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 Pl.CXXV. 
 
 -•Th-e markings on I'rus 
 
 tuli a S axoni c a . 
 
( 274 ) 
 
 II. — Note on the Markings of Frustulia Saxoniea. 
 
 By Assistant-Surgeon J. J. Woodward, U. S. Army. 
 
 (Read before the Royal Microscopical Society, November 3, 1875.) 
 
 Plates CXXIV. and CXXV. 
 
 On returning from my last summer’s vacation my attention was 
 directed a few days ago, for tlie first time, to a letter by Mr. W. J. 
 Hickie in the July number of tlie * Monthly Microscopical Journal 
 (p. 32), on the subject of the markings of Frustulia Saxoniea , 
 which seems to call for some comments from me. 
 
 Mr. Hickie refers to a paragraph in the ‘ Monthly Micro- 
 scopical Journal ’ (vol. ix., p. 86) headed “ Frustulia Saxoniea as a 
 Definition Test,” which he, however, thinks may possibly misrepre- 
 sent my views, and remarks — “ What is there said is by no means 
 very clear ; but it certainly does make him assert one of two 
 things : either (1) that Frustulia Saxoniea is a one-lined object 
 (i. e. has transverse, but no longitudinal lines) ; or (2) that, 
 though it undoubtedly has transverse, and may possibly have 
 longitudinal lines as well, no one as yet has succeeded in seeing 
 the latter, but that those who fancied they saw them, as Dippel 
 and others, have been deceived by ‘ diffraction phenomena.’ ” I will 
 frankly say that the second of these views very fairly represents 
 my opinions on this subject, although it is not precisely what I 
 said. 
 
 The paragraph quoted by Mr. Hickie is abridged from a “ Note 
 on the Frustulia Saxoniea as a Test of High-Power Definition,” 
 which I published in the ‘ Lens ’ for October, 1 872 (p. 233), and 
 of which I send herewith two copies, with the request that one of 
 them may be forwarded to Mr. Hickie. The original article was 
 illustrated by a Woodburytype plate, of which I have now no 
 spare copies ; I send, however, two silver prints (marked A) from 
 the negative used in its preparation. In this “ Note ” I quoted 
 Dippel’s description,* which attributes to Frustulia Saxoniea both 
 longitudinal and transverse striae, and estimates the first at 1 8 to 
 20,°the second at 34 to 35 in the one hundredth of a millimeter. 
 I stated that I myself found the transverse striae count 85 to 90 in 
 the one thousandth of an inch, which agrees substantially with 
 Dippel’s figures, but said, “ The longitudinal strip of Dippel, how- 
 ever, I must regard as diffraction phenomena,” and went on to 
 assert that as I observed them “ they varied too much in their 
 distance apart, with varying obliquity of illumination, to bear any 
 other interpretation.” It will be observed that I did not, in my 
 
 * From ‘Das Mikroskopund seine A.nwendung.’ ErsterTkeil. Braunschweig, 
 1867, s. 132. 
 
2 7° Transactions of the Royal Microscopical Society. 
 
 Note, speak generally, as Mr. Hickie does, of what “ Dippel and 
 others fancied they saw, but specifically of the longitudinal strife 
 ot Dippel. My reason for this limitation was, I confess, that I did 
 not at the time know that any authority but Dippel bad described 
 longitudinal strife on this diatom. So far as Dippel was concerned 
 I had not merely his description and his statement of the number 
 ot lines he observed to the inch, but the excellent and truthful 
 woodcut m his book* on which to form an opinion. This wood- 
 cut, moreover, enabled me both then and now to know that the 
 specimens I studied belonged to the same species as that which 
 Dippel described. 
 
 In m y “ Note,” then, I spoke only of the longitudinal strise 
 of Dippel, but now, in response to Mr. Hickie’s letter, I willingly 
 express my belief that the longitudinal lines which he describes are 
 ot the same character. At the same time I shall be very glad if he 
 can convince me, by satisfactory evidence, that this belief is erro- 
 neous, for analogy inclines me towards the opinion that in both 
 Fi ustulia Saxonica and Ampliipleura pellucida the striae are 
 really lows of beads, as is so easily to be seen in Navicula rhom- 
 houles, and that, consequently, we ought to be able to see longi- 
 tudinal strife when the illuminating pencil has the proper direction, 
 if only our glasses had the requisite defining power. 
 
