prism, suited to make the object appear just as far as the floor. CHAPTER VI. ON THE POLAKIZATION OF LIGHT. Origin of the Term. — " If," says Sir D. Brewster,* " we transmit a beam of the sun's light through a circular aperture into a dark room, and if we reflect it from any crystallized or uncrystallized body, or transmit it through a thin plate of either of them, it will be reflected and transmitted in the very same manner, and with the same intensity, whether the surface of the body is held above or below the beam, or on the right side or left, or on any other side of it, provided that in all these cases it falls upon the surface in the same manner, or, what amounts to the same thing, the beam of solar light has the same properties on all its sides, and this is true of light emitted from candles or any luminous bodies, and all such light is called common light." But if the same light be allowed either to fall upon a rhomb of Iceland spar, or upon a plate of * Treatise on Optics, page 157. ON THE POLARIZATION OF LIGHT. 237 glass at the angle of incidence of 56°, as was first discovered by Malus in 1808, the two rays or beams into which it is separated will have different properties on different sides, and in the case of the glass, supposing we hold another plate of glass over the first, it wiU be found that the reflected ray will pass through it when held in some positions, and not In others : if the glass be turned round without altering its angle to the horizon, the light wiU be reflected in one quarter, transmitted in a second quarter, reflected again in the third, and when the circle is completed it will be again transmitted ; that is, a beam of light has acquired the property of sides, on two of which it can, and on the other two it cannot be reflected ; and as these properties bear some analogy to the poles of a magnet, a ray of light so modified is said to be ■polarized. In the case of the Iceland spar the polarization is eflfected by double refraction, but In that of glass by reflection and transmission. In order to explain the polarization by reflection from glass, the apparatus represented by fig. 154 has been contrived: it ■W\- Fig. 134. consists of two tubes of wood, D C ; C having at one end a plate of glass. A, capable of being turned round an axis, so that it may form different angles with the axis of the tube ; another tube, D, a little larger than C, carrying a similar piece of glass, B, is made to fit over it, so that by turning either of the tubes the two plates may be placed in any position in relation to one another. If a beam of light, r s, be allowed to fall upon A at the polarizing angle of 56° 45', it will be reflected through the tubes and wiU fall upon the second plate, B ; If this plate be also placed at the angle of 56° 45', and the tube to which it is 238 USE OF THE MICROSCOPE. attached be turned round so that it occupies the position represented by the figure, the ray, r s, will be reflected to E ; if the tube be again turned slowly round, the light will be found to pass through the plate when it has arrived at a distance of 90° from the starting point ; if the tube be turned again, the light will get more and more faint until another 90° are arrived at, when the ray will undergo total reflection ; and so will the changes take place at every quadrant until the starting point is again reached, the ray, r s, being alternately reflected and transmitted. For the purpose of polarizing light, various substances have, from 'time to time, been employed ; amongst the most useful for the microscope wiU be found either glass, blackened on one side, or a bundle of thin glass plates, a crystal of Iceland spar, or a crystalline mineral termed tourmaline. It would be foreign to our purpose here to enter into any of the numerous theories that have been broached, to accoimt for the above described phenomena; for these the reader is referred to the works that are specially devoted to the subject ; but as one of the principal objects of this treatise is to teach those who are uninitiated in micro- scopic science, the use of the various kinds of apparatus sup- pHed with the best achromatic microscopes, we will only take notice of those polarizing instruments which, when applied to the microscope, have been found necessary, in order to aid the observer in his investigations into the structure of organic and inorganic substances. The polarizing apparatus most useful to the microscopist has been already described at page 121 ; it consists, as shown in figs. 67 and 68, of two prisms of calcareous spar, constructed after the plan of Mr. Nicol, of Edinburgh, who employed for the purpose a rhomb of the spar divided into two equal por- tions, in a plane passing through the acute lateral angles, and nearly touching the obtuse solid angles. The cut surfaces, having been carefully polished, were then cemented together with Canada balsam, so as to form a rhomb of nearly the same size and shape as it was before the cutting ; by this arrange- ment only one of the two rays into which a beam of ordinary light passing through a rhomb of this spar would be separated ON THK POLARIZATION OF LIGHT. 239 is transmitted, the other being rendered too divergent ; hence it has received the name of the single image prism. Two of these must be provided, one to be adapted to the under surface of the stage, and termed the polarizer, as shown by fig. 66, whilst the other, called the analyzer, is placed above the eye-glass. To effect the same purpose, some persons employ a bundle of glass plates as a polarizer, and a tourma- line as an analyzer ; but the colour of the latter is often ob- jectionable in the compound microscope, M'here everything that win diminish the brightness of the light and brilliancy of the coloiu-s should be avoided. It will be found that a tour- maline of a neutral tint forms an excellent analyzer, having one great advantage, viz., that when placed over the eye- piece, the field of view is not contracted as it is when a Nicol's prism is employed. " The best tourmaline to choose," says Mr. Woodward,* " is the one that stops the most light when its axis is at right angles to that of the polarizer, and yet admits the most when in the same plane." In order to illustrate some of the most striking phenomena of polarized light in a very simple way, by the achromatic compound microscope, the apparatus consisting of two prisms and a film of selenite, as described at page 121, will be nearly all that will be required as well as for the examination of minute animal, vegetable, and mineral structures; we will, therefore, proceed at once to the application of the same. Method of using the Polarizing Apparatus. — The polarizing prism represented by figs. 66, 67, and 68, having been adapted to the under surface of the stage, either another prism or a plate of tourmaline is to be placed over the eye-piece, or in the end of the draw-tube, as shown by fig. 75, and the light having been reflected through them by the mirror, the step that next becomes necessary is to make the axes of the two prisms coincide : this is done by revolving either the upper or lower one, and noticing whether, in some positions, the light is completely cut oflT, and in others wholly transmitted ; if the field of view be not darkened twice during one revolution of * Familiar Intrnditction to the Slndy nf Polarized Light. Second Edition. Page 3-5. 240 USE OF THE MICROSCOPE. the prism, the axes do not correspond; to remedy this, the compound body (if capable of being turned away from the stage, as in Mr. Ross's and Mr. Powell's instruments,) must be shifted either to the right or the left until this point is attained. If now a thin plate of selenite, or other doubly refracting crystal, be placed on the stage, and be brought into the focus of the object-glass, and the light be caused to pass through the prisms, the selenite will produce a colour according to its thickness: if one of the prisms be now revolved slowly, we shall find that more and more light will be transmitted, but the intensity of the colour will be diminished, and when a quarter of a revolution has been accomplished the brilliancy of the colour will re-appear ; but what was originally red wiU become green, and the green will again become red at a second quarter of a revolution. If the selenite be removed, and some very thin crystals of sulphate of copper, tartaric acid, or one of the other substances presently to be enumerated, be substituted for it, a most gorgeous set of colours will be seen ; and as the prism is being revolved, the same alternations of reds and greens will take place as with the selenite. If, however, a piece of glass, with some perfect crystals of iodide of potassium or common salt upon it, be placed under the same conditions, neither the light nor the colours will be seen: hence bodies may be divided into those that polarize and those that do not polarize ; to the latter class belong the iodide of potassium and common salt just named. The primitive form of crystal of these substances is the cube, and it has been found, by experiment, that these crystals, not possessing the property of double refraction, do not exhibit colour when placed between the prisms; but by far the greater number of other crystalline bodies possess this property, which is essential to the production of colour. One of these last must, therefore, be chosen in order to exhibit colours ; but it happens that there is one part or axis of the crystal in which the property of double refraction does not exist ; this is called the axis of [no] double refraction : in some crystals there are two such axes. In other bodies there are certain planes along which, if the refracted ray ON THE POLARIZATION OF LIGHT. 241 passes, it experiences no double refraction; this is termed the neutral axis, and no colour will be produced around it when polarized light passes through the crystal in the direction of this axis. Some persons, at the suggestion of Sir David Brewster, employ a rhomb of Iceland spar for an analyzer, instead of a Nicol's prism ; this should be so thick (when the selenlte is placed on the stage) as to separate the complementary colours by double refraction; and in order to protect the rhomb from scratches, a plate of thin glass should be cemented to two of its surfaces ; with this prism as an analyzer, and with one of Nicol's as a polarizer, a film of selenite of uniform thickness,^ and with a brass plate, three inches by one, per- forated with a series of small holes, from the one-sixteenth to the one-fourth of an inch in diameter, a variety of interesting phenomena may be exhibited, as described by Mr. M. S. Legg, in the Transactions of the Microscopical Society* If the brass plate be placed on the stage, with the smallest hole in the field of view, and an inch object-glass be employed as the magnifier, the rhomb, when placed over the eye-piece, will give two images of the aper- ture, as in fig. 154, a. If the eye-piece be now turned, the images will describe a circle; and if a larger aperture be brought into the field of view, the images will over- Fig. 154. , \ ■ A " lap, as shown in hg. 154, b and c. If the Nicol's prism be now adapted to the under surface of the stage, and the eye-piece revolved, it will be found that in certain parts of the revolution there will be two images, whilst in others there will be only one, fig. 155, ab cd. If now a film of selenite of a certain thickness be placed underneath the brass plate, the apertures will be coloured red and green, fig, 154, a' V d, and if the eye-piece be revolved, at every quarter * Vol. ii., part ii. 16 ^%gm 242 USE OF THE MICROSCOPE. of the circle the colours will be seen to change alternately from red to green, and from green to red, fig. 155, a' H c' d'. If the brass be slid along so as to bring the two largest aper- tures into focus, the images will overlap, and where they do so white light will be produced, as shown in fig. 154, 6' c'. rig. 155. The experiments may be varied by employing two double refracting crystals, placed one over the other, and by removing the NicoFs prism, but retaining the brass plate on the stage ; two distinct images will then be seen, but at twice the original distance from each other, as in fig. 156, a. If the crystal Fig. 156. nearest the eye be turned from left to right, two faint images will appear ; continuing the revolution, the four images will be equally luminous, and when the crystal has been turned round 90°, there will be only two images of equal brightness ; ON THE POLARIZATION OP LIGHT. 243 continuing the turn, two other faint images will appear ; fur- ther on, four images of equal brightness, and at 180" they will all coalesce into one bright image, c. If a film of selenite be placed between the two crystals, we shall see three coloured images instead of two colourless ones, fig. 156, a'; of the three, the two outer may be green, the middle red. By turning the crystal nearest the eye, the middle image will gradually divide until the completion of the quarter of a revolution, when four images wiU appear, as at b', two being red, the other two green; revolve the crystal another quarter of a circle, the three images, as at d, will re-appear, but with different properties — the two outer being now red, and the middle green; at another quarter four images will be seen, also with their colours reversed, as at d', and at the completion of the circle the original appearance will again occur. Wlienever polarized light passes through certain crys- tals in the direction of the optic axis, a series of beau- tifully coloured rings will be seen. Certain angles of any given crystal must be ground down to a plane sur- face and be polished, in order to exhibit the rings; when this is accomplished, there will be found in some positions of the analyzing prism a black cross, with a series of rings around it, and in others a white cross, with the colours of the rings reversed. In crystals having two axes of double refraction, a double system of rings will be seen. Nitre is a beautiful instance of this kind, and a transverse section of a prism of this substance, when ground down and polished, will, with polarized light, exhibit the double system of rings ; but if the prism be ground in some other directions, colour will be produced ; hence it becomes necessary, in order to exhibit the phenomena of colour, to have crystals cut either in the direction of the axis of double refraction, or in a plane inclined at certain angles to it. But when the same sub- stances, in a state of solution, are crystallized on glass, it frequently happens that many of the crystals will be arranged with their axes of double refraction in the direction of the beam of polarized light ; all such, therefore, will exhibit colours, as 16* 244 USE OF THE MICROSCOPE. will those also in which the thickness of the crystal is not below a certain standard, this for the selenite is the "00046 of an inch ; the red colour is always produced by the thickest films, the violet by the thinnest. It must, however, be borne in mind that the red and green are always complementary to each other, or together make white light. The exhibition on a screen of the coloured rings in crystals cut at right angles to their crystaUographical axes, was first effected by Mr. Woodward, by the gas polariscope; the crystal having been placed within the focus of a lens of low power, and a tourmaline used as an analyzer. Mr. Legg has successfully effected the same results by the achromatic compound microscope, and in order to afford an additional means of investigating these phenomena, he recommends the use of the erector, before described at page 126, by which the microscope is converted into a telescope of low power. When the Mcol's prism is used as a polarizer, the field of view will be too much limited for the action of the erector; in these cases it will be advisable to employ a bundle of thin glass plates, placed in such a manner that light may fall on them at an angle of 56" 45', when the light reflected will be polarized, and a large field illuminated, but not with so much brilliancy as by the prism and concave mirror. The usual mode of exhibiting microscopic objects by polar- ized light is to place them on the stage of the instrument with a Mcol's prism as a polarizer, adapted below the stage, and a similar prism above the eye-piece; in this way most crystalline bodies may be shown. Some few vegetable struc- tures may also be exhibited in the same manner, amongst the latter may be enumerated the hairs on the leaf of the Eloeagnus, the siliceous cuticle of the Equisetum, and of some of the grasses, together with starch of various kinds, all of which are beautiful objects for the polarizing microscope. Many animal structures, such as feathers, slices of quill, horn, hoof, and other cuticular appendages, are best shown by placing a thin film of selenite or mica beneath them, by which they become intensely coloured ; the selenite or mica should be of uniform thickness, in order to develop the true structure of ON THE POLARIZATION OF LIGHT. 