7b 85-B 9435 jular Photographic Series. No. lo. 6d. net. i ne X Rays Arthur Thornton, M.A. London : Percy Lund & Co., Ltd., Memorial Hall, E.C. THE X RAYS. Foot of boy, age i6, taken with boot on, showing bullet at back of leg. From photograph by the author. THE POPULAR PHOTOGRAPHIC SERIES. No. lo. THE X RAYS BV ARTHUR THORNTON, M.A., Senior Science Mast(?r, Bradford Grammar School. Bradford : — Percy I.und & Co., Ltd., The Country Press. London Memorial Hall, Ludgate Circus. i8g6. PERCY LUND AND CO., LTD PRINTERS AND PUBLISHERS THE COUNTRY PRESS, BRADFORD AND LONDON 5 CONTENTS. CHAPTER I. PAGE Sound. Longitudinal Vibrations .. .. g CHAPTER n. Ether Vibrations .. .. .. -.13 CHAPTER HI. Hertz Electrical Vibrations .. .. ..17 CHAPTER IV. Light . . . . . . . . . . . . 20 CHAPTER V. Electrical Discharges through Gases .. 25 CHAl^TER VL Discovery of X Ray.s. Fluorescent Effects 33 CHAPTER VH PiioTociKAi'Hic Methods .. .. .. ..40 CHAPTER VHP RaDIOGRAI'HS . . .. .. .. .. ..48 CILVPTER IX. Uses of RADiociRAPHv . . . . . . • • 53 CHAPTER X. The Nature of A' Rays . . 57 7 INTRODUCTORY. In December, 1895, Professor Rontgen, of the University of Wurzburg, published a paper " On a new kind of Rays." That paper, a model of scientific method and complete- ness, gave an account of experiments on fluorescence and photography, which have excited a popular interest far exceeding that usually accorded to the most brilliant scientihc investigations. The press has teemed with articles on " New Photography." A better title is " Radiography," while for the rays them- selves the discoverer's title, x rays, is suitable until we have fuller knowledge of them. We hope to show in this book not only how to use the new x rays, but also to give some indication of present theories about them. For whatever it is that Professor Rontgen has discovered, scientists, as a whole, agree that it is some form of vibration. It would therefore seem advisable to indicate how far this discovery is " new," and what was the previous work leading up to it. 8 For that purpose we shall give a brief account of the vibrations which cause sound, heat, light and chemical effedls. We shall, point out how our senses of hearing and seeing are limited to particular sets of vibrations, but science has long known other vibrations which we may describe as sound which cannot be heard and light which cannot he seen. We shall show how to produce and detedl this inaudible sound and invisible lights and indicate how Prof. Rontgen's vibrations are ''new" inasmuch as they are probably much more rapid than any previously known, and have remarkable power of penetration. 9 CHAPTER I. SOUND. LONGITUDINAL VIBRATIONS. In descril)in<^^ vibrations we shall proceed from the slowest to the quickest. The slowest we shall deal with are sound vibra- tions. A steel spring fastened at one end can be made to vibrate by plucking the free end. Its vibrations''' are, say, 4 or 5 per second. It produces no sound. Shorten the steel spring until the vibrations are 20 per second. A low musical note is heard. Shorten the spring again, its x ibrations are still more rapid and we hear a note of higher pitch. In each case the vibrations produced in the air are similar, but our ears cannot hear vibrations w hose rate is slower than about 16 per second. The long, slowly vibrating rod produces vil)rations which may be called '* inaudible sound " — inaudible because they are too slow. A Galton's whistle can make very rapid sound vibrations. It is a whistle having a narrow barrel of length adjustable from about an inch to zero ; the shorter the barrel the shriller is the whistle ; emitting 25,000 vibra- * Vibration means the complete swing-swung of the moving body or particle. 10 tions per second the whistle sounds Hke a sparrow's chirp, at 30,000 it is like a bat's screech, and is inaudible to many persons; above 40,000 it is inaudible to all. A sensitive flame readily detects these very rapid vibrations. This is a long, thin Hame emerging from a narrow, round pipe, the gas pressure being just below the roaring point. In this state air vibrations from a hiss or shrill note cause the flame to flicker and roar ; the high notes of the whistle affedl: it violently, and they continue to do so when inaudible to us. The flame can hear higher notes than we can. Hence our first point is clear. Our sense of hearing is limited to a particular set of vibrations ; there are others of precisely the same nature, but too slow or too quick for our ears. It is quite possible that the hearing of the larger animals extends to a lower range, and pracftically certain that the cries of many insecSls are too high for our ears. A Rough Table of Sound Vibrations. No. per Second. 