 In favour of his opinion that he has actually seen these longi- 
 tudinal strife on Frustulia Saxonica, Mr. Hickie states, in the first 
 place, that Herr Seibert showed him in the summer of 1872 “ a 
 couple of very beautiful photographs of that diatom, one of which 
 exhibited the transverse, and the other the longitudinal lines, with 
 far more clearness, sharpness, and distinctness than the printer will 
 be able to reproduce the words I have here written.” If I had 
 these photographs before me, they might, perhaps, though I confess 
 I do not expect it, show me that Herr Seibert has seen somethin 0, 
 
 I have been unable to see, and I must therefore express the hope 
 that Mr. Hickie, in return for the package of photographs I send 
 him herewith, will take the trouble to obtain copies of them for me 
 to study. I hope also that he will obtain copies of them for the 
 Eoyal Microscopical Society, to compare with those I send. In the 
 absence of Herr Seibert’s photographs, however, I would remark, 
 that spurious lines, due to diffraction and interference, can be photo- 
 graphed quite as readily as real lines, from which they can only be 
 distinguished by observing precautions which Mr. Hickie’s letter 
 does not show to have been considered either by himself or any of 
 the distinguished gentlemen to whom he tells us he exhibited the 
 longitudinal strife in question. 
 
 Mr. Hickie tells us further, that “ In Ehomboides also the 
 transverse lines are much closer than the longitudinal, whereas in 
 
 * Op. cit., fig. 103. 
 
 - • T He i : a v kin <£> s o nFru s tei Li a ' EUlu iurrcc- 
 
Markings of Frustulia Saxoniea. By Dr. J. J. Woodward. 276 
 
 Frustulia Saxoniea it is just the reverse.” This statement directly 
 contradicts the description of Dippel, whose measurements, quoted 
 above, make the longitudinal lines nearly twice as coarse as the 
 transverse instead of finer, and it might at first be thought that the 
 two gentlemen were describing different things, but, in point of 
 fact, as I stated in my “Note,” these spurious longitudinal lines 
 vary greatly “in their distances apart with varying obliquity of 
 illumination,” and also, I may now add, with varying positions 
 of the fine adjustment, so that I have had no difficulty in photo- 
 graphing them, as seen on the same frustule, and with the same 
 objective and distance, both finer than the transverse ones as 
 described by Mr. Hickie, and coarser than the transverse ones 
 as described by Dippel, and in both cases they appeared to me, as 
 Mr. Hickie says they did to him, “as plainly and visibly as any of 
 us are ever likely to see our own faces in our looking-glasses.” 
 The distinctness with wdiich these lines can be seen has nothing to 
 do with the question of their objective reality, for diffraction pheno- 
 mena are often quite as distinctly visible as the optical images of 
 actual objects. 
 
 The question of the real nature of these longitudinal lines 
 appears to me to be one of considerable interest, because it brings 
 up the matter of recognizing lines due to diffraction and inter- 
 ference, when observed in the field of the microscope, and because 
 these phenomena have been a fruitful source of error in the inter- 
 pretation of microscopic images. I have therefore thought it worth 
 while to prepare a series of photographs of Frustulia Saxoniea for 
 the purpose of illustrating my meaning. 
 
 The first of these photographs (marked B) represents a frustule 
 of Frustulia Saxoniea adjusted to show the transverse striae. This 
 frustule measured Foas of' an inch in length, with eighty-six striae 
 to the toVtt of an inch. The negative is magnified 1830 diameters 
 very nearly, and to this of course the paper print closely approxi- 
 mates.* 
 
 The second photograph (marked C) represents the same frustule 
 with its left end raised so as to bring it obliquely to the light. In 
 its lower half, besides the transverse lines, a series of longitudinal 
 lines can be seen, which give rise in places to a distinct appearance 
 of dots at their intersections with the transverse striae. The nature 
 of these lines will be best understood after a study of the third and 
 fourth photographs. 
 
 In the third photograph (marked D) the same frustule is shown 
 with its left-hand angle still more elevated. The transverse striae 
 have disappeared; but in the left-hand half of the frustule we have a 
 beautiful series of longitudinal lines which very closely resemble 
 
 * Silver prints after mounting are usually a trifle larger than the negatives 
 from which they are printed. 
 
277 Transactions of the Royal Microscopical Society. 
 
 those shown in Dippel’s woodcut above referred to. Such longi- 
 tudinal lines as these. I suppose, Mr. Hickie himself has observed and 
 recognized as spurious, for he says in his letter — “ I had also no 
 difficulty in bringing into view those wide-spaced, spurious lines 
 alluded to by Dr. Woodward.” 
 