245 the object as indicated by the various colours occasioned by the different densities of its parts. A list of those objects that win exhibit the most beautiful series of colours will be given in another part of the work. It may be here stated, that the film of selenite employed to give coloiu: to objects, should be mounted between two glasses for protection ; it may be even immersed in Canada balsam, in the same manner as an ordinary object. Some persons employ a large film mounted in this way between plates of glass, with a raised edge to act as a stage for supporting the object on, it is termed the " selenite stage." Cause of the Colours of Polarized Light. — In fig. 153 it was shown, that when a beam of light reflected from a plate of glass, at the angle of 56" 45', was received by another plate of glass at the same angle, it would be found (if this second plate were capable of being revolved) that in two positions in one revolution the light would be entirely stopped, whilst in two others it would be whoUy reflected; in the microscope the same effect can be shown by two prisms, one either above the eye-piece or between it and the object-glass, and the other between the object-glass and the mirror. It has been stated before, that if a thin plate of a doubly refracting crystal were placed between the prisms (when in the situation that the transmitted ray of polarized light from the lowest prism is stopped by the upper prism), it would not only cause the light to pass through the first prism, but, according to the thickness of the plate of the crystal, so should we have colours comple- mentary of each other, or which, together, would make white light. From a very early period, philosophers have been employed in the investigation of the nature of light, and two principal theories have been advanced, one by Newton, who maintained that light is material, and is emitted by all self- luminous bodies in minute particles ; the other by Huyghens, who supposed that an elastic medium, called ether, fills aU space, and occupies the intervals between the particles of aU substances, and that luminous bodies excite vibrations in this ether, which spread by waves, in the same manner as those formed in water when a foreign substance, such as a stone, is dropped into it. This latter theory is the one now generally 246 USE OF THE MICROSCOPE. adopted, and has received the name of the undulatory theory. Light, as is well known, is made- up of three colours, each colour being produced by a wave or undulation of a particular length; if all meet together in the same state of vibration, white light is formed, but if they meet under other conditions, darkness or colour may result from the waves interfering with each other. If the difference of length between two waves be an even number of half undulations, the two will coincide and produce a colour equal in intensity to the two combined ; but if the difference be an odd number, darkness will result. Let us now apply this principle to the selenite between the two prisms, and let us suppose the prisms to be arranged so that no hght may be seen when the eye is applied to the eye-piece ; the polarized beam, therefore, from the first prism not being able to pass through the analyzing prism, a plate of a double refracting crystal is introduced; this has the property of dividing the polarized beam into two rays, which are polarized at right angles to each other ; but at angles of 45" to the original ray, these falling on the analyzing prism, and being inclined somewhat nearer to its crystallographical axis than the polar- ized beam originally was, some of its vibrations will pass through, and colour will be produced by interferences in the undulations ; and if, in the first case, the colour be red, then, as the prism is turned, it will become green. The red and green colours are, as has been before stated, complementary, or the one is what the other wants to form white light ; if a double refracting crystal of Iceland spar be used instead of the Nicol's prism, it can be shown that where the red and green are super-posed, white light results. But as this subject will be best understood by reference to a diagram, the author has selected the following from the second edition of the valuable Introduction to the Study of Polarized Light, lately published by his friend, Mr. Woodward. In order to render the diagram more intelligible, it may be as well here to state that ordinary light is represented by a cross, which denotes that its vibrations are in planes at right angles to each other, whereas, when one set of such vibrations only is shown, the light is said to be polarized. Mr. Woodward's description of the production of colour by polarized light being in itself ON THE POLARIZATION OF LIGHT. 247 SO comprehensive, the author has been induced to copy it verbatim. E p H Fig. 157. f*"^\; " A B, C D, represent the rectangular vibrations by which a ray of common light is supposed to be propagated. " E, a plate of tourmaline, called in this situation the polarizer, and so turned that A B may vibrate in the plane of its crystallographical axis. " F, light polarized by E, by stopping the vibrations C D, and transmitting those of A B. " G, a piece of selenite of such a thickness as to produce red light and its complementary colour, green. " H, the polarized light F bifurcated, or divided into ordi- nary and extraordinary rays, and thus said to be dipolarized by the double refractor Gr, and forming two planes of polarized light o and e, vibrating at right angles to each other. " I, a second plate of tourmaline, here called the analyzer, with its axis in the same direction as that of E, through which the several systems of waves of the ordinary and extraordinary rays H, not being inclined at a greater angle to the axis of the analyzer than that of 45", are transmitted and brought together under conditions that may produce interferences. " K, the waves E," and E-' for red light of the ordinary and extraordinary systems meeting in the same state of vibration, occasioned by a difference of an even number of half undu- lations, and thus forming a wave of doubled Intensity for red light. " L, M, the waves Y° and Y' and B" and B^ for yeUow and 248 USE OF THE MICEOSCOPE. blue of the ordinary and extraordinary systems respectively meeting together, with a difference of an odd number of half undulations, and thus neutralizing each other by inter- ferences. " N, red light, the result of the coincidence of the -waves for red light, and the neutraUzation by interferences of those for yeUow and blue respectively. " H, in the lower part of fig. 157, represents dipolarized light. " I, the analyzer turned one quarter of a circle, its axis being at right angles to that of I, in the upper part of the same figure. " K, the waves E° and E," for red light of the ordinary and the extraordinary systems meeting together with a difference of an odd number of half undulations, and thus neutralizing each other by interference. " L, M, the waves Y" Y' and B" B" for yellow and blue of the two systems severally meeting together in the same state of vibration, occasioned by the difference of an even number of half undulations, and forming by their coincidences waves of double intensity for yeUow and blue Hght. " N, green light, the result of the coincidences of the waves for yellow and blue light respectively, and the neutralization by interference of those for red. " By using a plate of selenite of uniform thickness, the colour win be uniform, whereas a plate of different thicknesses will produce different colours, following the same order as those of Newton's rings ; red being produced by the thickest, violet by the thinnest, and intermediate colours by intermediate thicknesses of the plates of selenite." By substituting Nicol's prisms for the two plates of tour- mahne, and by the addition of an object-glass and eye-piece, the diagrams would then represent the passage of the light through an achromatic compound microscope. If, instead of the sele- nite, other crystals, such as Iceland spar, quartz, nitre, &c., cut in the manner described at page 240, be placed between the tourmahnes, coloured rings will be produced, which, in some cases, may be intersected either by a black or white cross, according as the light is stopped or transmitted. All ox THE POLARIZATION OF LIGHT! 249 substances, whether animal, vegetable, or mineral, which, by the unequal arrangement of their particles, possess the property of double refraction, will, when placed between the prisms, exhibit colours varying according to the otherwise unap- preciable difference of density of their various parts, and these differences may thus be distinguished and traced out much more satisfactorily than by common light. " Should, however," says Mr. Woodward,* « the doubly refracting properties of the tissue be too feeble to produce a sufficient difference of colour, the effect may be considerably increased by placing the object on a plate of selenite of uniform thick- ness, for which purpose a thickness capable of producing a bright purple or hght blue colour will be found to afford the most agreeable contrast, and, as a single plate, to be the most generally useful." In the preceding description of the colours produced by polarized light, those of which mention has been chiefly made are the red and the green ; it must not, however, be imagined that these are the only colours, for, in practice, it will be found that not only every colour of the spectrum, but every variety of tint of each of these primary colours, wiU be pro- duced by variations in the thickness of the doubly refracting substance, through which the polarized light passes; these tints may be classified into seven orders, as was done by Newton, when he ascertained the thickness of coloured plates, and particles of air, water, and glass. Selenite, from the cir- cumstance of its splitting easily into laminae, may be obtained of all thicknesses, and films of intermediate thicknesses between •00124 and '01818 will give all tints of colour between the white of Newton's first order and the white arising from the mixture of aU the colours. The same variety of tints may be produced by placing the films one over the other; for this purpose, Mr. Darker, who has paid considerable attention to the sub- ject of polarized light, and to whose ingenuity we are indebted for some of the most beautiful of our apparatus for exhibiting certain phenomena in connection with this subject, has con- structed for Dr. Leeson the instrument represented by fig. 158. Op. at, p. 31. 250 USE OF THE MICROSCOPE. It consists of a plate of brass, four inches long, an inch-and a- half broad, and one-fifth of an inch thick, having a piece of raised brass screwed to it, against which objects may rest when the microscope is inclined; in the centre of the brass plate there is a hole, one inch in diameter, into which is fitted a ring of the same metal, with a shoulder on its under side to receive certain "cells, into which plates of selenite are fitted ; this ring is capable of being revolved either to the right or the left of a central index or dart, by means of an endless screw, turned by the small handle seen on the right side of the Fig. 158. figure. P A i, P A |, P A i, represent three brass cells, into each of which are burnished two plates of thin glass, having between them films of selenite of difierent thicknesses. The dart at P A denotes the direction of the positive axis of the selenite, and the figures -i, |, &c., denote the parts of a vibra- tion retarded by each disc, which, by their super-position and variation in position, by means of the endless screw motion, produce all the colours of the spectrum. Advantages of Polarized Light to the Microscopist. — The application of this modification of light to the illumination of very minute structures has not yet been fuUy carried out, but stiU there is no test of differences in density between any two or more parts of the same substance that can at all approach it in delicacy. All structures, therefore, belonging either to the animal, vegetable, or mineral kingdom, in which the power of unequal or double refraction is suspected to be present, are those that should especially be investigated by GONIOMETER. 251 polarized light. Some of the most delicate of the elementary- tissues of animals, such as the tubes of nerves, the ultimate fibriUaj of muscle, &c., are amongst some of the most striking subjects that may be studied with advantage under this method of illumination. It would be impossible, in a work like the present, to give a long list of objects that require the aid of polarized light for their exhibition; every structure that the microscopist is investigating should be examined by this light, as well as by that either transmitted or reflected; objects mounted in Canada balsam, that are far too delicate to exhibit any structure under ordinary illumination, wiU often be well seen under polarized light; its uses, therefore, are manifold. Those who would wish to enter scientifically into this subject, should consult the works of Biot, Brewster, Herschel, and the excellent Lectures of Dr. Pereira; the Familiar Introduction to the Study of Polarized Light, by Mr. Woodward, may also be studied with very great advantage. The object of the author in the present chapter has been to show the nature of the apparatus employed, how it is adapted to the microscope, and the method of using it, together with as short an explanation as possible of some of its most important principles; the mode of preparing such objects as wUl best exemplify these principles will be fuUy described in a subse- quent chapter. CHAPTEE VIL GONIOMETER. Foe the purpose of measuring the angles of crystals, whether microscopic or others, a very beautiful instrument has been invented by Dr. Leeson, and has been termed by him, a Double Refracting Goniometer. A full description of it has been given in the Proceedings of the Chemical Society, Part XXIII., of which the following Is a transcript : — " The goniometer, invented by the author with a view to 252 USE OF THE MICEOSCOPE. remove many of the difficulties incident to the instruments heretofore in use, is represented in figs. 159 to 167. Amongst the peculiar advantages of this instrument may be enumerated its capability of measuring opaque and imperfect crystals, also microscopic crystals, and crystals in the interior of transparent media. It is equally apphcable to the largest crystals, and will measure angles without removing the crystal from a specimen, provided only the whole is placed on a suitable adjusting stage. The instrument depends on the apphcation of a doubly refracting prism, either of Iceland spar or of quartz, of such a thickness as will only partially separate the two images of the angle which it is proposed to measure. " Premising that the goniometer may be either mounted as a separate instrument or attached to a microscope, it is pror posed to describe it as when fitted to a body of the improved compound microscopes in common use. The same letters will be used for the corresponding parts in figs. 159 to 164, which represent the various parts of the goniometer fitted up as the eye -piece of a microscope, together with those of the adjusting stage to support the crystal which is intended to fix upon the traversing stage of the microscope. "Fig. 159 is a perspective view of the goniometer, and fig. 160 is a section of the same. At a is an achromatic prism of Fig. 159 double refracting Iceland spar — a Kochon's prism of quartz may be substituted; 6 is a brass tube containing the prism, with a round aperture forming the cap of the eye-piece, and sliding stiffly on the tube c attached to the arm d carrying the vernier of the graduated circle /*. At /is an achromatic eye- GONIOMETER. 253 ])iece, either Huyghenian or positive, sliding stiffly or screwing into the tube g which fits into the tubular body of the micro- scope. If the crystal be large, and no magnifying power re- quired, this eye-piece may be dispensed with ; a single lens of long focus may occasionally be substituted with advantage. The vernier has a clamping screw at i and an adjusting screw at h. A reading microscope, placed at c, is attached to the vernier. Fig. 161 is the adjusting stage to support the crystal whose angles are to be measured, and fig. 162 is a Fig. 161. Fig. 162. section of the same. The crystal may either be attached by Canada balsam or wax to a glass, or blackened ivory plate / dropping into the ring m, which again fits into the ring n, or the crystal may be conveniently held by the clamping arms in fig. 163, attached to a ring, also fitting into the plate n. Fig. 163. Fig. 164. Two of these arms have very small cups at their extremity 254 USE or the microscope. fitted with a piece of India rubber or cork, by means of which they can be pressed against a crystal without bruising it. The third arm has a small fork at its extremity, which tiu-ns round on the end of the arm at x in any direction to suit the crystal. The arrangement of screws and guides shown in the drawing is for the purpose of sliding the arms to and from the centre where the crystal is to be fixed. Fig. 164 is another ring fitting into n with two light springs to hold a slip of glass on which to fix a crystal when desired, or to carry one of the ordinarily mounted microscopic specimens. " p, figs. 161 and 162, is a semicircle attached to the ring n, which is centred on two screws, at t f, passing through two uprights, so that it may be inclined at any angle and fixed by the clamping screw r. This semicircle may be graduated, in which case it can also be employed for deter- mining the inclination of the optic axes by polarized light. Not only can the plate m be revolved in any direction within the ring n, and set to any inclination by the semicircle p, but the whole ring o carrying the upright supports may also be revolved around the short tube shown in the section at ?t upon the plate s, which fits upon the traversing stage of the microscope. The ring o may also be graduated, if thought useful, and thus rendered convenient for investigations by means of polarized light. " When a crystal, or any angle of the same, is viewed through the prism attached to the goniometer, two images thereof are produced by revolving the prism, which, with respect to each other, may be made to occupy various relative positions, as shown, for example, in figs. 165, 166, 167. Let a b c, fig. y Fig. 165. Pig. 166. Pig. 167. 165, be the angle to be measured, the crystal being adjusted properly, as hereinafter explained. Place the vernier at zero. ^' a'l / I' 6 I -Cc i'c / /° /J /i 7 /_ "^ / a/ / / GONIOMETER. 