16 Lowest audible. 32 Lowest musical note used. 128 Man's voice in conversation. ^^^l Woman's voice in conversation. 512J 2000 High soprano note. 4000 Highest musical note used. 30,000 Bat's cry, inaudible to many. 40,000 Highest audible note. 11 One note is an ocflave higher than another when its vibration frequency is double. Thus: — Musical notes extend about 7 oclaves ; audible notes extend about 1 1 ocftaves. We have still to describe the motion of the air particles conveying the sound. It is very like the motion along a line of men when the first pushes his neighbour, who passes the push on, and so to the end of the line. A figure makes this clearer. a b c d e f S Fig. I. The lo particles, a, b, r, etc., are e(|ui- distant until the first a receives the blow from the vibrating body. Then a moves towards h and makes h move on, a presently returning, and so on along the line. Our figure represents the particles in new lines each time, but in reality they do 12 not leave the original line ; consequently : — 1. A disturbance travels from left to right. 2. A condensation travels in the same line, a rarefadlion the reverse way. 3. Each particle makes a short excursion to and fro in the same line. Such vibrations are longitudinal^ and the wave motion is a condensation-rarefaction. Fig. 2 illustrates what the air would look like if we could see it whilst transmitting Fig. 2.— Sound waves in air. these waves from a bell to the ear. The condensations are not stationary, but are all moving on, with velocity about 360 yards per second, and the number that strike the ear per second is the vibration frequency which determines the pitch of the note. Prof. Rontgen surmises that his x rays are transmitted by extremely rapid longi- tudinal vibrations. 13 CHAPTER II. ETHER VIBRATIONS. We now leave the slow air vibrations of thousands per second to speak of ether vibrations of millions, billions, and possibly trillions per second. First, what is ether ? Science has ceased to regard space as empty ; it is filled with a medium called ether, which pervades all bodies as well as fills the space between them. Properties are ascribed to this ether to explain the phenomenaof radiation, electricity, magnetism, and the still more subtle difficulty of gravitation. But ether satisfies our needs at present if it merely a(5ts as a nominative case to the verb to undulate,''-'' And its undula- tions or vibrations shall be regarded from the simplest mechanical point of view ; i.e., such as would be produced in an elastic solid. Gases can only vibrate longitudinally, elastic solids can vibrate both longitudinally and transversely, * Lord Salisbury to the British Association at Oxford. H Hitherto longitudinal ether vibrations had not been recognised : their possibility was so evident that their absence needed explana- tion. It was supposed that ether was incom- pressible so that its longitudinal vibrations would travel with infinite velocity, i.e.^ pracflically be non-existent. Now Professor Rontgen suggests that his x rays are trans- mitted by longitudinal vibrations — the question is not yet settled — perhaps the a (r c £ { % \ \ \ i- Fig. 3 — Longitudinal Vibrations. majority are in favour of transverse vibra- tions. In a transverse vibration the particles move at right angles to the line along which the disturbance travels. A figure again makes it clearer. The disturbance starts hy a being moved downwards, presently dragging b after it, c and so on; a meanHme oscillating upwards past its first position, to be followed by h, and so on. The result is : — 1. A disturbance or wave travels from left to right. 2. Each particle makes a short excursion up and down. We are familiar with such waves along ropes, and prac51ically the waves on the surface of water are the same. In the etlier there is no surface, but the waves spread out in all direcflions, and we do not consider the particles of ether as necessarily moving up and down, but in any direction which is at ri^Iit angles to the dire(5lion along which the wave travels. The distance from crest to crest is called a wave length. Ether waves fall into the following sets : Ether vibrations. Per second. I. Hertz, eledrical waves. . a few millions. /"Heat waves, Infra red . loo billions and upwards. 2 J Light waves. |'^'^ 400 billions I (violet .. 700 billions. (^Adlinic waves, ultra \ iolet up to 2,000 billions. 