 Let us observe the character of these lines. They run parallel 
 to the midrib, handsomely following its sweeping curves, and it 
 will be seen that they become progressively closer and closer 
 together from the midrib towards the margin of the frustule. It 
 will also be seen that they do not terminate at the edge3 of the 
 frustule, but sweep off into the open space outside, where they 
 form a series of rhombic figures by crossing a fresh series of diffrac- 
 tion lines conditioned by the margin of the frustule, as the former 
 series was by the midrib. These characters would be sufficient to 
 show these lines to he spurious, but if, now, the fine adjustment be 
 toyed with, or the illumination changed, or both, they pass, by the 
 most insensible transitions, into new combinations, one of which is 
 shown in the next photograph. I would also call the attention of 
 those who examine this photograph to three shadows on the left of 
 the lower part of the frustule, which are cast by three out-of-focus 
 fragments of dirt adhering to the under surface of the glass cover 
 of the preparation. Each of these shadows is surrounded by a 
 series of lines due to diffraction and interference. The lower two 
 sets of these lines especially very closely resemble the longitudinal 
 lines on the frustule in number, distance apart, distinctness, and 
 general character. 
 
 In the fourth photograph (marked E) the same frustule is 
 shown standing vertically with the light coming from the right 
 side of the picture. On both sides of the frustule, but especially 
 on the right side — the left being in shadow — there are a series of 
 fine longitudinal lines, which I suppose to be similar to those Mr. 
 Hickie has described. They are rather closer together than the 
 transverse striae (counting eleven, on the negative, in the space 
 occupied on the negative of the photograph marked B, by ten of the 
 transverse striae), and their distance apart is more equable than is 
 the case with the longitudinal lines in the last photograph. Still, 
 if the paper print be examined with a hand-lens, I think it will be 
 plainly seen, as can readily be measured on the negative, that the 
 lines become progressively closer towards the margin, those nearest 
 the midrib being farthest apart. It will also be seen, on both sides 
 of the frustule, and especially at its upper end, that these longi- 
 tudinal lines are not limited to the surface of the frustule, but pass 
 off into space outside, where they cross a fresh series of beautiful 
 diffraction lines conditioned by the margin of the frustule. I would 
 particularly invite attention to this last series of lines, especially as 
 seen towards the top of the right-hand margin of the frustule. The 
 
Markings of Frustulia Saxonica. By Dr. J. J. Woodward. 278 
 
 fine longitudinal lines shown on the frustule in this photograph, 
 then, like the coarse ones shown in the last, have all the characters 
 of diffraction fringes ; but if any doubt remains, it is only necessary 
 to toy with the fine adjustment, when the number of the lines and 
 their distance apart will be found to vary continually, while in the 
 case of the transverse striae, or of any other real lines, the number 
 remains constant as long as they can be seen at all. 
 
 Besides the principal frustule shown in these pictures, part of 
 another frustule is shown in each, which affords still further illustra- 
 tions of the longitudinal lines in question. I may add that, although 
 I endeavoured to take the four pictures with the same power, 
 trifling differences exist due to the necessary variations in the focal 
 adjustment. 
 
 As a still further illustration of the spurious longitudinal lines 
 of this diatom, I add a print (marked F) of a negative magnified 
 1600 diameters, made November 10, 1872, by Tolles’s immersion 
 yjrth, in which the central frustule shows the transverse striae, 
 while portions of two others exhibit longitudinal lines, similar, as 
 I must suppose, to those Mr. Hickie sees. 1 would call attention 
 on this print, and, indeed, on the two others (marked A and B) 
 which show the transverse striae, to a curious series of diffraction 
 lines just outside of the margin of the frustule, which appear to be 
 conditioned by the transverse striae themselves. These spurious 
 lines are at exactly the same distance apart as the transverse striae, 
 but form a sharp angle with them. 
 
 I send copies of all these photographs for Mr. Hickie, and also 
 a set for the Royal Microscopical Society. If the longitudinal lines 
 which Mr. Hickie sees are, as I suppose, similar to those which I 
 have photographed, these pictures will enable those who examine 
 them to decide whether his interpretation or mine is correct. If he 
 thinks he sees something of a different nature, I shall be happy to 
 consider the evidence on that head when he presents it. 
 
 These photographs will, moreover, enable those interested in 
 the subject to decide another question raised by Mr. Hickie in his 
 letter, viz. whether what I have photographed and described is 
 really Frustulia Saxonica, and also, whether I have, as he suggests, 
 “ merely been wasting” my “time on a bad slide.” It is quite true 
 that I have been using Moller’s slides ; it is also quite true that, 
 like Moller, I suppose Frustulia Saxonica to he identical with 
 Navicula crassinervis. I do not pretend to any special personal 
 knowledge of the proper classification of the diatoms, and derive 
 my opinion entirely from my friend Professor Hamilton L. Smith, 
 of Hobart College, Geneva, New York, who I suppose to be more 
 thoroughly acquainted with the subject of the Diatomacese than 
 anyone on this side of the Atlantic, and who wrote me, January 9, 
 1872: “ Navicula crassinervis has long been recognized as = 
 
279 Transactions of the Royal Microscopical Society. 
 
 Frustulia Saxonica..” Mr. Hickie asserts that there is a difference, 
 but does not make clear in what the difference consists. I should 
 be happy to learn further from him on this head, if he has any- 
 thins? to teach. If, on examining my photographs, he thinks they 
 represent something different from his slides of Frustulia Saxonica, 
 I should he glad to receive one of these from him to study. I have 
 the less hesitation in asking this favour, as he tells us his collection 
 
 of these slides is a very extensive one. . 
 