255 and there clamp It fast; then revolve the tube b containing the prism until the lines forming one side of the angle to be measiu-ed coincide in both images, as, for instance, the lines a b, a' b', fig. 166; then release the vernier and revolve it on the graduated circle, until the two lines forming the other side of the angle b c, V d, also coincide, fig. 1 67. The amount of rotation thus obtained is the measure of the angle, or its com- plement, according to the direction in which the vernier is moved. Instead of starting from zero, it is of course suffi- cient to take the difference of the readings in the two positions. " There are two descriptions of angles, the one sort being the plane angles produced by the lines forming the edges of the planes, the other sort being the angles representing the mclination of the planes themselves to each other. "When a plane angle is to be measured, it is necessaiy that the two lines or edges forming it should be both situated in a plane perfectly horizontal, that is perpendicular to the axis of vision. The stage, fig. 161, furnishes every facility for this adjustment, which may be known to be perfect by using a suitable object glass and observing that every portion of both lines remains exactly in focus on traversing the stage. " When the inclination of two planes is to be measured, they must be so adjusted that their line of junction is parallel to the axis of vision, or, to use a famihar expression, they must be taken out of winding, as it is termed. A very little practice will satisfy the observer that these adjustments may be readily accomplished by the stage in question. " Similar adjustments may be eifected, although somewhat more difficult, by using the forceps commonly sold with the compound microscopes, more especially the three-pronged forceps made by Smith. " The author cannot too strongly insist on the importance of the microscope in examining the surface of the planes of crystals subjected to measurement, convinced as he is that obliquity in many cases arises not only from conchoidal frac- tures, but also from imperfect laminte elevating one portion of a plane, and yet allowing a very tolerable reflection when 256 USE OP THE MICROSCOPE. measured by the reflective goniometer. Another source of error sometimes arises from not observing that the planes measured are those of macled or aggregated crystals, and thus furnish angles which would not exist in a distinct crystal." Mr. Ross has also contrived a very excellent goniometer ; it is constructed somewhat on the same principle as the micrometer eye-piece, shown at fig. 134, page 212, by placing a cobweb in the lower part of the outer tube in the focus of the eye-glass, instead of a divided scale, and having the upper rim of the same tube divided into 360". By turning either the crystal or the cobweb, so that one of the sides of the former may lie in a line parallel with the cobweb, and having set the index at zero, or observing the degree it points to, and then bringing the cobweb in a line parallel with another side of the crystal, the number of the degrees passed over will give the angle required. PART III. MANIPULATION. 17 MANIPULATION. CHAPTER I. The microscope, and all the apparatus necessary for the investigation of every class of object, having been fully described in the preceding pages, it remains now to point out the different methods by which these objects may be exhibited, as well as mounted and preserved for future examination. As this work is chiefly intended to afford information to those who are young in microscopical science, and who, therefore, cannot be supposed to be familiar with many of the methods employed in the preparation of the different kinds of objects, the author must be excused for entering somewhat in detail into a few of the most important processes ; as it is requisite that the directions given should be sufficiently plain to enable even the most inexperienced manipulator to carry them into effect. All structures intended to be viewed by transmitted light requiring glass of some form or other, either for their support whilst being investigated, or when permanently mounted, it becomes necessary first to allude to the best mode of cutting such glass into the proper shape by a diamond. Diamonds for Cutting Glass. — The diamonds required by the microscopist are two in number, the plough or glazier's diamond, and the writing diamond. The former is necessary for cutting the thin plate or crown glass for slides, and for cells and boxes to mount objects in, whilst thg latter is required for cutting the thin glass for covers, and for writing the names of the objects on the end of the slides. The plough or glazier's diamond is represented by fig. 168, and consists of a shaft or handle, and of an oblong piece either of 17* 260 MANIPULATION. steel or brass, in which the diamond is set; this piece of metal is connected to the handle by means of a swivel-joint which works in the end of a brass ferrule attached to the handle of the instrument. The handle is generally composed of wood or ivory, the upper part is round and slight, whilst the lower part is much larger and flattened on two sides, for reasons presently to be assigned. The amateur, when taking a diamond in hand for the first time, must not be disappointed if he fail to cut a strip from a piece of glass : he may even persevere, and still some time elapse before he is certain of making a good cut — hence it has been thought that a few words of instruction on this point would not here be out of place. The diamond should be held in the hand thus : — The upper part of the shaft or handle should be placed between the fore and middle finger, and one of the square sides of the lower end pressed upon by the thtmib, and the opposite square side by the middle finger. The side of the oblong piece of metal in which the diamond is set should now be brought against the edge of the ruler, and by means of the swivel-joint it will readily accommodate itself to it. Supposing we wish to cut a strip from a piece of glass, we must first lay the ruler on the glass, in the position in which we wish the cut to be made; the diamond held as already directed must be placed against the edge of the ruler, at the spot where we wish the cut to commence, and should the operator have never cut with the same diamond before, he should rest it on its point, and move it backwards and forwards upon it, making the upper end of the handle describe a curve, until the diamond marks or takes readily to the glass. When the right position is»^t)btained, the diamond may be drawn carefully along the ruler to the opposite end of the glass, care being always taken that the pressure exerted on it be not great, and that the same degree of inclination of its Fig. 168. MANIPULATION. 261 handle to the ruler be preserved throughout, else in parts the glass wiU be cut, and in others only scratched. A true cut will be known by being very faintly visible, and by a particularly musical sound being produced by the cutting, whereas a scratch is known by its jagged edges, and by the rough and harsh sound made whilst the diamond is passing : when the glass is held towards the light, it will be seen that the scratch is merely superficial, but the true cut, although faintly seen on the surface, will show that the glass is cracked for some con- siderable distance below it. When we try to separate the slip which has been cut with the diamond, we shall find that it easily breaks off, whilst the scratched portion wiU be with difficulty, if at all, removed. The operation of cutting, I although at first sight very simple, will be found not to be so in practice. The best plan for the tyro is to procure a piece of soft glass, and make a number of cuts as near to each other as possible, and try how thin a slip can be broken off. Writing Diamond. — The writing diamond is repre- sented by fig. 169; it is not provided with a swivel, neither is the handle squared, but consists for the most part of a thin, tapering stem of ebony or ivory, and as it, like the glazier'rdiamond, will only cut or scratch in certain positions, it is requisite that the operator should ascertain this point, and when once found, a mark should be made on the handle, in order to show which part of it is to be held in front. Those diamonds having a portion of the handle squared on which to place the thumb or forefinger will be found to be by far the most convenient ; but when the operator has once fixed upon the best writing point of the diamond, he pi- jgg may, if he chooses, cut out a place in the handle for the reception of the thumb or forefinger. There are two descriptions of writing diamonds, one in which an irregu- lar stone has been ground or turned to a fine point, whilst the other is formed of a sharp-pointed fragment or splinter. The former is to be preferred, although its cost is considerably greater. 262 MANIPULATION. CHAPTER II. ON CUTTING GLASS. Glass. — The best kinds of glass for mounting objects upon, are those known in commerce as thin plate and flatted crown, both may be purchased either in sheets or cut up into slides ready for use ; if the former be preferred, it should be free from holes and flaws, whilst, in the latter, veins and air bubbles must be avoided ; should the operator wish to cut it himself, he must be provided with a diamond of the form represented in page 260, and a piece of apparatus termed a cutting-board. Cutting-board. — This is represented by fig. 170, it consists '^'L \ 9 Fig. 170. of a piece of mahogany or deal, a b, about a foot or eighteen inches long by eight or nine broad, and three quarters or one inch thick ; upon this, and close to one of the sides, is screwed a strip of similar wood, c, about an inch broad and one-fourth of an inch thick, but not so long as the bottom piece by two inches ; upon it may be marked lines, such as efg, to indicate the width of the more common sized slides. The rulers are generally made of the length of the board, and as broad as the sizes of the glass-slides required to be cut, minus half the thickness of the setting of the diamond ; the wood most suit- able for them is from one-eighth to one-fourth of an inch thick ; one of these rulers is represented by d. In aU, the ends should be cut perfectly square, for a purpose presently ON CUTTING GLASS. 263 to be named. Now, supposing we wish to cut the slides of the usual size, viz., three inches long by one inch wide, we must be provided with two rulers, the one not quite one inch broad, the other not quite three inches, as no diamond will cut close to the edge of the ruler, in consequence of its being set in the middle of a piece of steel or brass, therefore the distance of the cut, made by the diamond-point from the edge of the ruler, will be regulated by the thickness of the setting, and as no two are of the same thickness, it becomes necessary to have the rulers, at first, of the respective breadths of one inch and three inches, and then to have one edge planed, until the cut made by the diamond is exactly the right measure. When those who use plate- glass prefer having the edges of their sUdes either ground or polished, it is as well to keep the rulers of the exact size of the slide, viz., one of an inch and the other of three inches broad, by these the slides will be cut larger than is required, but the loss the edges sustain in the grinding will, in all probability, bring them down to the proper size. Process of Cutting. — In the cutting of slides, it is advisable first to cut the glass into strips three inches broad, this is done in the following manner : — One edge of the glass, if not quite straight, is to be made so by cutting a thin strip off; this straight side is to be placed against the thin raised edge on the cutting-board, and pressed firmly towards it, the broad ruler is then to be laid upon the glass, and also pressed against the raised edge, and a cut, made with the diamond in the manner previously described ; we shall now have a strip of glass three inches broad. The next step is to make one of its ends square, this may be done by laying the strip against the raised edge of the cutting-board, with one extremity ex- tending a little beyond it, the broad ruler, being perfectly square at both ends, is now to be placed with one of these against the raised edge and a cut made, this will render the end of the strip of the glass square ; this squared end is then to be brought against the edge of the cutting-board, and, with the narrow ruler, it can be cut up into as many slides of three inches by one as its length will admit of; when these are 264 MANIPULATION. placed in a bundle, they will be found to be all of one size, which would not be the case were any other plan adopted. The small strips cut off from plate-glass, which are not large enough for slides, should not be thrown away, as they will be found very usef J in making cells or boxes. It will be noticed that slides of three inches by one have- only been spoken of in the preceding description, of course any other sizes may be cut by the same process, by having rulers of the requisite breadth. Edging the Slides. — All glass that has been cut with the diamond will present a rough edge, to obviate this, and at the same time to improve the appearance of the slide, the edges must be ground smooth, this is done by rubbing them on a perfectly flat plate of metal, with fine emery and water. As one of these plates will be found of great use for other pur- poses than that just mentioned, it will be as well here to speak of the best form to be adopted. Several metals will answer the purpose, but the best is a mixture of lead and pewter ; cast-iron and brass, especially the former, will answer very well, but lead is rather too soft ; in order to get the surface flat, it may be planed, or, if the plate be not very large, three of the same size should be procured ; they may all be kept per- fectly true by a process well known to most mechanics, this is nothing more than grinding one alternately against the two others, and these two against each other, whereby a perfectly flat surface may be always kept. If cast-iron be employed, three plates, about seven or eight inches in diameter, will answer the purpose exceedingly weU. The process of grinding the edges consists in holding the slide in a vertical position on the plate, and rubbing it either round and round, or else backwards and forwards, until perfectly flat; if the edges require to be bevelled, the slide must then be inclined at an angle of 45° with the plate, and be rubbed in the same manner. The emery should be fine, and the slide dipped in clean water occasionally, and then wiped to see how the grind- ing proceeds. To receive the excess of emery and water, which is sure to escape over the side of the plate, a cloth may be placed around it, or, what is best, is to have the plate ON CUTTING GLASS. 265 fixed into a block of wood in the same manner as a hone. If the edges require to be polished, after having been ground, they may be rubbed upon a flat piece of wood covered with buff leather that has been impregnated with putty powder, water being used in the same manner as with the emery. If a large quantity of slides be required, the process of grinding will be facihtated by employing a lapidary's wheel or mill, charged with emery ; the polishing may also be done in the same apparatus by using a wooden wheel charged with putty powder. If the slides be intended to be covered with paper, the grinding of the edges may be dispensed with, as the paper wiU hide all inequalities of surface. ON CUTTING THIN GLASS FOR C0VEE8, ETC. The thin glass employed to cover microscopical preparations is manufactured only by Messrs. Chance, of Birmingham, it may be procured of various degrees of thickness, from the one-twentieth to less than the one-hundredth of an inch; being unannealed, it must, on account of its brittleness, be handled with care. For cutting it, the board described at page 262 for the thick glass may be used, or a smaller one, consisting of a piece of mahogany about eight inches long, three broad, and a quarter of an inch thick, with a raised edge, as repre- sented at c. The lines e f g, va. front of the raised edge, should also be present, to indicate the length and breadth of two slides of the size most commonly used, and to form a guide for cutting the covers. As it matters little if the covers be not cut exactly of the same size, one narrow ruler will generally suffice for all purposes. The diamond to be employed must be the writing one, having a sharp point, but the pressure exerted ought to be slight, as the thin glass, from not being annealed, is apt to break with the slightest touch ; should the operator, however, not be provided with a writing-diamond, the glass may be cut with a plough-diamond by laying it on a piece of plate-glass that has been wetted, a plan first adopted by Mr. Warington. This will fill up all inequalities and cause an adhesion between them, and, with a little care, the diamond-point will cut it. 266 MANIPULATION. but if the first stroke fail, the operation must be repeated until a scratch- is made, as it often happens that, from the hardness of the surface of the unannealed glass, a plough- diamond will not readily mark it. To Cut Circular and Oval Covers. — This may be done either by a machine in which a plane surface of wood, on which the glass is laid, is made to revolve in a circle or oval underneath a diamond-point, or by the writing-diamond ; in which latter case a model of the size of the cover will be required. In order to cut circles, the following plan has been adopted with great success by the author: — Six or more flat pieces of brass, each about 1^ inch square and -^ of an inch thick, are to be provided, each having a hole in its centre, the smallest hole to be about \ an inch in diameter, that in a second Jj of an inch larger, and so on, increasing by the same amount up to the sixth ; each piece of brass should be marked with a number. One of these being laid upon the thin glass, and pressed firmly down with the finger, the diamond is to be passed round the margin of the disc, care being taken that, in the passing round, it be made to revolve on its own axis, otherwise the beginning and end of the cut will not join, a little practice wiU soon overcome this obstacle. By this plan any odd pieces of glass may be cut up and put away in boxes, each being marked with the number of the brass em- ployed in the cutting. When oval covers are required, pieces of brass with oval holes may be used in the same manner as the others. When a number of discs of the same size are required, it is advisable to have the thin glass cut into strips a little broader than the circles are to be, the circles when cut can then be readily separated from the surrounding glass ; but should any difficulty arise, a few strokes made by the diamond through the largest and most attached parts, will readily cause their separation. Much of the thin glass used for covers is slightly curved, care, therefore, is required to select those pieces that are the flattest, especially when the covers are destined for cells containing fluid ; if they be not flat, the gold-size, or other material employed to cement their edges, will be certain to run underneath them. When the ON CUTTING GLASS. 