3. Kontgen waves possibly trillions. All the waves tra\el at the same rate through the ether, 186,000 miles per second, the short waves as fast as the long ones, they make up for their shortness by vibrating more rapidly. The hrst and last sets are produced by eledtrical means, the middle set probably so. The first and second sets are certainly transverse, the third set probably so. i6 Of all these sets only the light waves, ex- tending less than an ocflave, direcftly afFedl the eye — the rest are invisible light — in that respecft our sense of sight is more limited than our sense of hearing. We notice large gaps, representing vibra- tions of which we know nothing. Rontgen has added one set, probably others will soon be added to fill up the gaps. Indeed, since Rontgen's discovery another set between ultra-violet and x rays has already been indicated by Mons. Becquerel (see page 6i). CHAPTER III. HERTZ ELECTRICAL VIBRATIONS. A Leyden jar consists of a glass jar with inner and outer layers of tinfoil reaching nearly to the top. It is charged by electrical machines, and when discharged gives a bright, loud spark of exceedingly short duration, certainly less than ytjouoo second. Nevertheless, even in that short time the discharge is oscillatory^ several sparks passing backwards and forwards at the rate of some few millions per second. The exa(5I number depends on the size of tlie jar. With a quart jar it would be about (So, 000,000 per second. With a jar as small as the chemists' molecule the rate would be billions per second, about the same fre(iuency as light vibrations : a result in accordance with the generally accepted theory of light, the electro- magnetic theory of Clerk Maxwell. These Hertz oscillations resemble the more rapid light oscillations; Hertz first showed they could be radiated to a distance. The transmitter consists of two plates, A and B, each carrying a thick wire ter- 10 — B i8 minated in polished knobs about half an inch apart, which form the "spark gap" for dis- charging the inducSlion coil connecSled to the plates. The receiver may be a simple loop of wire terminated in two bright knobs, whose distance apart is adjusted to the proper amount (perhaps in.) When properly adjusted a spark passes at the receiver every time one passes at the transmitter. Eledlrical oscillations have Fig. 4. — Hertz's Radiator and Receiver. been sent across from the transmitter to the receiver. These oscillations are transverse; they can be reflecfted, refracfted, polarised, and in fa(5t undergo all the modifications that light oscillations can undergo. They differ from light :— (1) In a much slower rate of vibration. (2) In producing no effecfl on the eye. They are our first example of " invisible light," and we only call them " eledlrical" because they are produced and detedled ele(5lrically. 19 They further differ from Hght in pene- trative power. It is not surprising that a substance which is " opaque " to one set of vibrations, should be transparent " to another set of slower or quicker vibrations. In the use of such terms as "opaque" and transparent " we have been too much influenced by our eyes dealing with light vibrations ; for example, the so-called opaque substances — wood, pitch, stone walls, are transparent to the comparatively slow Hertz vibrations. If these slow vibrations could direcflly affe(5l photographic plates, they would, to some extent, have anticipated the Rontgen photography by a few years. 20 CHAPTER IV. LIGHT. Of all the interesting facfts connedled with light we must confine our attention to a few connecfted with reflecSlion and refradlion. Light proceeds in straight lines in one medium, till it strikes the surface of another Fig. 5. — Reflexion and Refradlion of Light. medium, when it is refle(5ted or refradled into some new diredlion. Refle(5lion and refracflion, are both shown by the experiment illustrated in fig. 5. Light from the sun, or a lantern, passes though the sht at L and strikes a mirror at M, it is then refle(51ed into the circular vessel along the dire(51:ion lO, LI and I O being equally 21 inclined to the mirror. At O the hght strikes the water, and some is reflected along OR, but some enters the water in the direcflion OG. The path of the light is broken at O, so it is said to be refracted. Refracftion always takes place at the sur- face of two media of different densities, the path of the light in the denser medium being more nearly perpendicular to the surface. With a well polished mirror ninety per cent, or more of ordinary light may be refledled. At the very best it is difficult to refle(5\ more than two per cent, of the x rays. And the