 I conclude these remarks with the hope that Mr. Hickie will 
 find them as courteous as I acknowledge his own to be. 
 
 List of Photographic Prints accompanying Dr. Woodward's “ Note on the Markings of 
 
 F rustulia Saxonica . 
 
 A.— Print from the negative used to illustrate Dr. Woodward’s paper in the 
 
 j$ . Frus tule 3 photograph ST'for the present paper to show the transverse strife, 
 
 0.— Same frustulefsame power, showing both transverse and longitudinal lines. 
 
 D Same frustule, same power, showing the longitudinal lines of Dippel. 
 
 pj; Same frustule same* power, showing longitudinal lines similar to those 
 
 dpscribod by Mr. Hicki.6. . 
 
 F.— Print from a negative made in 1872, showing the transverse atria on one 
 frustule, and longitudinal lines on parts of others. 
 
 EXPLANATION OF PLATES CXXIV. AND CXXV. 
 
 These Plates contain figures the same size as those in the several admirable 
 photographs which Dr Woodward wSo^in 
 
 diatom is occasionally introduced, in the Plate 
 but a single frustule is represented. 
 
 [The following remarks are contained in the ‘ Lens, October, 
 1872.] 
 
 Note on the Frustulia Saxonica as a Test of High-Power 
 
 Definition. 
 
 The genus Frustulia (Agardh) includes several species of diatoms 
 which possess bacillar or navicular frustules, “ immersed m an 
 amorphous gelatinous substance.”* The species are divided by 
 Shard into two groups, the first with “ evident strip, while 
 in the second the striae are “ wanting or very indistinct. In the 
 second -roup Pritchard places the species Saxonica (Babenhorst), so 
 called from having first been noticed in Saxony, where it “forms 
 dirty, olive-brown, tremulous jelly-like masses m little cavities of 
 damp rocks.” The ‘ Micrographic Dictionary (second edition) 
 
 * ‘ A History of Infusoria.’ By Andrew Pritchard. 4th edit., London, 1861, 
 p. 924. 
 
Markings of Frustulia Saxonica. By Dr. J. J. Woodward. 280 
 
 gives a brief description of this species, but does not mention the 
 striae. 
 
 The first description of the striae with which I am acquainted 
 was given by Professor Reinicke,* who saw fine transverse lines, 
 but found no objective capable of bringing them out clearly enough 
 to count them (He used immersion objectives of Hartnack.) 
 
 The markings on the Frustulia Saxonica are described and 
 figured by Dippel, | whose description is as follows : “ The Frus- 
 tulia Saxonica possesses longitudinal and transverse striae, of 
 which the first are tolerably far apart (from eighteen to twenty to 
 the hundredth of a millimeter), the latter, on the contrary, some- 
 what closer than those of Grammatophora subtilissima (from 
 thirty-four to thirty-five to the hundredth of a millimeter). Both 
 systems of striae are very pale ( schwach gezeichnet), so they cer- 
 tainly require an excellent objective for their resolution. Never- 
 theless, with oblique light, and on bright days, they can be seen 
 with objectives of the highest power almost as 'well as those of the 
 Grammatophora, if the object is only properly prepared, the valves 
 separated ( gespalten ), and mounted dry. In balsam, on the other 
 hand, the transverse striae are very difficult to see : nevertheless, I 
 have recognized them, even in this case, with the System No. 1 0 of 
 Hartnack.” 
 
 During the summer of 1871, a slide of Frustulia Saxonica, 
 mounted dry, was presented to the Museum by Dr. J. J. Higgins, 
 of New York, and subsequently two other slides, also dry, were 
 obtained from J. D. Moller, of Wedel, Holstein. I found no diffi- 
 culty, on these slides, in seeing and counting the transverse striae, 
 both with monochromatic sunlight and with the light of a small 
 coal-oil lamp. The longitudinal striae of Dippel, however, I must 
 regard as diffraction phenomena, similar in character to the longi- 
 tudinal lines which some have described in the central portion°of 
 Grammatophora trustifies ; they varied too much in their distance 
 apart, with varying obliquity of illumination, to bear any other 
 interpretation. The transverse striae, on the other hand, I found 
 very definite in character. I counted on different trustifies from 
 eighty-five to ninety to the thousandth of an inch, which agrees 
 substantially with the results of Dippel, whose figures correspond 
 to from eighty-six to eighty-nine to the thousandth of an inch. 
 The frustules themselves varied in length from '0018 to -0029 
 inch. 
 