267 edges of the covers, whether circular or otherwise, are not cut sufficiently well to allow of the fragments being separated from them readily, but little pieces of glass are left behind, a fine file rubbed over the edges, will make them smooth, and any of the cements will adhere more firmly to them. For this purpose the cover should be held between the thumb and first finger, and only a small portion of the edge allowed to project above the fingers for the file to act on at a time ; by these means there will be but very little risk of fracture. The methods above described are very useful when the operator is in possession of a writing diamond only ; but the following very ingenious instruments have lately been con- trived for the express purpose of cutting circular covers. The first of these is the invention of Mr. Shadbolt, and is repre- sented in fig. 171. It consists of a cen- tral stem of steel wire, a a, sliding freely in a tube of brass, b. To the upper end of the wire is attached a disc of ivory, c, and to the lower a small cylinder of box- wood, d; to the lower end of the tube, b, is fastened a piece of brass, e, through which is made to slide a small triangular bar,^ carrying at one end a semicircular piece of brass, ff, through which slides a steel pin, h, armed with a diamond point; j7i. V§>^ *^/ the pin may be fixed at any required height by the screw, i, and the bar also (when regulated for the size of circle to be cut) by the screw,,/. The use of the instrument is obvious, the cylinder, d, being placed in an upright position upon the piece of thin glass, and the arm, /, adjusted to the required size of the cover to be cut, a finger of the left hand is to be placed upon the disc, c, and one of the rio-ht upon the edge of the larger disc of wood, k ; if this last be revolved, it wiU carry with it the arm, bearing the diamond, and a little practice will enable the operator so to regulate the pressure of the latter upon the glass, as to make a perfectly Fig. 171. 268 MANIPULATION. even ctit. Mr. Shadbolt has lately done away with the disc of wood, k, and made the tube, h, much larger at that part, and given it an hexagonal figure, like that of D in fig. 172, for the fingers to act upon. This plan answers as well as the disc, and the instrument in consequence can be packed in a much smaller compass. An instrument somewhat similar to the above, but less costly, has for a long time been furnished by Mr. Darker, who has also contrived one of a more complete kind, which is fitted to the top of a box, and the central stem is supported by an arm, by which it is kept in a vertical position. The diamond is connected with an adjustable hori- zontal arm, and is made to revolve by means of two pulleys and a connecting band. The glass to be cut is laid upon a piece of plate glass, blackened on its under smrface, which forms part of the cover of the box, and the diamond is kept away from the thin glass by means of a spring ; but in order to render the instrument more effective, an extra finger and a central stem, to keep the thin glass in place, are required. On this account, the apparatus represented by fig. 172, as contrived by Mr. Thomas Ross, wiU be found to perform more readily. A bent arm of brass. A, is supported by means of a square block of wood upon a base, B. Through the collar, C, passes a stem, which is of octagonal figure at D for the fingers, and cylindrical at E. Upon this latter part shdes a spring box, F, having a piece of cork or India rubber at its free extremity. An horizontal arm, G, supporting the spring box, H, through which slides the stem, I, carrying the bar, K, of the diamond, L, is capable of being adjusted for the diameter of the cover by means of a screw. The use of the instrument is obvious ; the glass is laid upon the foot, B, and the proper size having been fixed on by means of the scale, M, the stem, E, is pressed down upon the glass firmly by the fingers applied at D. In order, however, to prevent the prcssm-e from cracking the glass, the spring boxes, F and H, have been employed, so that however much force be applied at D, the diamond and central piece, F, only press upon the glass by the force of the two springs contained in the boxes. ON CUTTING GLASS. 269 As soon as a complete circle is marked by the diamond point, the fingers are removed, and the stem is lifted from the glass by the heliacal spring. By this instrument, as by Mr. Shad- bolt's, the glass to be cut is firmly fixed in the centre, and on this account a piece a little larger than the circle to be cut is only required. Fig. 172. 270 MANIPULATION. CHAPTER III. METHODS OF CEMENTING CELLS. There are three principal methods of mounting microscopical preparations : — first, in the dry way ; second, in some kind of preservative fluid, such as spirit and water, or Goadby's or other solution ; and third, in Canada balsam. In the first method, aU that is generally required is a slip of glass or a slide (as it is commonly termed), and a cover of thin glass ; but, in the second method, when the object is of any thickness, the fluid requires to be contained in some kind of reservoir, which is most frequently made of glass, and is called by micro- scopists a cell. Of these, various shapes and sizes wiU be found necessary, but as aU require to be fastened to a slide, by means of some kind of cement, it wUl be as well here to describe the most approved methods of conducting the operation. Method of Cementing Cells without Heat. — The thin glass, the tubular, the drilled, and aU the other forms of cells pre- sently to be described, may be fastened to a bottom glass or slide by means of one or other of the following cements, viz., a mixture of gold-size and lamp black, or gold-size and litharge or red-lead, or a solution of asphaltum in turpentine j in aU these cases, the parts of the cell that are to be cemented must be simply painted over with a tolerably thick layer of one of the above cements, and then be laid on the slide, and be put aside until the cement is suflSciently hard for the cell to be used, which wiU often require many days. Cementing Cells by Heat. — The most efficacious plan, how- ever, is to employ one of two substances known in commerce as Canada balsam or marine-glue, both of which must be liquefied by heat, and, to effect this, a simple apparatus, such as a small plate of sheet-iron, supported in some way or other over a lamp, must be provided. The size of the iron-plate should be large enough to hold slides three inches long by one wide ; it can be supported in various ways ; if the plate be METHODS OF CEMENTING CELLS. 271 about five inches square, it may very well be heated, either upon the ring over a chemical argand lamp, or upon one of the rings of a retort-stand, and a spirit lamp used. By far the best plan is to have a piece of wrought iron about six inches long, two and a half inches broad, and one-eighth thick, with small legs to support it, about three inches from Kg. 173. the table ; the legs for convenience of carriage (as shown in fig. 173) are jointed, and under the table a spirit lamp is placed to heat it. Whilst the plate is being made warm, the cell and the slide to which the cell is to be cemented, are to be laid upon it, and on the sides of the cell small pieces of marine- glue, cut either into lumps or shavings, are to be placed ; these must be watched, and as soon as they begin to melt, they may be moved one towards the other with a sharp-pointed instru- ment, so as to cover the whole of the surface which is to be cemented ; as soon as this is done, and the glue is seen to boil, the cell may be taken up with a pair of forceps and turned over upon the slide, and when it has been adjusted to its right place^ it may then be firmly pressed down upon the slide with a piece of flat wood, so that all the superfluous glue, with any air bubbles that are present, may be squeezed out. The cell may now be removed from the plate and placed upon a piece of wood ; before it gets cold the superfluous glue may be 272 MANIPULATION. scraped away easily with a small chisel, such as represented by fig. 174, or by a knife, but if this operation be left until the Fig. 174. cell is cold, the task then will be more difficult. As soon as the cell is cold, a small quantity either of a weak solution of potaah, or even spirits of wine, may be poured into it, and all the small particles of glue removed first with the chisel, and, secondly, with a chisel-like piece of wood, having a thin piece of rag over it, so that the cell may be freed both from the cement and from any greasy matter that may be present (care being always taken that the glass be not scratched). The cell may now be rinsed out with clean water and wiped dry, first with a cloth or an old cambric handkerchief, and finished with chamois leather ; it is now ready for use. When the glue has been heated too much, it turns black, becomes tough, and wiU not stick to the glass ; when this has happened, it is better always to take away the cell, scrape oflF all the old glue, and begin afresh, than attempt to cement it with glue that has been over-heated and lost its fluidity. As soon as the glue boils, no time should be lost in laying down the cell in its proper place. Sometimes small black gritty masses are present in the glue, these should be re- moved whilst the melting is going on, as when they are left, unless they can be crushed by pressure, they will prevent the cell from coming down flat on the glass, and often large air bubbles will appear when the pressure is taken oiF. There are many kinds of marine-glue in use, but the best for cementing cells is that known in commerce as G. K. 4, this melts at a temperature some few degrees higher than that of boiling water, the harder kinds get brittle by keeping, and their melting point is much higher. While the plate is warm, and all the materials are at hand, it is best to cement a number of cells, all of which may be cleaned and put by ready for use, as it will be found in practice necessary to METHODS OF CEMENTING CELLS. 273 have several sizes in store, that an object may be mounted speedily, for many valuable things are laid by and forgotten when the trouble of making and cleaning a cell has to be gone through before the mounting can be accomplished. To Cement Cells with Canada Balsam. — The iron plate, the spirit lamp, and the tools mentioned in the article on cement- ing cells with marine-glue, will all be as requisite for the Canada balsam as for the former material. The plate is also to be warmed, and the cells laid on it, as there described, but upon the sides of the cell some old but semifluid Canada balsam is to be placed. As soon as it shows the least symptom of boiling, which is known by the disappearance of aU air bubbles, the cell may be taken up with the forceps and laid upon the slide, pressed to get rid of the surperfluous balsam, and then set aside to cool ; the superfluous, balsam may be removed with the knife or chisel, and the ceU may be cleaned with a rag, dipped either into turpentine or ether. To get rid of these, the potash or alcohol may be substituted, and when the cell is rinsed out with clean water, and wiped dry with a rag and chamois leather, it is ready for use. Canada balsam is not so good for the cementing of cells as the marine-glue, in consequence of its getting brittle by age ; some consider- able care even is required in handling preparations that have been mounted only a few years, as the least jar or bending of the slide to which the cell is cemented is often attended with a separation of the balsam. It should always be borne in mirid that the heated slide to which the cell has been cemented must not be touched with a damp hand, neither should it be laid on metal or anything wet, as the glass, when suddenly cooled, is very apt to crack. If the upper part of the cell be uneven, it should be rubbed on the metal plate described at page 264 until it is perfectly level, as the success of the operation of cementing down a cover will niaterially depend upon this point, 18 274 MANIPULATION. CHAPTER IV. ON CEMENTS. The cements employed to fasten down the covers of cells and boxes for containing microscopical preparations mounted in fluid, are of many kinds, the most useful of them being as follows : — Japanneri Gold- Size. — This consists of a mixture of boiled linseed-oil, dry red-lead, litharge, copperas, gum animi, and turpentine; the first ajid last ingredients being its principal constituents, it can be purchased at most shops where varnishes are sold, but, as its drying properties increase with its age, it is necessary that it be two or three years old before it is employed. It should be laid on with a camel or sable-hair pencil, and be kept in a wide-mouthed or other vial, closely corked. A thin coating should be laid on at first, and when this is dry another rather thicker. If the gold-size be very thin, a small quantity of lamp-black or litharge may be mixed with it on a slab by means of a palette knife, and as this mixture very soon dries it should be used quickly. The brush employed with either of the above cements may be cleaned with turpentine. Sealing-wax Varnish. — This is made by dissolving sealing- wax of any colour in alcohol, it having previously been broken into small pieces. This cement is not so good as the gold-size, in consequence of not sticking readily to damp surfaces, but it forms an excellent material for giving a shiny coloured coating as a finish to the mounting of an object. It is laid on with a small brush, and should be tightly corked to prevent the spirit from evaporating ; the brush may be cleaned with alcohol. Some persons prefer shell-lac to sealing-wax for a cement, but it wUl be found rather too brittle ; the method of using and keeping it is the same as in the case of the seal- ing-wax. OM CEMENTS. 275 Asphaltum. — This, which forms a very good cement, is made by dissolving Egyptian asphaltum in boiling linseed-oil or in turpentine ; it answers very well for the first coating, but has not suificient body, unless mixed with some solid material, to form the entire mass of cement around the covers of objects ; it IS of a fine black colour, and as such will serve for the last or finishing coating. A solution of this substance in tur- pentine will make a good cement for fastening cells to the glass slides, instead of the marine-glue or Canada balsam, having this advantage, that spirit may be employed as the preservative fluid without injury to it. Canada Balsam. — A solution of this substance, either in ether or turpentine, evaporated to such a consistence as is suflScient to allow of its being laid on with a camel's-hair pencil, has been recommended by Dr. J. W. Griffith as a very good substitute for the gold-size; a mixture of lamp-black and white hard varnish, when laid on immediately, he also considers a good cement. Marine-Glue. — This most useful cement, the invention of Mr. Jeffery, is composed of a mixture of shell-lac, caoutchouc, and naptha ; many kinds are made, but that known in com- merce as G K 4 is the best for microscopic purposes ; a solvent for it is supplied at the manufactory,* and wUl be found of great service to those who wish to construct any of the larger kinds of ceUs or boxes. When about to be used, the glue must be cut into thin slices, laid on the glass, and heated until it begins to boil, or a hot iron may be placed on one of the surfaces of the glass until the same effect is produced ; if any gritty particles be present, they must be removed. To ensure a firm connection between two surfafces of glass, both must be well warmed, otherwise the glue will stick to one and not to the other. The excess may be removed readily before it is quite cold by means of the chisel described at page 272, and all trace of the remainder by the employment of caustic potash, care being taken that none of the latter be left in contact with the glue, as it is apt to insinuate itself between * Commercial Road, Limehouse. 