X rays have not yet been satisfactorily refra(5led ; they enter and pass through the new medium without changing their dire(flion at all. But ordinary light is not only vefvaded, it is usually dispersed as well. This is best shown 22 by Newton's experiment when white light is refracfted through a prism, ABC (fig. 6). Not only is the light bent from its original course, DE, but it is spread into a coloured band, which can be received on a screen MN. This series of colours is called the spectrum, and is generally said to consist of seven colours — red, orange, yellow, green, blue, indigo and violet : but there is an imper- ceptible gradation with every intermediate shade. All these colours are due to the same kind of ether vibrations, differing only in wave lengths and vibration frequencies. All travel with the same speed. The shortest and most rapidly vibrating waves are the violet and these are the most refracSled. In the spectrum the position of any band of colour depends on its vibration frequency, the more rapid being the more refracfted, and therefore the nearer to the violet. The following table gives some of the numbers : — Colours. Wave Lengths in ins. Number per second. Red Yellow Blue Violet 00000270 00000227 00000196 00000167 450 ^ 535 622 727 . millions of millions. Average 00000225 540 billions This set of vibrations is the only set dire(5lly perceptible by our eyes. But we can prove that there are :-- 1. Vibrations slower tlian the red whose position in the spe61:rum is helow the red, hence their name infra-red. 2. Vibrations quicker than the violet whose position is above the violet, ultra-violet. r. The Infra-Red Rays. — Any form of delicate thermometer placed in the infra-red portion of the spaclrum indicates a heating cffecl:, and this effeil exte.ids far below the red. In the visible specflrum itself there is a similar heating effe^, so that radiant light and radiant heat do not essentially differ, but that radiant heat which is below the red is "dark heat," or -'invisible light." This in- visible light can be reflected, refracfted and "focussed" just hke visible light. If it be focussed on a piece of platinum, that piece becomes red-hot, i.e., gives out visible light. The platinum has somehow increased the vibration frequency so far as to make the rays visible. Once again our terms, opaque and transparent, do not apply to the infra-red rays. To these rays black ebonite is more transparent than clear glass. If our eyes were so constructed that we saw with vibra- tions of fre(iuency below 400 billions, we should use ebonite in our windows in prefer- ence to glass. 2. The Ultva-Violet A'^7;'5.^Referring to fig. 6 we see a region marked ultra-violet. In it there are no visible rays : vibreitions are 24 present but they are too rapid for our sense of sight. But if paper soaked in quinine sulphate be placed at the upper part of the screen, the violet will shine out brighter than before and the region beyond the violet will also appear, shining with a bluish light. By some means the quinine can reduce the vibration frequency from invisibility above 700 billions per second, to visibility below that number. This property is called fluovescence^ as fluor spar exhibits it to a marked degree. The ultra-violet, "invisible light," quickly afFecfts a photographic plate. If the plate be exposed to the whole specftrum we should find little or no efFecft at the red end, an increasing effedl towards the violet, and most effedl in the ultra-violet. Indeed ordinary photography is mainly accomplished by ultra-violet rays. Such rays are called chemical or actinic. They differ from ordinary light — (1) In a quicker rate of vibration. (2) In producing no effedl on the eye. Photography extends the ultra-violet spedlrum through a length many times that of the visible specftrum. Most scientists believe that x rays vibrate much faster than the extremest ultra- violet known ; yet they are not refradled by a prism but pass straight through to E in fig. 6. X rays are ultra-ultra-violet in vibration frequency, but have a place in the prismatic spe(5lrum as if infra-infra-red ! 25 CHAPTER V. ELECTRICAL DISCHARGES THROUGH GASES. As it is by means of electrical discharges that we get x rays, we must now consider those discharges. Photograplis of hghtning flashes show tracks hke winding rivers, not sharp zig-zags. The small elecftric sparks between the knobs of our ele(51:ric machines are similar. (See The elecftric discharge through a rarefied gas, in an exhausted tube is very different. There is no actual spark, but the gas, as a whole becomes luminous, with flickering patches or striaj of light. Fig. 8 shows Fi^. 