 I subsequently removed the cover of one of the dry slides 
 obtained from Moller with the diatoms adherent to ‘it, and 
 
 * See a review of his “ Beitrage zur neuern Mikroseopie,” in the ‘ Quarterly 
 Journal of Microscopical Science,’ vol. ii., new series, 1862, p. 292. 
 
 t ‘Das Mibroskop und seine Anwendung.’ Erster Tlieil. Braunschweig 
 
281 Markings of Frustulia Saxenica. By Dr. J. J. Woodward. 
 
 mounted the specimen in Canada balsam. The striae were then 
 paler than before, but I cannot say that I found them more 
 difficult to resolve. Both in balsam and dry I could get resolu- 
 tion by the Tolles’s immersion |th belonging to the Museum, and 
 that by lamplight as well as by monochromatic sunlight. With 
 immersion objectives of higher powers the lines were still more 
 distinctly separated, and I obtained the finest results with the 
 immersion front of the of Powell and Lealand, and with 
 
 the new immersion Wth recently made for the Museum by 
 Mr. Tolies. 
 
 On the whole, the Frustulia Saxonica is an easier test than 
 the Amphipleura pellucida, as may he inferred from the above 
 measurement of its strife, and the difference is especially marked by 
 lamplight. Those therefore who work by lamplight only will find 
 this test more extensively useful than the Amphipleura. 
 
 The Woodbury print [see Plate CXXIV., Fig. A] illustrating 
 this article is copied from a negative made by the immersion front 
 of the Powell and Lealand Wth belonging to the Museum. The 
 power used was 1750 diameters. It displays the transverse strife 
 as seen when the utmost pains are taken to avoid the longitudinal 
 diffraction lines. 
 
 London : Printed by William Clowes k Sons, Stamford Street and Charing Cross. 
 
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 The Monthly Micro scopical Journal, Junel, 1876 . 
 
 A. Gnats Body Scale, 
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 W West & c° imp. 
 

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TheMoiitJilyMici’oscopical Journal, June 1,1876 
 
 ?] C XI, 
 
 WPVesfc H C? 
 
( 253 ) 
 
 I . — On the Markings of the Body-scale of the English Gnat and 
 the American Mosquito. 
 
 By Dr. J. J. Woodward, U. S. Army. 
 
 ( Bead before the Royal Microscopical Society, May 3 , 1876 .) 
 
 Plates OXXXIX. and OXL. 
 
 My attention was first directed to the markings on the body-scales 
 of the English gnat by a letter received last summer from Mr. John 
 Mayall, jun., enclosing a mounted slide of these scales, and a photo- 
 graph of a drawing of one of them by Dr. John Anthony, of Bir- 
 mingham, representing the scale as marked by longitudinal beaded 
 ribs, having three uniform parallel rows of smaller beads in every 
 interspace betiveen two adjoining ribs. (See PI. CXXXIX.) 
 
 Mi-. Mayall stated that Dr. Anthony had made the drawing to 
 represent an appearance of the scale glimpsed by central light, but 
 had not been able to show this appearance to him in the microscope 
 as definitely as it appears in the drawing, and requested me to 
 undertake to photograph the scale as seen under the microscope. 
 This I did at such leisure hours as I was able to command during 
 the latter part of last year, and in so doing arrived at results which 
 may perhaps be of interest to some of your Fellows. 
 
 I at once observed the very great similarity between the scales 
 of the English gnat and those of the American mosquito, with 
 which I had been familiar for a number of years ; a similarity 
 which relates to all the details of surface markings, as well as 
 to the size and general outlines of the scales. In the case of the 
 mosquito, I had seen that the scales are crossed transversely by fine 
 markings, probably ridge-like corrugations of the thin double mem- 
 brane composing them, and that these transverse markings crossing 
 the longitudinal ribs at regular intervals, gave to the latter a beaded 
 appearance ; but I had not believed that the transverse markings 
 were also beaded. I must add that the ribs and transverse mark- 
 ings exist on both surfaces of the scale, though much more boldly 
 on one than on the other, and that the longitudinal ribs of the 
 opposite sides unite at the broad end of the scale, where they 
 generally project as bristle-like appendages beyond the general 
 contour. 
 