18* 276 MANIPULATION. the two cemented surfaces, and cause their separation in a short space of time. Electrical Cement. — A very useful cement for some purposes hereafter to be described, is made by melting together ten ounces of resin and two ounces of bees'-wax, adding two ounces of red ochre and a teaspoonfull of plaster of Paris ; this cement is generally employed for fastening brass or wood to glass in all kinds of electrical apparatus ; it must be used when hot, and can readily be fashioned into any shape before it gets quite cold. Another very excellent but less brittle cement is made by melting together two ounces of black resin, one ounce of bees'-wax, and one ounce of vermilion ; this will be useful for making the thin flat cells described at page 288, as well as for many other purposes. Several other cements will be required occasionally by the microscopist, viz., a thick solution of gum arabic in water, to which a small quantity of essential oil has been added, to prevent it from fermenting and becoming sour; also, the same powdered and dissolved in acetic acid or distilled vine- gar ; as a substitute for these, the liquid sold by Messrs. Ackermann as diamond cement will be found of great value, both as a cement and as a fluid for mounting some kinds of objects. Mastich varnish is also very useful for cementing opaque objects to discs of cork or leather, or to any of the other kinds of surfaces on which they are intended to be mounted. For covering slides with paper, common paste is a good cement, but as it soon gets mouldy and sour, the following mixture, employed by Mr. Jackson, is a good substitute : — Powdered gum tragacanth, 1 oz. ; powdered gum arabic, 2 oz. ; white sugar, 2 oz. ; mixed and kept in a dry state. When wanted, mix up a small quantity with water by means of a brush, called by painters a "Jitch tool" and if what is not used be kept in an open shallow vessel, it will dry before it becomes mouldy, and may readily be rubbed up again with water. White-lead, ground in linseed-oil, will be required for making the cells described at page 286 ; this should be old, ON CEMENTS. 277 but free from all rough particles ; it may be mixed up on a slab with a small quantity of gold-size, and if a little litharge be added, it will dry more readily. Besides the above described cements there are a few others so useful as to require mention here. The first of these is known in commerce as Suffffitt's Liquid Jet; it is said to be a solution of bituminous shale in naptha: this cement dries in a few minutes and is admirably suited for making thin cells as well as for securing their covers, neither spirit nor turpentine act upon it. It shotdd be kept tightly corked, and only a small quantity exposed at a time. Coachmakers' Black Vaitiish may be employed as a substi- tute for the above, but it does not dry quite so quickly ; this can be obtained of any varnish maker ; but that sold by Mr. Penney, 251, Tottenham Court Koad, is very excellent. Black Japan is a good cement for making thin cells by Mr, Shadbolt's machine, described in page 288 ; it possesses the property of becoming very hard and tough by exposure to a gentle heat in an oven. The cells of Mr. Topping, also described in page 288, are prepared of this material. In all cases where any of the above described cements are used, it must be borne in mind that too much should not be laid on at once, a thin coating at first, and a thicker when the first is dry. 278 MANIPULATIOX. CHAPTER V. ON PRESERVATIVE FLUIDS. The preservative fluids, like the cements before described, require to be varied according to the nature of the structures they may be employed to conserve ; thus, for instance, a solu- tion of salt, alum, and corrosive sublimate in water, that will keep most fleshy substances, will destroy others that contain bone ; hence it becomes necessary to select a diflferent fluid for each of these structures, and of all the various kinds that have hitherto been employed, with the single exception of spirit, there is no one kind that is universally applicable; for large preparations, proof spirit will answer every purpose ; but as this is rather too strong for most of the cements used by the microscopist, it has been ascertained that for all delicate structures there are certain proportions of alcohol and distilled water that will be sufficient to preserve them, and will not act in any way either on the marine-glue, gold-size, or asphal- tum. The following fluids will be found the most generally useful. Spirit and Distilled Water. — In the proportion of one ounce of alcohol 60° above proof, to five of distilled water, a fluid may be made that is capable of preserving not only injections, b^it the elementary tissues both of animals and vegetables, but all the colours of the latter wiU be destroyed. Acetate of Alumina. — This, when dissolved in distilled water in the proportion of one ounce of the former to four of the latter, wiU preserve even very delicate colours, and when injected into the blood vessels of animals. Is said to prevent decomposition, and forms the so called Gannal process, em- ployed very much on the Continent for the preservation of animal structures on a large scale. Goadhy's Fluids. — The first of these, and the one for which Mr. Goadby was rewarded by the Society of Arts, consists of four ounces of bay salt, two ounces of alum, four grains of corrosive sublimate, and two quarts of boiling water ; these ON PRESERVATIVE FLUIDS. 279 ingredients are to be well stirred, and the solution very finely filtered. These proportions form a strong fluid, but, if neces- sary, a much larger quantity of water may be added without any diminution of its preservative qualities. Another kind consists of three pounds and a half of bay salt, seven grains of corrosive sublimate, and six quarts of water; this fluid, from containing no alum, is not so liable as the former to act on Huch structures as shell and bone. Another kind, called the arsenical solution, is made by mixing together two drachms of arsenious acid, three pounds of bay salt, and six quarts of water; to dissolve the arsenic, it should be boiled with a portion of the water in a tin saucepan. All the above fluids should be carefully filtered before they are used. Solution of Creosote. — Creosote does not readily mix with water, but if in a very minute state of division it may be suspended in it : one way of getting a solution is to mix it with water in a retort and distill, the water will come over highly charged with it. One objection, however, to the use of this solution is its tendency to quit the water and adhere as a deposit to the sides of the vessel in which it is contained. Mr. Thwaites, of Bristol,* recommends a fluid into which creosote enters as an ingredient for the purpose of mounting preparations of algte ; it is made as foUows : — Mr. Thwaites^ Fluid. — To sixteen parts of distilled water add one part of rectified spirits of wine and a few drops of creosote, sufficient to saturate it, stir in a small quantity of prepared chalk, and then filter : with this fluid mix an equal measure of camphor-water (water saturated with camphor), and, before using, strain ofl" through a fine piece of linen. For the same purpose Mr. Ealfs recommends the following, viz., bay salt and alum of each one grain, to be dissolved in an ounce of distilled water. Mr. Ealfs also informs us,t that Mr. Sidebotham employs distilled water alone for mounting delicate specimens of algae, and when the last coat of cement is nearly dry, he applies a fine bronze with a camel's-hair pencil. * Kalfs' Desmidiece, page 40. t Op. at., page 42, 280 JIANIPULATION. Glycerine.— TWis fluid (the sweet principle of fats and oils), lately recommended by Mr. Warington for preserving delicate animal and vegetable tissues, has many advantages over other fluids, not only in keeping the green colours of infusoria, but, as it evaporates very slowly, it may be employed in cells without so much care being necessary in the cementing of their covers. It is miscible with water in all proportions; the most convenient strength for use will be one part of glycerine to two of distilled water ; if made weaker than this, confervas will readily grow in it when exposed to the air, but not when sealed up in cells. If pure glycerine be employed it wUl act on the marine-glue, and from its highly refracting properties, many delicate structures wiU be entirely lost in it, as in Canada balsam. The best glycerine is procured in the manufacture of lead plaster (the emplastrum plumbi of the Pharmacopceia) ; it also remains in great abundance after the formation of soap ; but in this latter case it always is mixed with some free alkali, which renders it unfit for use. Castor Oil was first recommended by Mr. Warington, in 1844, as a medium for mounting certain classes of objects, especially crystals; he has also found that it answers equally well for minute fungi and parasitic insects ; before being em- ployed, it should be carefully examined, to see that it be per- fectly free from all crystalline deposit. Chromic Acid is useful for mounting some very delicate preparations; it was first used by Mr. Warington, in 1842, as a preservative agent, being part of a patent for a new mode of tanning ; it has since been employed by Mr. Bowman and M. Briicke to harden the vitreous humour of the human and other eyes, in order to detect its real structure. Chromic acid may be procured in the crystalline state, and should be dissolved in so much water as will render the tint of the solution a pale straw colour. A much weaker solution than this wiU keep most animal tissues. Salt and Hater. — A solution made in the proportion of five grains of salt to an ounce of distilled water will answer for keeping very many animal and vegetable structures ; it was first recommended by Dr. Cook, more than twenty years ago. ON FRESEUVATIVE FLUIDS. 281 We are told that Mr. J. T. Cooper,* in the course of a series of experiments to determine the best fluid for preserving coloured tissues, also found that salt and water, to which acetic acid had been added, succeeded extremely well for this purpose ; such a mixture will also answer for most minute vegetable tissues. The great objection to the use of all saline fluids is the growth of confervse which takes place in them; this may, in a great measure, be avoided by the addition of. a few drops .of creosote, or a small quantity of camphorated water. Naptha. — This, when mixed in the proportion of one part to seven or eight of water, will make a good preservative solution; the author, by accident, having placed in water some portions of skin to macerate, that had been coarsely injected with a material into which naptha entered rather largely as an ingredient, was surprised to find them perfectly free from decomposition even after the lapse of many weeks; the water was impregnated with naptha, and the specimens were in excellent condition, having undergone little or no change. General Directions. — It may here be stated, that for all large specimens, such as injections, the spirit and water, or Goadby's first solution, may be used ; and for others, either the creosote or glycerine solutions, as those containing saline matter, when placed either between glasses simply, or in the thin glass cells, are apt to crystallize slowly, and interfere with the objects that are mounted in them. Goadby's solu- tion, containing salt, alum, and -corrosive sublimate, will keep animal structures that have been injected with size and ver- milion exceedingly well ; but those in which the vessels are filled with flake-white will have that substance destroyed in a few hours; in these cases, either the arsenical or the spirit and water only should be employed. The glycerine fluid, when kept for some time, is apt to become mouldy; it should, there- fore, be mixed in small quantities, and then only a few hours before it is required. When objects are to be mounted in either of the above fluids, it must be laid down as a rule that they should have been soaking for some hours in the same * Microscopical Journal, vol. i., page 184. 282 MANIPULATION. fluid, or in a fluid of a similar kind ; this should be more par- ticularly attended to when the preparation has to undergo dissection in water previous to its being mounted. It has often happened to the author to find a preparation that had been dissected in water, and mounted in a cell in spirit and water immediately after, completely covered over with small air bubbles in a few hours, from the slow admixture of the two fluids. With Goadby's solution it does not so often happen, but with this a white sediment will be sometimes deposited in the bottom of the cell when the preparation has been soaking in spirit for some time previously. When the operator has more than one specimen of a rare kind, he should not confine himself to mounting them all in one fluid, but should try such others as in his opinion may be likely to succeed ; a note of this should be made at the time on the glass slide, and the date of the mounting also placed thereon ; such records will be of great service as guides to future operations. In addition to the fluids employed for the preservation of objects, there are certain agents now coming into use which are solid when cold. The first of these was suggested by Mr. Wenham, and consisted of a mixture of gelatine and treacle ; rthis has since been improved upon by Mr. H. Deane,who, in a paper lately read before the Microscopical Society of London, recommends the following : — Gelatine, 1 oz. ; water, 4 oz. ; honey, 4 oz. ; rectified spirits of wine, g oz. ; creosote, 6 drops. The gelatine is to be soaked in water until soft ; the honey is to be raised to the boiling heat in another vessel and added to the moist gelatine ; the whole is then to be made boiling hot; when it has somewhat cooled, but is still perfectly fluid, the creosote and spirits of wine, previously mixed together, are to be added; the whole is then to be filtered through fine flannel; when cold, the composition is in the form of a very stiff jelly, which, on being slightly warmed, becomes perfectly fluid. A mixture of gelatine and glycerine, as suggested by Mr. De la Hue, will also answer the same purpose. The mode of using these agents will be described in the chapter devoted to the mounting of objects in Canada balsam. METHOD OF MOUNTING OBJECTS IN ELUID. 283 CHAPTEE VI. METHOD OF MOUNTING OBJECTS IN FLUID. All very delicate animal and vegetable tissues, to exhibit their structure clearly, should not be mounted in the dry way or in Canada balsam, but in some preservative fluid, such as spirit and water, Goadby's solution, or one or other of the fluids which have already been enumerated. The most minute structures, such as the vessels of plants, the muscular and other tissues of animals, requiring in aU cases exceedingly high powers for their due exhibition, must of necessity be preserved in very thin cells, with a small amount of fluid. The best method is as follows : — Take a slip of thin plate glass, of the size adopted by the Microscopical Society, viz., three inches long by one broad, or any other convenient size, and after having cleaned it thoroughly by washing with a dilute solution of caustic potash, to remove all grease, let it be laid flat, and a drop of one or other of the preservative fluids presently to be enumerated placed upon it ; in this the object is laid, and after having been properly spread out with the needle point, it is ready to receive its cover of thin glass. This cover should be selected with care, and be as thin and flat as possible; when freed from all grease by being rubbed with caustic potash, it should be wiped with a clean cloth or chamois leather, and when flnished must be held in the middle, not with the fingers, but with forceps. The next part of the process is to touch its edges slightly with one or other of the cements before enumerated (the simple gold-size wiU be found to be the best, as it is most free from colour), the cover being held with the forceps, the brush or finger with a very small quantity of the gold-size, may be gently passed around each edge. This being done the next step is to lay the cover upon the object, which 284 MANIPULATION. IS i effected by dropping the cover gently upon the fluid, and jjressing it lightly to exclude all the excess, and to leave only a thin stratum intervening between the two lasses ; the excess may be removed either by (rawing it away by the sucking tube, fig. 175, r by small slips of blotting-paper. After this peration is finished, a thin layer of cement is to I le placed where the edges of the cover come in ontact with the bottom glass ; when this is dry, .nother thin layer may be put on, until the angle I letween the two glasses is nearly filled up. The ise of anointing the edges of the cover with the lement before laying it on the fluid is to prevent ts getting wet, and by that means hindering the lement from sticking. Care must be taken to sxclude all air bubbles from between the cover md bottom glass, otherwise the cement wiU run n, especially when the bubbles are near the sdge; should too much of the fluid have been Irawn out from between the glasses, and an air bubble be left, it is then necessary to add a little more of the fluid at the edge where the bubble is, and it will then run in and occupy the place of the bubble, and the excess of fluid may be again taken away in the manner before described. Objects mounted in this way seldom keep very long, and when once an air bubble has made its appearance, it becomes necessary that a watch should be kept upon it, lest the cement also run in and spoil the specimen. When an object possesses any appreciable thickness, it is by far the best plan not to mount it in this manner, but to adopt one or other of the following methods, in all of which a small reservoir is employed to contain both the fluid and the preparation ; this reservoir is termed a cell; various forms of these, which wiU be particu- larized as the thin glass cell, the concave, tubular, and drilled cells, with many others, here require separate mention. In order to prevent the cement employed in the flat cell Fig. 175. iliij|UAu*i Kg. 176. METHOD OF MOUNTING OBJECTS IN FLUID. 