7.— Eledtric Sparks. 'ig- 7-) 26 roughly the discharge through a Geissler's tube, a sealed tube from which air has been pumped until the residual air has a pressure perhaps less than ^jyoo P^^^ ^^^^ ordinary air. The current, from a powerful indu(5fion coil, is led to the tube by two platinum wires, fused through the glass, called electrodes, respecflively anode and kathode, marked + and — , or A and K. Anode = road up, where the current enters. Kathode = road down, where the current leaves. Note that the luminous striae extend through nearly the whole tube, but near the kathode there is always a dark space. We shall only consider the adlion in this dark space. Improve the vacuum by pumping out more air — the dark space increases. Fig. 9 shows this, the kathode is the middle wire, the anode can be at either end. Fio;. 8.— Discharge through Geissler's Tube. Fig. g. — The Dark Space. 27 Improve the vacuum still more, and the dark space extends throughout the whole tube. Such high vacuum tubes are named after Crookes, who investigated their proper- ties. The residual air may be at a pressure less than one millionth that of the atmo- sphere. In Crookes' tubes the eleclric discharge scarcely illuminates the gas, but illuminates Fig. lo. — Low and High Vacuum. the glass walls of the tube, which shine with a green light if made of German glass, but with a blue light if made of English lead glass. The gvecn fliiovcsccnt patches are the starting points of the x rays. Crookes established results, some of which may be summarised thus : — I. The kathode attracts particles of gas, elecflrifies them, and then repels them 28 with great velocity (as much as 120 miles a second). The particles are shot out at right angles to the kathode, and cross the dark space " in straight lines till stopped by the particles of gas or solid bodies in their path. In a low vacuum tube the free straight path is small ; the particles are soon stopped by others, and all of them Fig. II.— Shadow of a Cross. become luminous ; hence the luminous halo round the dark space " of a Geissler's tube. In a high vacuum tube the particles pro- ceed straight across the whole tube until they bombard the opposite walls, which become hot and luminous, or fluorescent. Any solid body put in their path is also bombarded, and may become hot and fluorescent. 29 6. A magnet outside the tube deflecfls the particles inside the tube so that they no longer move in straight lines. It must be understood that even in high vacuum tubes there are still many million particles of air. These particles, discharged straight from the kathode, form rays, often called ^''kathode rays.'' Crookes called the Fig. 12. — Radiant Matter Heating Platinum. residual gas "radiant matter," and showed its properties by various tubes. Fig. lo represents two tubes, respectively loii^ and high vacuum. In each the kathode is the disc at the right. The anode may be the top, middle, or side wire. 30 (a) In the low vacuum tube the luminous discharge proceeds from the kathode to whichever wire is the ^node : the posi- tion of the anode has some effecSl. {b) In the high vacuum tube the discharge ignores the anode, proceeds straight from the kathode, bombards the glass opposite and makes it fluoresce : the position of the anode has no effedl. Fig. 1 1 shows the efFe(5f of an aluminium cross in the path of the kathode rays. Fig. 13. — Fluorescence of Crystals. The tube fluoresces, especially at the end, except where it is protecSled by the cross, which casts a black shadow. Fig. 12 shows how the kathode rays emitted by the cup-shaped kathode can make a piece of platinum red hot : the particles are focussed " on the platinum which is at the centre of the cup. A modification of this tube is the so- called focus" tube, the best form known for emitting the x rays, which seem to start from the central platinum. 31 Fig. 13 illustrates the fluorescence of a solid body, a diamond or other crystal placed in the path of the rays. Fig. 14 illustrates actual rotation caused by the particles, and their deflexion by a magnet. The particles shot from the kathode hit the vanes of the " mill wheel," but there is no rotation until the magnet pulls the particles up to the upper vanes, for in the Fig. 14.— Rotation and I)efl{.'