 My examination of the slide received from Mr. Mayall has led 
 
254 Transactions of the Royal Microscopical Society. 
 
 rue to the opinion that this description applies to the gnat scale as 
 well as to the mosquito. Nevertheless, on examining a gnat scale, 
 as requested by the donor of the slide, with the immersion yVth of 
 Powell and Lealand, by central illumination, I succeeded, after 
 some trying with the right-angled screws of the achromatic con- 
 denser, in obtaining, as I suppose, the very appearance Dr. 
 Anthony’s drawing is intended to represent, and the three parallel 
 rows of minute intercostal beads started out suddenly into view 
 between each pair of longitudinal ribs over the whole surface of 
 the scale. 
 
 This appearance was so realistic that at first I inclined to the 
 opinion that it represented truly the actual markings of the scale, 
 and accordingly I endeavoured to photograph it as requested, with 
 the objective named, by monochromatic sunlight, and after several 
 failures succeeded in obtaining a fair representation of what I saw. 
 
 I send herewith a print (marked A) from the resulting negative. 
 
 I have, however, since then been led to form the opinion that 
 these clearly seen beads are a spurious appearance, produced by 
 longitudinal diffraction lines, conditioned by the longitudinal ribs 
 and parallel to them, which cross the true transverse markings 
 at right angles, and thus give rise to the optical appearance of 
 beads at the point of intersection ; the whole series of phenomena 
 being similar in character and origin to the diffraction phenomena 
 observable in many diatoms, &c., as described by me in my “ Note 
 on the Markings of Frustulia Sctxonica ” in this Journal, December 
 1875, p. 274. 
 
 My chief reasons for this opinion in the present case are — 
 firstly, that the longitudinal diffraction lines are clearly seen, both 
 in the microscope illuminated by lamp or sunlight, and in the 
 photographs (as, for example, in the print A) to extend into empty 
 space beyond the contour of the scales almost as far as the ends of 
 the bristles in which the parallel ribs terminate ; and secondly, that 
 they vary in number with varying obliquity of illumination, so that 
 in the same scale two, three, four, or five rows of beads can be seen, 
 and photographed at pleasure, in each intercostal space. 
 
 Since arriving at this conclusion I have had no difficulty in 
 producing at will, either the beaded appearance, or that which I 
 conceive to represent correctly the surface markings, on any scale 
 I have tried, whether of the gnat or mosquito. 
 
 If the selected scale is illuminated with the light thrown per- 
 pendicularly to the transverse markings, by means of an Abraham s 
 prism, the beaded ribs and smooth transverse markings will be 
 clearly shown ; and if now the stage be rotated so as to turn the 
 long diameter of the scale more and more obliquely to the illumi- 
 nating pencil, the spurious lines, and with them the beads, will 
 start into view ; the number of spurious lines, and consequently 
 
Markings of the English Gnat , <&c. By Dr . Woodward. 255 
 
 the number of rows of beads, varying with the angle of the illumi- 
 nating pencil. Or the true appearance may be produced by the 
 achromatic condenser adjusted so that the light is either truly 
 central, or slightly oblique in the direction of the length of the 
 scale ; and then a very moderate degree of obliquity in the illumina- 
 tion transversely to the scale, obtained by means of the right-angled 
 screws of the condenser, will bring out the rows of beads, varying 
 in number as in the former case, in accordance with the degree of 
 obliquity attained. 
 
 I submit these results without further comment at the present 
 time, with a few additional photographs intended to represent some 
 of the chief appearances obtained. Two of these pictures, in addi- 
 tion to that mentioned above, are from the slide of gnat scales sent 
 by Mr. Mayall, and are taken with the immersion T Vth of Powell 
 and Lealand ; the others represent a mosquito scale as seen with an 
 immersion T Vth, constructed for the Museum by Mr. Tolies, of 
 Boston. I selected for this series a different lens from that used 
 for the gnat scales to show that the diffraction appearances dis- 
 cussed in this paper result from the optical conditions under which 
 the scales are viewed, and not from any peculiarity in the objectives 
 of any particular maker. 
 
 In conclusion, I would refer those who desire preliminary 
 information as to the character and distribution of the gnat scales 
 to the paper by Mr. Jabez Hogg, “ On Gnat Scales,” in this 
 Journal for October, 1871, p. 192 ; or to his work on the Micro- 
 scope, the first edition of which was published in 1854. The 
 description there given of the various forms of gnat scales, and of 
 their distribution on the insect, is very nearly accurate for the 
 mosquito also. 
 
 List of Photographs. 
 
 A. Gnat scale, showing three rows of intercostal beads. Magnified 1350 dia- 
 
 meters by Powell and Lealand’s immersion T ^th. (Neg. 771.) See PI. 
 CXXXIX., Fig. A. 
 