285 from running in to spoil the object, the author's late brother, Mr. Edwin Quekett, adopted the plan represented by fig. 176, which was to take a piece of writing paper, about one-eighth of an inch smaller ' " ■■ "l^""nr'"=" 1 ■ ,. each way than the cover to be em- ployed, and from the middle of this to cut a square or circular hole suf- ficiently large to hold the object. After the fluid had been placed on the slide, and the object deposited in it, this paper cell was also placed in the fluid, and when adjusted to the centre of the slide, the cover was laid on in the usual manner, the paper preventing the cement from running beyond it to obscure the object. Following out this principle, Mr. Darker has ingeniously con- trived a cell of the form represented in plan by A B, fig. 177, and in section by C. These cells are cast in glass of the size represented by A B, and, in order to prevent the cement from running in, the sides are constructed as shown by G D B ; B being the outer margin of the cell, D a flat surface for the cover to rest on, and G a gi'oove between it and the inner margin. These cells are cemented to the shdes in the usual manner, either with marine-glue or asphaltum dissolved in turpentine, the method of cementing down the cover is the same as in the other forms, a small quantity of gold-size being applied around its edge either with a brush or the finger before it is placed on the fluid within the cell ; should there be any tendency in the cement to run under the cover, it must first fill up the groove before it can get into the cavity where the object is. The under surface of this form of ceU can be ground sufficiently thin to enable a quarter of an inch object-glass to view any object; contained within it. Fig. 177. 286 MANIPULATION. The Concave CeW.— This consists of a slide of plate-glass, rather thicker than ordinary, in the centre of which a concave cell or pond has been ground out, and either left in the rough state or polished ; it may be either of the form represented in fig. 178 or fig. 179, and the method of mounting objects in it p. ^yg is the same as in the flat cell; a thin layer of the gold-size or other cement is to be placed around the margin of the cover as in the case of the flat cell, and the excess wiped off with the finger. The fluid is to Fig- 179. u 1 J • ..1. ° be placed in the concavity, and the cover dropped on as in the preceding descriptions, but the cover should be so much larger than the cell, as to leave a margin of one-eighth of an inch around it. This form of cell, when polished, does very well for many objects where accurate definition of the surface only is required ; and, when unpol- ished, is most useful for thin opaque objects, such as pieces of injection, leaves of plants, &c., &c., that are too thick to be mounted between two flat pieces of glass. White Lead Cell. — A very convenient and durable form of cell may be made by spreading some old white lead, that has been ground in oil, on one of the slides that measure three inches by one, taking care to leave an aperture or pond in the centre a little larger than the object; the lead may be laid on of a thickness equal to that of the object. Spirit and water, or other fluid, is to be placed in the pond, and the object de- posited in it ; the thin glass cover, which should be as large as convenient, is to be put on obliquely, and pressed firmly down into the white lead, taking care to exclude all air bubbles. The author has objects in his possession, quite perfect, which were mounted eleven years ago. Mr. William Valentine, who has adopted this plan for many years with complete success, uses a little trowel of box or other hard wood of the shape represented by fig. 180 to plaster down the white lead and press the glass, and a more convenient instrument for the purpose cannot be devised. When Mr. Valentine first com- METHOD OF MOUNTING OBJECTS IN FLUID. 287 menced this plan of mounting, thin glass was not to be met with, he was therefore obliged to use a stout piece of mica ; Fig. 18). Fig. 180. with this his objects have kept perfect for many years. Mr. Holland, whose name has been frequently mentioned as the inventor of the triplet object-glass, has recommended* a cell of the form represented by fig. 181, as useful for many objects that require a high power. a b exhibits a glass slide, on which is painted, with white lead (worked up with one part linseed-oil and three of spirit of turpentine), a pond or cell,c c, enclosing a space, d. Glasses so pre- pared with cells of any size or shape must be allowed to dry before they are used. The method of mounting objects within this form of cell is thus described: — " A drop of fluid containing the object is placed within the space d, and a piece of mica of the same size as the part painted dropped on the fluid; but care must be taken that the drop be not in sufficient quantity to touch the inner margin of the cement. That being accomplished, take some almond oil in a hair pencil, and pass it lightly and slowly round the edges of the mica: the oil will insinuate itself under it, and will surround the object without mixTng with it. When the oil is cleaned off, a coating of the white lead may be laid round the edges of the mica, extending about one-tenth of an inch within and without it." The two following methods of mounting very delicate objects, such as Desmidiese, have been recommended by Mr. Thwaites.f The first consists in marking out on the glass slide a cell of the required shape with gold-size, thickened either with litharge, red-lead, or lamp-black: these materials are to be mixed * Vol. xlviii., Transactions of the Society of Arts, page 123. f Kalfs' DesmidiecB, page 40. 288 MANIPULATION. together on a slab and laid on the slide as soon as possible, as this mixture quickly becomes hard. In the second, where deeper cells are necessary, marine-glue is used : this must be melted and dropped upon the slip of glass, and flattened when warm with a piece of wet glass, and what is superfluous cut away with a knife, so as to leave only the walls of the cell ; these, if they have become loosened, may be made firm again by warming the under surface of the slip of glass. The sur- faces of these cells may be made flat by rubbing them on the metal plate with emery and water. The plan of laying down and of cementing the thin cover is the same as that for the flat and other cells before described. Mr. Topping prepares cells for receiving minute prepara- tions in the following manner : — He takes a slip of glass and lays on it two thin pieces of mahogany of the size of the glass ; each has a hole of the required figure cut in the centre ; in one piece the hole is the size of the outer margin of the cell, in the other of the inner margin. These, when laid over the glass, afford the means of marking out with a writing diamond the space to be occupied by the cell, which must be filled up with black japan. The glass is now to be transferred to an oven, the heat of which should be raised gradually to prevent the japan from blistering, and if care be taken in this part of the process, a cell so made will resist the action of proof spirit. For the construction of cells for mounting objects in fluid, the following very simple and efficacious method is adopted by Mr. G. Shadbolt, having the threefold advantages of neatness, rapidity of execution, and great economy. It frequently happens that it is desirable to preserve in fluid an object of such extreme thinness, that even the thin glass cell is too thick, such as the cuticles of some vegetable pre- parations, desmidieae, &c. To make cells adapted for such pur- poses, and others somewhat thicker, as a complete substitute for thin glass cells, a little instrument contrived by Mr. Shad- bolt, and shown at fig. 182, will be found highly useful. It consists of a piece of weU -seasoned mahogany or other wood, about an inch in thickness and of about 9 inches by 3^. To this are attached two flat mahogany wheels, a a, both | inch MKTHOD OF MOUNTING OBJECTS IN FLUID. 289 thick, and 3| inches in diameter, which are made to turn together, and in opposite directions, by means of a silken cord / — / . '^-— -,__ a. h / € ^ <^ CZ>e \\\ ^ Fig. 182. crossed. A handle, b, is attached to one wheel, and a little spring, c, formed like the letter Y, to the other ; this spring is capable of being raised by pressing the tail of the Y. An- other piece of mahogany, d, is attached to the base, so as to surround about one-third of the wheel carrying the spring, and should be sufficiently thick to be slightly elevated above the said spring; this is intended as a support to the hand. The screws or centres on which the wheels move, e e, must not project in the slightest degree beyond the wood. A few cir- cular lines, from an inch, as the extreme, to any smaller size, should be drawn on the wheel with the spring. Having raised the Y spring, by pressing the end, e, place on the wheel a slip of glass of the usual size, and adjust it so that none of the cor- ners project; if the wheel be made of the size named (3| inches), it will then be centred in one direction, and may be readily centred perfectly by means of the largest ring drawn on the wheel. Having dipped a camel's hair pencil into some appropriate cement, hold it to the glass, at the same time causing the latter to revolve, by means of the second wheel with the handle, a very neat circular cell will thus be formed. The best kinds of cement for this purpose are either the asphalte cement, described page 275 of this work, or the japanners' gold size, or a mixture of the two in equal propor- tions. If one coating be not thick enough for the purpose intended, another can be laid on as soon as the first is dry, as there Is no difficulty in centering. In making cells with the asphalte cement, a coating of gold- size should always be laid on at last, in order to prevent its becoming too brittle, and the 19 290 MANIPULATION. first coating is always best of asphalte alone, as it adheres more strongly to the glass than the gold-size, which is apt to peel off unless it is baked, as soon as the first coat is laid on. This apparatus is also of great assistance in cementing on the covers, both of the cells made by it and also of the ordinary glass celts, as they are not only cemented on much more neatly than by hand, but what is of more importance, with, less dan- ger of disarranging the contents. Mr.- Shadbolt has also a method of making cells of greater thickness, of marine-glue, mentioned at page 275 ; his process is as follows : — Having procured two pill slabs of about 6 inches square, of marble, glass, or earthenware, as flat as possible, 5 gun punches, the largest being | of an inch in diameter, and the smallest | of an inch, the others intermediate, also a small pipkin ; a little of the glue is gently melted in the pipkin, the two pill slabs are then wetted with cold water, and some of the melted glue is dro23ped on one of them, and the other is immediately pressed upon it, so as to force it into a thin sheet, the thickness depending on the amount of pressure. In a few moments the slabs may be separated ; sometimes they adhere so strongly, notwithstanding the water, that the assistance of a large dinner knife is required to disunite them. When a sufficient number of sheets has been thus formed, lay one on a piece of flat wood or cardboard, and with the largest punch pressed on it, cut it into a number of discs ; with the second sized punch cut out the centre of these, leaving only rings of glvie ; the third sized punch may be used on the smaller discs cut out of the large ones, and so on to the smallest ; thus four sized rings will be formed. It is now necessary to hardeji the rings of glue, and to fix them to the glass slides, (vhich may be both done at one operation. Place the requisite number of glass slips on a tray or piece of wood, and make them tolerably hot in an oven; as soon as they are taken out, carefully drop a ring of glue on each, in the exact place where required, the rings of glue will immediately melt, and by keeping up a gentle warmth may be made as hard as is desired ; they will also be very firmly attached to the glass by this process. The slides must be kept the whole time in a METHOD OF MOtlNTING OBJECTS IN FLUID. 291 horizontal position, to prevent more of the glue getting on one side than the other. When allowed to cool, but before becoming quite hard, a piece of cold glass should be pressed on the top of each ring, and left there till cold ; this is to cause the ring to be quite flat at top, ready for the reception of the cover. On pulling the glasses apart, it will be found that the glue will readily separate from the glass that was cold, and remain firmly fixed to the other. The covers can be cemented on when- the cells are filled, by means of the apparatus pre- viously described. The Thin Glass Cell. — The cell represented by fig. 183 was first employed by Mr. Goadby, and consists, as its name implies, of a piece of thin glass, such as is used for covers, about three-quarters of an inch square, out of which a round hole, varying from one-half to five-eighths of an inch, has been drilled. In order to make this useful, it is to be cemented to one of j,.^ the shdes of plate-glass with marine-glue, in the manner previously described at page 271. After the cell has been properly cleaned, as there directed, it is ready to receive the intended object, which is to be mounted as follows: — A small quantity of gold-size is to be placed upon the margins of the ceU, and as much as possible wiped off with the finger (taking care that the size is wiped away from the hole of the cell and not towards it) ; the fluid in which the object is to be mounted must now be placed in the cell (and it is always a good plan to put in rather more than is used), and after the object has been properly arranged, the cover previously cleaned and anointed on its edges, as in the case of the flat cell, is to be laid on the fluid and pressed down, and the excess removed from its edges by the blotting paper or sucking tube, and the cement laid on in the manner before described. When these cells are not sufiiciently thin, they may be readily made so by rubbing them down on the metal plate described in page 264, with some fine emery and water. It is a good plan always to give them a rub on the metal, not only in order that they may be rendered flat, 19* 292 MANIPULATION. which they rarely are, but because a ground surface is cal- culated to give firmer hold to the cement than one which is polished. The author prefers this form of cell to any other, even for the most delicate tissues, and both cell and cover can be made so thin, that an eighth of an inch object-glass can be used. The flat cell, however carefully prepared, is almost certain to leak after a time, and, from its containing such a small amount of fluid, a very short period will elapse before the object within it is found perfectly dry ; but in the thin glass cell there is more fluid, and any leaking is made evident by the formation of an air bubble; when the bubble gets very large, the cover can easily be removed, and the object re- mounted without its having first been allowed to get dry. When thicker objects, such as injections or other opaque animal structures, require to be mounted, it is necessary to have a much deeper form of cell than any of the preceding ; these may be made of all depths and diameters by having transverse slices cut from glass tubes, and may be denominated the tube cells; one of them is shown in fig. 184, and when cemented to a slide in fig. 185. The tube should be rather Fig. 184, Fig. 185. thick, at least one-eighth of an inch, in order that the cells may hold firmly to the bottom glass. These cells may be made of all diameters between the one-fourth of an inch and an inch-and-a-half — they may be even made larger, if required; the author has had some cut from stout bottles and from the necks of decanters, that are as much as two inches -and-a-half in diameter. Some exceedingly good and useful cells may be made from glass which has been moulded or cast into rectangular tubes ; slices cut transversely from these, of the shapes and sizes METHOD OF MOUNTING OBJECTS IN FLUID. 293 shown by the following figures, will be found the most con- venient, figs. 186, 187, 188, being for the glass slides of one Fig. 186. Fig. 187. Fig. 188. inch in width, whilst fig. 189 is intended for the slides, which Fig. 189. are one inch-and-a-half or more in width. These cells are cheaper than those which are drilled in plate-glass, and are quite as neat in appearance. Fig. 190 represents one of this form cemented to a slide and ready for use. Fig. 190. Drilled Cells. — These are composed of pieces of plate-glass of any convenient size, out of the middle of which either a circular or oval hole has been drilled, the depth of the cell depending in all cases on the thickness of glass used ; when required to be very thick, two or more cells of equal size may 294 MANIPULATION. be cemented together, either with the marine-glue or Canada balsam; figs. 191, 192, 19.3, represent three convenient shapes, and figs. 194, 195, 196, one of each cemented to a slide or bottom plate of glass ; these cells have many advan- tages over others, as any number may be made of one thickness, they may also be made perfectly square outside, and yet the cavity or cell within may be either oval or circular, which is _ often desirable. I ' """"^^ fjjg method of cementing them to I ' m the bottom glass is the same as that for IB»iiiii iiiiiiw II III Fig. 192. Fiff. 191. Fig. 193. Fig. 196. other forms of cells. Being made of plate-glass they are very flat, and from not being ground their surfaces will allow light to pass through when Canada balsam or very thin marine - glue is the cement, hence they may be employed with the Lieberkuhn. This and the succeeding form of cell were first suggested and used by Mr. Goadby. Built-up Cells. — These consist of four pieces of glass of con- venient size, which are cemented to form an oblong or square cell. The simplest form, and one which will answer the , A -.1- ■II- e b ■111- e ■I^^H METHOD or MOtJN-TlNG OBJECTS IN FLUID. 