(ftion. absence of the magnet the particles hit the upper and lower vanes equally. We have illustrated the actions inside the tube ; is there a continuation of the actions outside the tube ? Lenard showed that the kathode rays could pass through a thin "aluminium window " placed opposite the kathode. He thus passed kathode rays from one vacuum tube into another and e\ en for a short distance into air. 32 But though there must have been x rays mingled with these transmitted kathode rays, yet Lenard only recognised kathodic proper- ties. His transmitted rays were deflected by magnets {x rays are not), could excite fluorescence, even affedt photographic plates, but could only traverse small distances in air [x rays traverse several yards at least). Lenard led the way for Rontgen's brilliant investiga- tions of a(5lions outside the Crookes' tube. 33 CHAPTER VI. DISCOVERY OF X RAYS. FLUORESCENT EFFECTS. We have described what occurs inside the Crookes' tube, enumerating some results known during the last 20 years. It was whilst experimenting on the same lines as Lenard, that Professor Rontgen made his great discovery of the atftions outside the tube. He knew that many salts were capable of fluorescence, amongst others the platino- cyanide of barium. He coated paper with this salt to determine the action of a Crookes' tube on it. To shut off the ordinary light emitted by the tube he completely surrounded it with a jacket of thick black cardboard, a material quite impervious to bright sunshine, or the light from the elecflric arc. The tube as usual was excited by the discharge from a large inducflion coil, worked by a few secondary cells. The experiment was done in a darkened room, the paper being held near the enclosed tube. As soon as the current was turned on, the platino- cyanide shone with a bright fluorescence, 10 — c 34 whichever side of the paper was offered to the tube. The fluorescence was brightest when the paper was near the tube, but it was still visible when the paper was two yards distant. Something was emitted from the tube, capable of passing through ^'opaque" black cardboard, and two yards of ordinary air (this distance has been since much increased). It was a *'new" light, for it passed through the cardboard ; it was not Kathode rays for it passed through air. If there be interposed between the Crookes' tube and fluorescent screen a playing card, or a thin ebonite sheet, or even a thin aluminium sheet, the fluorescence is scarcely diminished. A book of a thousand pages or a block of wood a foot thick diminish the brightness, but still the screen shines. But denser metals, such as a copper coin, cut off* so much of the new " light as to cause " shadows " on the fluorescent screen. This castmg of shadows suggests the idea of rays " of light travelling in straight lines; and their unknown chara(5feristics were, and are still, fitly indicated by the title X rays. If these X rays fall on the eye they pro- duce no visible effedl whatever ; when they fall on the fluorescent screen they excite 35 other rays which are visible to the eye. This is quite analogous to the visible fluor- escence excited by the invisible ultra violet hght falhng on quinine (see page 24). In making the experiment it is evident that the brightest fluorescence is produced when the paper is opposite that portion of the tube on which the Kathode rays fall, a {3.61 which led Rontgen to conclude that this portion is the starting place of the x rays. Fig. 15. — Shadows by x rays. In the ordinary Crookes' tube the Kathode rays, projecfted in straight lines, strike the opposite glass walls and cause green fluorescencu (see Chap. V). It is from this patch of green fluorescence the X rays start, travelling out thence in straight lines which are not necessarily con- tinuations of the Kathode paths. A suitable arrangement for casting the shadows is shown in fig. 15. The tube is on the left; the Kathode rays proceed 36 from K and strike the glass, causing the green fluorescence. The position of the anode A is here immaterial. From the end of the tube the X rays proceed in straight lines, dotted in the figure. They pass through the book, but some are stopped by the coin in the book. They then pass through the thin ebonite sheet to the fluorescent screen ; this screen shines brightly except where the coin casts its shadow by cutting off" the x rays. The ebonite is used to shut off* the ordinary light emitted by the luminous tube. The shadow image may be seen in a darkened room, or by looking through a black paper cone, fitting close to the screen so as to cut off" all extraneous light. Professor Salvioni is said to have invented a so-called cryptoscope " for seeing hidden objecfts." He made the black cone and fastened the fluorescent screen, salt inwards, at one end. This cone is pointed at the Crookes' tube, and the substance holding the hidden obje(5f " is placed between the tube and the cone ; exacftly as in the above figure. The credit of this " invention " lies entirely in the black cone used to shut out side light. Any photographer looks at the image on the ground glass of his camera, with his head under a dark cloth, or even uses a focussing lens in a tube, without claiming to have made a great invention. 