 A smaller gnat scale, showing smooth transverse markings. Magnified 
 
 1500 diameters ; same objective. Achromatic condenser ; central light. 
 (Neg. 7S1.) See id., Fig. B. 
 
 0.— The same scale, with same objective and power, but moderate obliquity of 
 illumination obtained by means of the right-angled screws of the con- 
 denser. (Neg. 782.) See id., Fig. C. 
 
 D. — Mosquito scale, showing smooth transverse markings. Magnified 1350 dia- 
 
 meters by an immersion T qth of Tolies. Achromatic condenser ; nearly 
 central light. (Neg. 765.) See PI. CXL., Fig. D. 
 
 E. Same scale, same objective, but light oblique laterally as well as trans- 
 
 versely, showing two rows of beads in each intercostal space. 1350 dia- 
 meters. (Neg. 768.) See id., Fig. E. 
 
 F. — Same scale, &c., showing three rows of beads in each intercostal space. 
 
 1300 diameters. (Neg. 778.) See kl.. Fig. F. 
 
 G. — Same scale, &c., showing four rows of beads in each intercostal space. 
 
 1350 diameters. (Nog. 766.) See id.. Fig. G. 
 
256 Transactions of the Royal Microscopical Society. 
 
 Note by John Anthony, M.D. 
 
 Having read Dr. Woodward’s paper on the gnat and mosquito 
 scales, and looked carefully at the set of photographs in illustration, 
 which that gentleman has had the kind courtesy to forward to me, 
 
 I can come to no other conclusion but that what I have hitherto 
 regarded as real bead markings on the membrane in the intercostal 
 spaces on the scales from the body of the gnat are really and truly 
 spurious images, or, in the words of Dr. Woodward, “ diffraction 
 appearances.” 
 
 Some two years ago, in the examination of a large number 
 of gnat scales, principally with the fine |th and T Vth objectives of 
 Messrs. Powell and Lealand, such results were obtained that I 
 thought I had found in the scale from the body of the gnat an 
 excellent test for the “ definition ” of high-power objectives, inas- 
 much as there seemed, with moderately oblique and well-corrected 
 light, what appeared to me triple rows of clearly defined beads 
 between the beaded longitudinal ribs of the scale ; and, as the same 
 appearances of beads were always manifest, and as those beads 
 always seemed to come out clearer when viewed with objectives 
 of well-known excellence, I trust to be pardoned for believing that 
 what I saw were not only real appearances, but that such objects as 
 the gnat scales might be of the greatest service to the microscopist 
 as tests for the defining qualities of high-power objectives. Under 
 such impression I made a careful drawing of the markings on the 
 gnat’s scale under the most favourable conditions, and that drawing 
 I copied by means of photography; the photograph would have 
 been made directly from the scale of the gnat itself, but I had no 
 heliostat. These photographic copies of the drawing have gradually 
 passed into the hands of one or other of my microscopical friends 
 until the specimen forwarded with this paper is the only one left 
 to me. However, I have the negative, and if possible impressions 
 shall be printed for distribution among the members present at the 
 next meeting of the Society.* (PI. CXXXIX.) 
 
 Taking the E, F, and Gr photographs of Dr. Woodward to be 
 the most characteristic, inasmuch as they show respectively two, 
 three, and four rows of beads as seeming to exist upon the same 
 scale under different conditions of illumination, I think one can 
 only look upon my drawing as a representation of very clearly seen 
 spurious beads. Of course it is not very flattering to one’s amour 
 propre to have it shown so convincingly that one has taken the 
 shadow for the substance ; but I am assured that I shall have erred 
 in good company, inasmuch as the analysis of these diffraction 
 images will strike at the root of a vast number of descriptions of 
 
 * Some were sent by Dr. Anthony to the meeting. 
 
Markings of the English Gnat, dec. By Dr. Woodward. 257 
 
 quasi beaded tissue seen in all sorts of objects examined with high- 
 power lenses. 
 