295 purpose of either the thin glass cell or the tubular or drilled cell, when these are not a,t hand, may be thus made : Take a piece of glass of the required shape and thickness, say one inch long and three-fourths -wide, and mark out on it, with a writing-diamond or ink, the size of the cell you wish to make, as e in fig. 197, continue the lines to the edge of the glass as shown by dots at a fi e d. Now, with a cutting-diamond, make four cuts in the direction of the lines ah, ac; b d, c d; reject the middle piece, e; and cement the four outside pieces to the slide in the same manner as one of the other forms of cell, taking care always to put the pieces in the order in which they were before they were cut off; this is known by making little marks or lines in each corner, so that, when the pieces are separated, one half of the mark may be on one side and the other half on the opposite, as seen in the above figure ; this will serve Fig. 197. as a guide to fit the pieces pro- perly together, and when a little marine-glue is placed between the joints, the four pieces will be held as firmly together as if they were a solid mass. Should, however, the pieces not be brought down to a uniform level, the cell may be rubbed on the metal-plate with emery, and be thus reduced to any convenient thinness. Cells made in this way of the thin glass answer exceedingly well, and, when properly cemented, will form an excellent substitute for any of the other kinds ; they may be made of aU thicknesses of glass, from that used for covers up to the thickest plate that the operator can cut slips from with the diamond. Thicker cells, as represented by fig. 198, may be made of four narrow strips of stout plate-glass, cemented together as in the preceding specimen, upon a bottom piece of thinner plate; but care must be taken that the ends of the sides, c d, and the edges of « 5 be ground flat, and that the joints be firmly cemented. Strips of plate-glass, from one-eighth to half-an- inch, may be obtained at the looking-glass makers, or may even be cut by the diamond, which will answer very well for 296 MANIPULATION. this purpose, as all inequalities of surface may be ground down on the metal-plate. ■" ^.^aaaa— ■ Oy — -■». Ze^^ ^y/i^^y ^\y(yr€/(^ ACHROMATIC COMPOUND MICROSCOPES FOR STUDENTS. JM.CSoiverb]/. st Ioyi,S.on.-M BaOiiire, IlLbtisher. ZlSJiegent St. &2S0,Bna^v/ay, rmJirkfu.SjjSSl. ."'.■.' -4 ^ '.^ ■,/-^-' ' U^-y^ /v -V" .■■ n>-/-<.(?>. ':.I'^ur7i-Lr .1P,7Lc^V^-n, '/^'■'',B?va. 9 Ti^I Ti^ 3 wr.£Of%ard €Ul. ^^_^ .^_ ^ ^^r^.^/!' 3/yyp £^ Ie??zdo?v.-S-£cu,Ui,er&, FicTilisher. ZJ^.Jte^entSt. 3::2^0.3}va^wa^.27'twTor7c fj7SjlS61. X p^ ===-fi=^=?-, ^^^^^^^^^%?4^m^ o ■n*'- 'tYw'T*^ '-^^^Sl^^^^ ^^^^^3^^^—^ CO CJ 'tf'?r??>- ^^'h ■^**-?¥^: gctrtsi'^i^i^v HW-^?. ^V^-P^f^'^' «5S III Z ' 'J*-^ -,-«-, --., -,,.,-,a.-. .-,• .-^-'-■a.,w^t-M-.« -.-1. ;>>^j„r-tH^-^ J~ """^^ > p=; -y I " WZeon^r^, d.e-1 r.-^J'J^a.y.zy. jc:.J.v ZondOTt-MBazHUr&.I'ublishcr, Z79,It&ffsn;tSt ScZP6l^7va^way.2fe,wYor7c/l''sj2SSl Quekett on the Microscope. PI. 12 Messrs. SMITH & BECK'S IMPROVED SMALLER ACHROMATIC MICROSCOPE. LIST OF WORKS ON THE MICROSCOPE & THE COLLATERAL SCIENCES, TO BE HAD OF H. BAILLIERE, 219, REGENT STREET, LONDON; * AND 290, BROADWAY, NEW YOEK. ADAMS. Essays on the Microscope ; containing a Practical Description of the most improved Microscopes, and a General History of Insects. Second edition, by Kanmacher, with thirty-two plates. Quarto. London, 1798. 18s. Micrographia lUustrata ; or, the Microscope explained. Fourth edition, octavo, seventy-two plates. London, 1771. 6s. i BAILEY. Microscopical Examination of Soundings on the Atlantic Coast of the United States. Quarto, with one plate. Washington, 1851. 2s. 6d. Microscopical Observations made in South Carolina, Georgia, and Florida. Quarto, with three plates. Washington, 1851. 6s. BAKER. The Microscope made easy, and Employment for the Micro- scope, with Observations and Remarks. Two volumes, octavo, calf neat, illustrated with copper-plates. 8s. ^d. ■ Essai sur I'Histoire Naturelle du Polype Insecte, traduit par Demours. In 12mo, avec vingt-deux planches. Paris, 1744. 4s. The Microscope made easy. Second edition, octavo, fifteen plates. London, 1743. 5s. 6d. BAYARD. Examen Microscopique du Sperme Desseche sur le Linge ou sur les Tissus de Nature et de Coloration Divers. Octavo. Paris, 1839. 2s. 6d. BISCHOFF. Entwieklungsgeschichte des Kaninchen-Eies. With 16 plates. Quarto. Braunschweig, 1842. £1 Is. Traite du Developpement de I'Homme et des Mammiferes. Octavo et atlas quarto, de seize planches. Paris, 1843. 15s. BOEHM. DeGlandularumlntestinaliumStructuraPenitiori. Quarto, with two plates. Berlin, 1835. 2s. BREWSTER (Sir D.) on the Microscope; from the Encyclopjedia Britannioa. Post octavo, plates and wood-cuts, cloth. 1837. 6s. CHEVALIER (Ch.). Des Microscopes et de leur Usage. Paris, 1839. Octavo, six planches. 9s. Trois-cent Animalcules Infusoires dessines a I'Aide du Micros- cope, d'apres Pritchard. Paris, 1839. Octavo, avec 6 planches coloriees. 3s. DONNE (A.). Cours de Microscopic complementaire des Etudes Medicales : Anatoraie Microscopique ct Physiologic des Fluides de I'Economie. Paris, 1844. Octavo, de 550 pages. 7s. 6d. I FOREIGN WORKS ON THE MICROSCOPE DONNE (A.). Atlas du Conrs de Microseopie, execute d'apres Nature an Microscope-Daguerreotype, par A. Donne et L. Foucault. Paris, 1845. Atlas folio de vingt planches, contenant quatre-ringt figures gravees avec le plus grand soin, avec un texte descriptif. £\ 10*. DUCHARTRE. Observations Anatomiques et Organogeniques sur la Clandestine d'Europe. In quarto, avec sept planches. Paris, 1847. 6«. 6 13 degrees £ 3 s. d. 0' 15j. J inch Tube closed ... Add for each inch of tube. . . 7 105 12 180 20 Y 27 degrees 3 3 11 J. iV inch Tube closed Add for each inch of tube 120 12 210 20 360 35 - 65 degrees 6 5 IOj. Ditto ditto do. do. do. 65 degrees 6 6 10». J inch Tube closed Add for each inch of tube 205 25 360 35 620 60 [• 70 degrees 5 5 6s. i inch Tube closed Add for each inch of tube. . 240 30 430 46 720 80 >• 85 degrees 6 6 Ditto ditto do. do. do. 1 00 degrees 7 7 i inch Tube closed . ... Add for each inch of tube. . . . 450 40 760 60 1300 116 [ 90 degrees 7 7 Ditto ditto do. do. do. 110 degrees 8 8 iV inch Tube closed . ... Add for each inch of tube. . . . .500 50 920 70 1500 130 V 120 degrees 10 10 * With the § inch object glass and the erecting glasses, the magni^ing power will range employing eye-pieces Nob. 1 and 2, from 5 to 150. APPARATUS Fob the Microscopes Nos. 1 and 2 ;0f lower price, on account of their more simple adaptation to those instruments). £ s. d. Brasswork of Achromatic Condenser, with adjustments . . .10 Combination of Lenses for ditto . . - from £1. 10s. to 4 Wenham's Parabolic Reflector, and fittings 112 6 Amici's Prism for oblique light 1 10 Polarizing Apparatus from ^62. 10s. to 3 15 Bundle of Thin Glass for polarizing by reflection or transmission . 110 ADDITIONAL APPARATUS FOR THE MICROSCOPES IN GENERAL. Extra Eye piece from 13s. to Indicator to Eye -piece Erecting Glasses, for varying power, and for dissection . Brasswork for Achromatic Condenser . . from 10s. 6c?. to Combination of Lenses for ditto from Right-angled Prism Nachet's Prism Side Condensing Lenses from 12s. to Side Silver Reflector ... Polarizing Apparatus, with selenite . , Double Image Prism, with fitting to eye-piece .... Two ditto, ditto, with selenite, and brass plate with holes Crystals to show rings round the optic axis, fitted to eye-piece, each Tourmalines . . . from Wollaston's Camera Lucida, and fittings .... Steel Disc, ditto . . . . Micrometer for Stage, mounted in brass Ditto for Eye-piece, with fittings and adjusting serew Three Dark Wells, and holder Compressorium from 7s. 6d. to Screw Live Box . . . . from 6s. 6d. to Glass Trough from 6s. to Best Argand Lamp, with blue chimney Small Camphine ditto, on improved principle Iron Table, with revolving top for the Microscope .... 17 5 10 10 10 18 1 1 1 1 2 10 15 2 2 10 10 1 15 10 MICROSCOPIC OBJECTS. , Recent and Fossil - — Desmidiese and Algse ; Diatomaceae, recent and fossil ; Spicules and Gemmules of Sponges and Gorgonias; Zoophytes,Bection3 of Shells and Echinus Spines; Entomological Preparations Hairs and Feathers; Anatonlical Preparations, Sections of Bones and Teeth ; Injected Preparations; Mineralogical and Polarisoopic Objects. CABINETS FOR OBJECTS. In which the Specimens lie flat, and with porcelain Labels to the Drawers. To hold 1000 objects from £6 6s. to 7 Ditto 750 ditto from £5 to 5 15 Ditto 600 ditto .... from M 10s. to - 4 i CABINETS MADE TO ANT SIZE AND OF EVERY DESCRIPTION. INSTRUMENTS USED IN PREPARING OBJECTS. Materials used in Mounting Objects. Woodward's Table and Hydro-oxygen Polariseope and Microscope. Just Published, Second Edition, revised by the Author, A PAMILIAE INTEODUCTION TO THE STUDY OF POLAEIZED LIGHT, By Charles Woodward, Esq., F.K.S., &c. DISSOLVING VIEWS. AMUSEMENT and INSTRUCTION by means of CARPENTEE & WESTLEY'S improved PHANTASMAGORIA LANTERNS with the CHROMATROPE and DISSOLVING VIEWS, and every possible variety of Sliders, including Natural History, Comic, Lever, Moveable and Plain, Astronomical, Views in the Holy Land, Scriptural, Portraits, &c. No. 1 Lantern with Argand Lamp, in a Box, 2Z. 12«. &d. No. 2 ditto, of a larger size, il. lis. 6d. A pair of Dissolving- View Lanterns, No. 2, with Apparatus, 11/. lis. The Lamp for the No. 2 Lanterns is very superior. (The price of the Lanterns is without sliders.) Lists of the Sliders and Prices upon application to the Manufacturers, Messrs. CARPENTER & WESTLEY, Opticians, 2i, Regent-street, Waterloo- place, London. RETT'S INJECTED MICROSCOPIC PREPARATIONS For which the Prize Medai was awarded at the " Great Exhibition, 1851." Mr. HETT, M.R.C.S., bega to inform medical gentlemen and others engaged in microscopical researches, that he has for sale a variety of specimens of the most beautiful and perfect injections of various parts of the human body, and other classes of animals. Mr. HETT also prepares every variety of microscopic object required to illustrate the more important and interesting facts of minute Anatomy and Physiolog'y, and such Pathological specimens as admit of being mounted as permanent objects. Parties residing at a distance from London may have a box of pre- parations sent for inspection, on giving a respectable town reference and paying carriage both ways ; by this means they are enabled to examine the objects before purchasing, and to select such specimens only as they may require ; three clear days being allowed for this purpose. 24; BRIDGE STREET, SOUTHWARK. ftanilarft Irifntifir Wuh, PUBLISHED BY HIPPOLYTE BAILLIERE, 169, PULTON STREET, NEW VORK, U.S., AND 219, RJEGESIT STREET, IjOlSItON. J. BAILLIERE, LIBRAIRE, RUE HAUTEFBUILLE, PARIS. BAILLY BAILLIERE, LIBRAIRE, CALLE DEL PRINCIPE, MADRID. e'er^nlld Tn^S^^^i''' '^'"P^? « mtabmhmmt at the above aMress.for the sale of Frenoh, Na^Z mst^u^lf^ ^«J, ,'\"r7T ^«.P"*"™«» "/ Science (Anatomi, Medicine. Chemistry, IJ™ J tiistory, e,c.), a well selected stock of wdch he now offers to the Scientific Puldir Hnmni with the JeatSt «™w//,T ""f f/'^if * 1° "',™*^ Commissions for all Books he mas, not have in Stock ^ththe^e^fst'^e^li!^ H n 1 ^f^A <^'K'"<'« "■'^ E'^SH^^ ■'ournals and Periodicals applied with M- C*»rwL wi ^/^i. ^-^ ■• <"* "'^ **«,?''«'«w™ to announce that he has made arrangements wun M. Uianter, the celebrated instrument maker of Paris, for the suanly of all kinds of Surreal Instruments, an extensive assortment of which is now open forimpectioT^'^^ * -^ Surgical a few 'excfplL'L'^bfen^ei'LblfshT^ "° extendedin Foreign Currencies, the following rates have (with THE FRENCH FHANC, 25 CENTS. THE ENGLISH SHILLING, 30 CENTS. THE GERMAN DOLLAR, 100 CENTS. ON THE PUBLISHED PRICE. he wilffi^ ^h!fl^!.?ff"l"'; '" ^'^f °^'! ".' """'^ '^°°'^^ Published by him, and which have been reprinted, he will fix the price at such a rate ,as to insure the preference to the original editions. J^ttrsl ©conoittg, ^c. Blakey (B.) History of Logical Science froDi the Earliest Times to the Present Day, by Robert Blakey, Professor of Logic and Metaphysics, Queen's College, _ Belfast, Author of the History of the Philosophy of Mind, in 1 vol. demy 8vo. Boussingault. Kural Economy; in its Relations with Chemistry, Physics, and Meteorology. By J. B. Boussingault,' Member of the Institute of France. 2nd Edition, with Notes, carefully revised and corrected, 1 vol. 8vo. cloth boards, London, 1845 . . . . 18 Brewster (Sir David). The Natural History of Creation, in 1 vol. royal 8vo. Illustrated with Engravings and Woodcuts. In the press. Campbell. A Practical Text-Book of Inorganic Chemistry, including the Preparations of Substances, and then- Qualitative and Quantitative Analyses, with Organic Analyses. By Dugald Campbell, Demonstrator of Practical Chemistry to the University College. 12mo. London, 1849 . .056 Chapman. A Brief Description of the Characters of Minerals ; forming a famihar Introduction to the Science of Mineralogy. By Edward J. Chapman. 1 vol. 12mo. with 3 plates, London, 1844 . . . .040 - — ^ Practical Mineralogy ; or, a Compendium of the distinguishing Cha- racters of Minerals : by which the Name of any Species or Variety in the Mineral Kingdom may be speedily asceirtained. By Edward J. Chapman. 8vo. illustrated vrith 13 engravings, showing 270 specimens, London, 1843 .070 Chemical Society (Quarterly Journal of the). 2 vols. 8vo. London, 1848—49 . . . . . .16 ; ;— Vol. III., Parts I, 2, and 3, Published Quarterly. Each .0 30 Cook. Historical Notes on the Discovery and Progressive Improvements of the Steam Engine ; vrith References and Descriptions to accompany the Plates of the American condensing Steam Engine for River Boats. 18mo. and a large fol. plate on a roller and canvas. New York, 1849 . .0180 Dumas and Boussingault. The Chemical and Physiological Balance of Organic Natine : an Essay. By J. Dumas and J. B. Boussingault, Members of the Institute of France. 1 vol. 12mo. London, 1844 . . .040 Fau. The Anatomy of the External Forms of Man (for Artists). Edited by R. Knox, M.D., with Additions. 8vo. and 28 4to. plates. 1849. Plain .140 Coloured . . . . .220 The Text separately vrith the four additional Plates, for Persons possessing the French edition. Plain . . . .0120 Coloured . . . . . 14 Hippolyto Bailliere's Publications. STANDARD SCIENTIFIC WORKS. £ s d Gordon (L.) Treatise on the Steam Engine, and on the Motive Power of Heat, with every improvement to the present day. By L. Gordon, Professor of Engineering in the University of Glasgow. 8vo. Illustrated with Wood Engravings and Steel Plates. In the press. ■ A Synopsis of Lectures on Civil Engineering and Mechanics. 4to. London, 1849 . . . . . " . .076 Gordon and Liddell. Exposition of a Plan for the Metropolitan Water Supply, showing that the Thames at Maple-Durham is the most eligible source from which a supply of pure soft water can be brought for the inha- bitants of London and its suburbs. 8vo. London, 1849 . .010 Graham. Elements of Chemistry ; including the Application of the Science in the Arts. By T. Graham, F.R.S. L. & E., Professor of Chemistry at Uni- versity College, London. 2nd Edition, entirely revised and greatly enlarged, copiously illustrated with Woodcuts, vol. 1. 1850 . . .110 ^ Part IV. Humboldt. Kosmos : a General Survey of the Physical Phenomena of the Universe. By Baron A. Humboldt. The original English Edition, 2 vols, post 8vo. London, 1848 . . . . . .12 Vol. II. separately, 1848 . . . . . 12 Esemtz. A Complete Course of Meteorology. By L. P. Ksemtz, Professor of Physics at the University of Halle. With Notes by Ch. Martins, and an Appendix by L. Lalanne. Translated, with Additions, by C. V. Walker. 1 vol. post 8vo. pp. 624, with 15 Plates, cloth boards, 1845 . . 12 6 Enapp. Chemical Technology, or Chemistry applied to the Arts and to Manufactures. By F. Knapp, Professor at the University of Giessen. Edited, with numerous Additions, by Dr. E. Konalds, Professor of Che- mistry at the Koyal College, Galway ; and Dr. Thomas Richardson, of Newcastle-on-Tyne. Illustrated with 600 large Woodcuts, 3 vols. 8vo. London, 1848—1850 . . . . . .330 Vol. III. separately . . . . .110 Enipe. Geological Map of the British Isles ; in a case. London, 1848 .440 Xieon (John A.) The Art of Manufacturing and Refining Sugar, including the Manufacture and Revivification of Animal Charcoal. With an Atlas of 14 Plates, illustrative of the Machinery and Building. Large folio. London, 1850 3 3 Iiiebig. Chemistry and Physics, in relation to Physiology and Pathology. By Baron Justus Liebig, Professor of Chemistry at the University of Giessen. 2nd Edition, 8vo. London, 1847 . . . . .030 Mansfield. Benzole ; its Nature and Utility. 8vo. London, 1849 . .0 16 Mitchell (J.) Manual of Practical Assaying, intended for the use of Metallurgists, Captains of Mines and Assayers in General. With a copious Table, for the purpose of ascertaining in Assays of Gold and Silver the precise , amount, in Ounces, Pennyweights, and Grains, of noble Metal contained in one ton of Ore from a given quantity. 1 vol. post 8vo. London, 1846 . 10 6 Treatise on the Adulterations of Food, and the Chemical Means em- ployed to detect them. Containing Water, Flour, Bread, Milk, Cream, Beer, Cider, Wines, Spirituous Liquors, Coffee, Tea, Chocolate, Sugar, Honey, Lozenges, Cheese, Vinegar, Pickles, Anchovy Sauce and Paste, Catsup, Olive (Salad) Oil, Pepper, Mustard. 12mOi London, 1848 . . .060 MnUer. Principles of Physics and Meteorology. By J. MuUer, M.D. Illustrated with 530 Wood-cuts, and 2 coloured Plates, 8vo. Loudon, 1847 . . 18 Nichol. Astronomy Historically and Scientifically developed, showing the Rise of the Science from its Growth, and the Character of the illustrious Men who have contributed to it. By J. P. Nichol, Professor of Astronomy in the University of Glasgow. 2 vols. 8vo. Illustrated by Plates and Woodcuts. In the • » Our Planetary System, its Order and Physical Structure. 12mo. With Wood-cuts. London, 1850 . . . . .066 Views of the Grander Revolutions of our Globe. 12mo. With Woodcuts. London, 1850 . . . . . . .066 The Arrangement of the Fixed Stars. 12mo. With Plates. In the press. Htppolyte BatlUere'a PiibllcatlonB. aiAnifAKD SCIENTIFIC WORKS. Quekett (J.) Practical Treatise on the Use of the Microscope. Illustrated with Steel and Wood Engravings, 8vo. London, 1848 . . .110 ' A few copies, with the plates coloured . . .220 Practical Treatise on Minute Injection, and the Application of the Microscope to Diseased Structure. 8vo. Illustrated with Engraved Plates and Woodcuts. In the press. Begnault. An Elementary Treatise on Ciystallography, Illustrated with 108 _ Wood Engravings, printed on black ground. 8vo. London, 1848 . .0 30 Reiohenbach (Baron Charles). Physico-Physiological Researches on the Dynamics of Magnetism, Electricity, Heat, Light, Crystallization, and Chemisra, in their Relations to Vital Force. The complete Work from the German Second Edition, with Additions, a Preface and Critical Notes, by John AsHBURNER, M.D. 8vo. With Woodcuts and One Plate. London, 1850 .0150 Hldiardson. Geology for Beginners ; comprising a Familiar Exposition of the Elements of Geology and its Associate Sciences, Mineralogy, Fossil Con- chology. Fossil Botany, and Paleontology. By G. F. Richardson, F.G.S. 2nd Edition, post 8vo. with 251 Woodcuts, 1843 . . . . 10 6 Bichardson and Ronalds. Metallurgy ; and the Chemistry of the Metals. In 3 vols. 8vo. Illustrated with numerous Wood Engravings. In the press. Stars and the Earth. The Stars and the Earth ; or. Thoughts upon Space, Time, and Eternity. 4th Edition, Eighth thousand, 2 Parts in 1, 18mo. London, 1850 . . . . . . .020 Stephens. Manual of Practical Draining. 3rd Edition, with Woodcuts, 8vo. Edinburgh, 1848 . . . . . .050 Thomson. Chemistry of Organic Bodies— Vegetables. By Thomas Thomson, M.D. F.R.S. L. & E., Regius Professor of Chemistry in the University of Glasgow, Corresponding Member of the Royal Academy of Paris. 1 large vol. 8vo. pp. 1092, boards, London, 1838 . . . .14 Heat and Electricity. 2nd Edition, 1 vol. 8vo. Illustrated with Wood- cuts, London, 1839 . . . . . . 15 Chemistry of Animal Bodies. 8vo. Edinburgh, 1843 . . 15 Thomson (R. D.) British Annual and Epitome of the Progress of Science. By K. D. Thomson, Assistant Professor in the University of Glasgow. 3 vols. 18mo. cloth boards, lettered, each . . . . .036 First Year, 1837. Contains numerous Practical Tables of Weights, Measures, and Coins. The Popular Papers are by the Kev. B. Powell ; C. Tomlinson, Esq. ; W. S. B. Woolhouse, Esq. ; T. S. Davies, Esq. ; K, D. Thomson, M.D. Secmd Year, 1838. The Popular Papers are by T. Thomson, M.D., Eegius Professor of Chemistry in the University of Glasgow ; R. E. Grant, M.D., Professor of Comparative Anatomy in the University College, London ; R. D. Thomson, M.D. ; Life of James Watt, illustrated with a Portrait ; H. H. Lewis, Esq. Third Year, 1839. The Popular Papers are by J. S. Russell, Esq. ; Professor E. E. Grant; H. Garuier, Esq. ; R. D. Thomson, M.D. Wiesbach (J.) Principles of the Mechanics of Machinery and Engineering. 2 vols. 8vo. Illustrated with 200 Wood Engravings, London, 1848 . . 1 19 Vol II. separately . . . . . 18 ^natomg, ftltlrtctnf, Surstrfi, anir ilatttfal iD^tsitora, Ashburner. OnDentitionandsomeCoincidentDisorders.l8mo. London, 1834 40 Canton (A.) A Short Treatise on the Teeth, 12mo. with Wood-cuts. In the press. Coiirtenay. Pathology and Rational Treatment of Stricture and Urethra in all its Varieties and Complications, vrith Observations on the Use and Abuse of Urethral Instruments. The whole illustrated by numerous Cases. By F. B. CouRTENAY, M.R.C.S., &c. 4th Edition, 8vo. London, 1848 .050 l«», Fnlton Street, Hfew Tork, ana aio. Regent Street, l,oudou. STANDARD SCIJBNTIFIG WORKS. Cotirtenay. A Few Words on Perineal Section, as recommended by Pro- fessor Syme, for the Cure of Stricture of the Urethra. 8vo. London, 1850 .010 Practical Observations on the Chronic Enlargement of the Prostate Gland in Old People: with Mode of Treatment. By Francis B. Courtenay, 8vo. with numerous Cases and Plates, boards, London, 1839 . .076 Cruveilhier and Bonamy. Atlas of the Descriptive Anatomy of the Human Body. By J. Cruveilhier, Professor of Anatomy to the Faculty of Medicine, Paris. With Explanations by C. Bonamy. Containing 82 Plates of Osteology, Syndemology, and Myology. 4to. London, 1844. Plain .300 Coloured . . . . . . 5 15 Dungllsoil. A Dictionary of Medical Science, containing a Concise Account of the various Subjects and Terms, with the French and other Synonymes, Notices of Climate and of celebrated Mineral Waters, and Formulae for various officinal and empirical preparations. By Kobley Dunglison, M.D. Sixth Edition, greatly enlarged, 8vo. Philadelphia, 1846 . . . 1 10 Eberle. A Treatise on the Diseases and Physical Education of Children, by John Eberle, M.D. Third Edition, 8vo. Philadelphia, 1848 . . 15 Fau. The Anatomy of the External Forms of Man (for Artists). Edited by R. Knox, M.D., with Additions. 8vo. Text, and 28 4to. Plates. Loudon, 1849. Plain . . . . . ..140 Coloured . . . . . .220 Gerber and Gulliver. Elements of the General and Minute Anatomy of Man and the Mammaha ; chiefly after Original Kesearches. By Professor Gerber. To which is added an Appendix, comprising Researches on the Anatomy of the Blood, Chyle, Lymph, Thymous Fluid, Tubercle, and Addi- tions, by C. Gulliver, F.K.S. In 1 vol. 8vo. Text, and an Atlas of 34 Plates, engraved by L. Aldous. 2 vols. 8vo. Cloth boards, 1842 . .14 Grant. General View of the Distribution of Extinct Animals. By Robert E. Grant, M.D., F.E.S. L. & E., Professor of Comparative Anatomy at the University College, London. In the " British Annual," 1839. 18mo. London, 1839 .036 On the Principles of Classification, as applied to the Primary Divisions of the Animal Kingdom. In the " British Annual," 1838. 18mo. Illustrated with 28 Woodcuts, London, 1838 . . . . .036 Outlines of Comparative Anatomy. 8vo. Illustrated with 148 Woodcuts, boards, London, 1833—41 . . . . .18 Part VII. with Title-page . . . .016 Gross. Elements of Pathological Anatomy. Illustrated by coloured Engravings and 250 Woodcuts, by Samuel D. Gross, M.D., Professor of Surgery in the Medical Institute of Louisville. Second edition, 8vo. Philadelphia, 1845 . 112 Hall (Marshall). On the Diseases and Derangements of the Nervous System, in their Primary Forms, and in their modifications by Age, Sex, Constitution, Hereditary Predisposition, Excesses, General Disorder and Organic Disease. By Marshall Hall, M.D., F.R.S. L. & E. 8vo. with 8 engraved Plates. Lon- don, 1841 . . . . . . . 15 The following as an Appendix to the above WorJc. On the Mutual Relations between Anatomy, Physiology, Pathology, Therapeutics, and the Practice of Medicine ; being the Gulstonian Lectures for 1842. 8vo. with 2 Coloured Plates and 1 Plain. London, 1842 .050 New Memoir on the Nervous System, true Spinal Marrow, and its Anatomy, Physiology, Pathology, and Therapeutics. 4to. with 5 engraved Plates, London, 1843 . . . . . .10 Hassal. The Microscopic Anatomy of the Human Body in Health and Disease. Illustrated with upwards of 400 Original Drawings, many of them coloured, 2 vols. 8vo. London, 1850 . . . .250 Henriques. Etiological, Pathological and Therapeutical Reflections on the Asiatic Cholera, as observed in Europe, Asia Minor, and Egypt. 8vo. London, 1848 . . . . . . .016 Hufeland. Manual of the Practice of Medicine ; the result of Fifty Years' Experience. By W. C. Hufeland, Physician to the late King of Prussia, Professor in the University of Berlin. Translated from the Sixth German Edition, by C. Bruchhausen and R. Nelson. 8vo. bound. London, 1844 . 15 Ulppolyle llaiUlcrc'8 Piiltlicatlous. HTANUAKD SCIENTIFIC WORKS. £ t d Jones (W.) An Essay on some of the most important Diseases of Women, with a Description of a Novel Invention for their Treatment and Relief. Second Edition. 8vo. London, 1850 . . . . 0, 1 Practical Observations on the Diseases ofWomen, showing the nefcessity of Physical Examination, and the Use and AppUcation of the Speculum. Illustrated by Cases, Woodcuts, and Coloured Plates. 8vo. London, 1850 .070 Laennec. A Treatise on the Mediate Auscultation, and on Diseases of the Lungs and Heart. By R. T. H. Laennec, Professor to the Faculty of Medicine of Paris. With Notes and Additions by M. Laennec, and M. Andral. Trans- lated and Edited by Theophilus Herbert, M.D., from the last edition ; with Practical Notes, by F. H. Ramadge, M.D., Oxon. 8vo. with Plates. London, 1846 . . . . . . . 18 Xiebaudy. The Anatomy of the Regions interested in the Surgical Operations performed upon the Human Body ; vrith Occasional Views of the Patholo- gical Condition, which render the interference of the Surgeon necessary. In a Series of 24 plates, the Size of Life. By J. Lebaudy. Folio. London, 1845 14 Xiee. The Anatomy of the Nerves of the Uterus. By Robert Lee, M.D., F.R.S. Folio, with 2 engraved Plates. London, 1841 . . .080 Mapdclock. Practical Observations on the' EfiScacy of Medicated Inhalations in the Treatment of Pulmonary Consumption. By Dr. Maddock, 3rd Edition, 8vo. with a coloured Plate. London, 1846 . . . .056 Martin. A General Introduction to the Natural History of Mammiferous Ani- mals : with a particular View of the Physical History of Man, and the more closely alhed Genera of the Order " Qnadrumana," or Monkeys. Illustrated with 296 Anatomical, Osteological, and other Engravings on wood, and 12 fuU-plate Representations of Animals, drawn by W. Harvey, 1 vol. 8vo. London, 1841 . . . . . . . 16 Miner. The American Bee Keeper's Manual ; being a Practical Treatise on the History and Domestic Economy of the Honey Bee, embracing a full illustra- tion of the whole Subject, vrith the most approved Methods of Managing this Insect through every Branch of its Culture, the Result of many Years' Experience. By T. B. Miner. 12mo. London, 1849 . .. .0 86 Moreau (Professor). Icones Obstetricae ; a Series of 60 Plates and Text, Illustrative of the Art and Science of Midvrifery in all its Branches. By Moreau, Professor of Midwifery to the Faculty of Medecine, Paris. Edited, with Practical Remarks, by J. S. Stretter, M.R.C.S. Folio. Cloth boards. London, 1841. Price Plain . i . . . .330 Coloured . . . . . .660 Owen. Odontography ; or, a Treatise on the Comparative Anatomy of the Teeth, their Physiological Relations, Mode of Development, and Microscopical Structure in the Vertebrate Animals. By Richard Owen,F.R.S., Correspond- ing Member of the Royal Academy of Sciences, Paris and Berlin ; Hun- terian Professor to the Royal College of Surgeons, London. This splendid Work is now completed. 2 vols, royal 8vo. containing 168 plates, half- bound russia. London, 1840 — 45 . . . .660 — A few copies of the Plates have been printed on India paper, 2 vols. 4to. 10 10 Phillips. Scrofula: its Nature, its Prevalence, its Causes, and the Principles of Treatment. By Benjamin Phillips, F.R.S., Surgeon and Lecturer on Surgery to the Westminster Hospital. 8vo. vrith an engraved Plate. London, 1846 . 12 — A Treatise on the Urethra ; its Diseases, especially Stricture, and their Cure. 8vo. boards. London, 1832. . . . .080 Priehard. The Natural History of Man ; comprising Inquiries into the Modi- fying Influence of Physical and Moral Agencies on the different Tribes of the Human Family. By James Cowles Priehard, M.D., F.R.S., M.R.I.A. Corre- sponding Member of the National Institute, of the Royal Academy of Medi- cine, and of the Statistical Society of France ; Member of the American Philosophical Society, &c. &c. 3id Edition, enlarged, vrith 50 coloured and 5 plain Illustrations, engraved on steel, and 97 Engravings on wood, royal 8vo. elegantly bound in cloth. London, 1848 . . 116 ^ Appendix to the First and Second Editions of the Natural History of Man, large 8vo. with 6 coloured Plates. London, 1845 & 1848. Each .036 »09, Fulton Street, Wew York, and 919, Begent Street, Iiondon. STANDARD SCIENTIFIC WORKS. £ td Prichard. Six Ethnographical Maps, as a Supplement to the Natural History of Man, and to the Researches into the Physical History of Mankind, folio, coloured, and 1 sheet of letter-press, in cloth boards. 2nd Edition. London, 1850 .14 Illustrations to the Kesearches into the Physical History of Mankind. Atlas of 44 coloured and 5 plain Plates, engraved on steel, large 8vo. half- bound. London, 1841 . . . . . . 18 On the Different Forms of Insanity, in relation to Jurisprudence. (De- dicated to the Lord Chancellor of England.) 12mo. London, 1842. .0 5 Rayer. A Theoretical and Practical Treatise on the Diseases of the Skin. By P. Rayer, M.D., Physician to the Hopital de la Charity. Translated by R. Willis, M.D. 2nd Edition, remodelled and much enlarged, in 1 thick vol. 8vo. of 1300 pages, with Atlas, royal 4to. of 26 Plates, finely engraved, and coloured vfith the greatest care, exhibiting 400 varieties of Cutaneous Affections. London, 1835 . . . . .480 The Text separately, 8vo. in boards . . .18 . The Atlas 4to. separately, in boards . . . 3 10 Kyan. The Philosophy of Marriage, in its Social, Moral, and Physical Rela- tions ; vrith an Account of the Diseases of the Genito-Urinary Organs, with the Physiology of Generation in the Vegetable and Animal Kingdoms. By M. Ryan, M.D. 4th Edition, greatly improved, 1 vol. 12mo. London, 1843. 6 Shuckard. Essay on the Indigenous Fossorial Hymenoptera; comprising a Description of the British Species of Burrowing Sand Wasps contained in all the Metropolitan Collections ; with their habits, as far as they have been observed. 8vo. vrith 4 Plates. London, 1837 . . . 10 Plate I. is wanting. Elements of British Entomology. Part 1. 1839 . . .080 Syrne. Principles of Surgery. By J. Syme, Professor of Clinical Surgery in the University of Edinburgh. 3rd Edition, much enlarged, and illustrated with 14 Plates on India paper, and 64 Woodcuts, 1 vol. 8vo. London, 1842 .110 Vogel and Day, The Pathological Anatomy of the Human Body. By Julius Vogel, M.D. Translated from the German, with additions, by George E- Day, M.D., Professor to the University of St. Andrews. Illustrated with upwards of 100 plain and coloured Engravings, 8vo. cloth. London, 1847 . 18 Wardrop. On Blood-letting; an Account of the Curative Effects of the Abstraction of the Blood ; with Rules for employing both local and general Blood-letting in the treatment of Diseases. 12mo. London, 1835 . .0 40 Wardroper. A Practical Treatise on the Structure and Diseases of the Teeth and Gums, and on a New Principle of their Treatment. 3rd Edi- tion. 8vo. London, 1844 . . . . . .036 Waterhouse. A Natural History of the Mammalia. By G. R. Waterhouse, Esq., of the British Museum. Vol. I, containing the Order Marsupiata, or Pouched Animals, with 22 Illustrations, engraved on steel, and 18 Engrav- ings on wood, royal 8vo. elegantly bound in cloth, coloured Plates . 1 14 6 Plain . . . . . .19 Vol. II. containing the Order Rodentia; or, Gnawing Mammalia: vrith 22 Illustrations, engraved on steel, and Engravings on wood, royal 8vo. elegantly bound in cloth, coloured Plates, London, 1848 . . 1 14 6 Plain . . . . . .19 ITie Natural History of Mammalia is intended to embrace an account of the struc- ture and habits of all the hnovm species of Quadrupeds, or Mammals ; to which will be added, observations upon their geographical distribution and classifica- tion. Since the fossil and recent species illustrate each other, it is also in- tended to include notices of the leading characters of the extinct species. The Genera, and many of the species, are illustrated by Engravings on steel, and by Woodcuts. The modifications observable in the sti^cture of the skulls, teeth, feet, and other parts, are almost entirely illustrated by steel Engravings. Williams. Elements of Medicine : Morbid Poisons. By Robert Williams, M.D., Physician to St. Thomas's Hospital. 2 vols. 8vo. London, 1836 — 41 '.186 Vol. II. separately. 1841 . . . . 18 Hlppolyte BallUere'B FuMlcatlons. 19 i-An JUAWM scMJfitf TIFI V WORKS. Willis. Illustrations of Cutaneous Disease : a Series of Delineations of the Affections of the Skin, in their more interesting and frequent forms ; with a Practical Summary of their Symptoms, Diagnosis, and Treatment, including appropriate Formulae. By Robert Willis, M.D., Member of the Royal College of Physicians. The Drawings ai-e after Nature, and Lithographed by Arch. Henning. These Illustrations are comprised in 94 Plates, folio. The Drawings are Originals, carefully coloured. Bound in cloth, lettered. London, 1843 ....... On the Treatment of Stone in the Bladder by Medical and Mechanical Means. London, 1842 . . . . . * . Wood. A Treatise on the Practice of Medicine, by George B. Wood, M.D. Second edition, 2 vols. 8vo. Philadelphia, 1849 6 5 1 10 Babington. Primitise Florae Samicae; or, an Outline of the Flora of the Channel Islands of Jersey, Guernsey, Alderney, and Sark. 12mo. LondoUj 1839 . . . . . . .040 Fielding and Gardner. Sertum Plantarum; or. Drawings and Descrip- tions of Rare and undescribed Plants from the Author's Herbarium. By H. B. Fielding ; assisted by G. Gardner, Superintendent of the Royal Botanic Gardens, Ceylon. 8vo. London, 1844 . . . .110 Gray and Spragae. Genera FIor« Americas Boreali Orientalis. lUustrata 200 Plates. 2 vols. 8vo. Boston, 1848—49 . . . . 3 10 Hassall. A History of the British Fresh-water Algae, comprising Descriptions and Coloured Delineations of nearly 500 Species, including the Desmidae and Diatamacea. This Work is being re-issued in 12 Monthly Parts. 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