 This brings me naturally to the observation that I think micro- 
 scopists, who are not too proud to learn, owe to Dr. Woodward a 
 debt of gratitude for the trouble he has taken, and the skill he has 
 displayed, in teaching us the precautions we ought to take in high- 
 power investigations to distinguish between the false and the true. 
 I have worked long enough at the microscope to feel that as a rule 
 one ought, like the late Lord Eldon, to “ doubt ” everything ; and 
 have often amused myself in producing, on well-known diatoms, a 
 series of permutations of “ diffraction phenomena,” and therefore 
 can appreciate most fully the truthful excellence of Dr. Woodward’s 
 article and illustrations in the December number of the ‘ Micro- 
 scopical Journal.’ I regard these papers on spurious appearances as 
 among the most valuable contributions to microscopical literature. 
 Dr. Woodward points out that detail, however clearly seen upon a 
 scale, may be more than suspected of being unreal if it seems not to 
 be confined to the limits of the scale or shell itself, but to “ pass 
 off into space.” A question arises as to how you are to deal with 
 the appearances, such as a fairly careful observer might get, and 
 such as I myself observed on the gnat’s scale, where there was no 
 projection of the image into space ; a phenomenon with which, as I 
 stated, I am very familiar. The only suspicious point noticed 
 was an apparent alteration in the character of the detail on revo- 
 lution of the object, but the employment of fight more or less 
 oblique has in most cases, and particularly in very diaphanous 
 objects, accustomed one to look at certain scales always in the 
 same position with respect to the plane of the illumination as being 
 “ best seen ” ; and it is to be feared that very many of us, in our 
 employment of the microscope, are apt to be led away by beauty 
 of image. 
 
 It appears to be clear that no perfection of “ condenser,” and no 
 superior quality in the objective, can save us from acquiring erro- 
 neous impressions of what we see in the microscope, if we have not 
 a very distinct notion of the “ pitfalls ” which await us in the shape 
 of diffraction images. Dr. Woodward evidently has grasped this 
 difficult subject with a master hand ; he has used photography as a 
 witness — which to me is most satisfactory ; he has given us a most 
 valuable lesson, and I for one beg to thank him for it. 
 
 Washwood Heath, Birmingham, 
 
 April 3 , 1876 . 
 
 London : Printed by W, Clowes & Sons, Stamford Street and Charing Cross. 
 

 
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THE MASTER OE PHOTO-MICROGRAPHY 
 
 270. WOODWARD, J(OSEPH) J(ANVIER). Photo- 
 micrographs of the Mosquito, &c. Washington(?): 
 Army Medical Museum, 1872 $4000.00 
 
 Small folio, orig. half morocco, cloth sides with title 
 printed in gilt on front cover. With 4 printed leaves of 
 text and 34 mounted albumen micro-photographs (160 x 
 160 mm), each on a printed card mount with gold 
 border and combined manuscript and printed caption. 
 In very fine condition throughout. 
 
 A rare series of very beautiful micro-photographs of insects, e.g. the 
 mosquito, bed bug, head louse, dog louse, soldier beetle, beetle, 
 parasite fly, wine cellar fly, water bug, etc. These images are approxi- 
 mately 6 inches on a side and are of startling beauty; though they were 
 originally made in the aid of science, we see them today as art. Wood- 
 ward was the foremost practitioner of his craft . . . “the foremost 
 place among those who have aided in this progress will readily be con- 
 ceded to Lieutenant-Colonel J. J. Woodward, a distinguished surgeon 
 in the United States Army, whose photographs of difficult test- 
 diatoms and of artificially ruled lines will long serve as models which 
 less accomplished microscopists can scarcely hope to equal . . .” 
 (Sternberg, Photomicrographs , p 2). The D.A.B. states that Wood- 
 ward attained an international reputation in photo-micrography. This 
 volume is very rare; there were probably no more than 25 and at most 
 50 copies prepared. 
 
m 
 
 m 
 
 Very good copy. 
 
 “There were two further great works of chromolithography produced 
 during the ‘fifties’, and one in the ‘sixties’, all by J. B. Waring 
 (1823-75), an architect, a Swedenborgian, and in his own later estima- 
 tion, a prophet. The larger (indeed one of the largest chromolitho 
 books ever produced) was The Arts connected with architecture . . . 
 published in 1858 by Vincent Brooks. The page size was slightly 
 smaller than Lewis Gruner’s Specimens of ornamental art. It starts 
 with a fine ornamental colored title-page and contains 41 plates with 
 accompanying text. Waring drew all the plates himself and Vincent 
 Brooks lithographed them. Some of the plates, e.g. of marble inlays, 
 are in only one or two colours, like plates in The Grammar of Orna- 
 ment, but there are several plates of stained glass windows and fres- 
 coes which were more ambitious (since they represent depth, shadows 
 and perspective as well as design and colour) and were perhaps the 
 finest chromolithographed plates yet produced in Britain.’’ — 
 McLean, Viet. Book Design, pp 124-5. 
 
 THE FIRST WEBSTER 'S DICTION AR Y 
 
 268. WEBSTER, NOAH. An American dictionary of 
 the English language. New York: S. Converse, 1828 
 
 $ 2000.00 
 
 2 vols in one. Lg. 4to, new marbled boards, calf spine, 
 orig. 4-1/2 inches wide morocco lettering piece pre- 
 served indicating that this copy was originally bound as