37 Using either Rontgen's method or Salvioni's modification it is easy to find which bodies transmit and which stop the x rays. As the hst will be more fully discussed in the photographic secSlion, we shall only mention here that lighter bodies transmit the x rays, the heavier ones are more opaque. But it is well to state at once that there are no substances so opaque that the x rays cannot pass through thin sheets of them — there is only relative opacity. To obtain the same amount of absorption we must use thick or thin sheets according to the density, small or great, of the material. Wood, leather, paper, ebonite are very transparent, flesh fairly so, bones less so ; the heavier metals are much more opaque. Consequently dark shadows are cast by metallic objects, even when enclosed in wooden boxes, the nails of the box also cast their shadows. The method has been utilised for dete(5ling the presence of money, or other articles in postal wrappers and cases. Hence there have arisen such phrases as seeing the invisible," " looking through a block of wood," etc., which are, however, scarcely justifiable. We do not really see the obje(5ls, but only their shadows cast through the wood and other 'transparent" materials. The hand is a remarkable objecl:, for the flesh casts a faint shadow, the bones a deeper 38 shadow, metallic objedls on or in the hand, a still deeper shadow. The shadow picflures are precisely similar to the photographic picftures to be presently described. But it is necessary to add that Rontgen saw the shadows of his bones before he photographed them ; the credit has been wrongly assigned to Salvioni and others. We may add that, with better tubes and screens, shadows of all the joints, backbone, head, etc., have been cast and examined ; and X rays have been transmitted through the whole body, or even through the bodies of two men. One can make a good fluorescent screen thus : Grind pure barium platino-cyanide to finest powder ; mix it with some varnish, such as magilp. Spread it uniformly on a glass plate to a thickness of one-tenth of an inch ; cover the varnish with black paper or ebonite, it need not be dry first. The ebonite should be the thinnest sheet procur- able. The observer must place the screen between himself and the Crookes' tube ; the ebonite being nearest to the tube and the glass nearest to the observer. The objedls are placed betw^een the tube nnd ebonite, as near the latter as possible. Other materials have been suggested, such as potassium platino-cyanide and calcium tungstate. The former is said to 39 <^ive better results, but must be kept moist ; the latter is Edison's " invention " and must be properly crystallised (methods not yet described and otherwise the results seem no better than with the platino-cyanides). 40 CHAPTER VII. PHOTOGRAPHIC METHODS. Professor Rontgen is said to have dis- covered the photographic effecfts of x rays accidentally, but even so it was the sort of accident that only a man of genius would turn to good account ; in any case the analogies between ultra-violet rays and x rays would have soon led to experiments on photo- graphic plates. For photographic purposes the fluore- scent screen must be replaced by a box con- taining a photographic plate. Fig. 1 6 shows a suitable arrangement. The tube has its axis vertical, the kathode rays are shot downwards from K, render- ing the glass fluorescent at G G. From G G the X rays diverge outwards as in the doited lines ; some of them are stopped by the ohjecfh. They pass through the lid of the! box, and cast the shadow of the object on the plate. As the box is impervious to ordinary light, the operation need not be conducted in the dark. The a(5linic or chemical intensity of the X rays is comparatively feeble, so 41 that the "exposure" needs to be somewhat prolonged. The time of course depends on the distance between the source of the rays and the plate — for the intensity of " illumination " follows the usual rule, viz., it is inversely as the square of the distance ; doubling the distance quadruples the exposure. At a distance of six inches, exposures were often, ^ oeaec . '\ .^-^ \ . ' ■ 1 ' • * ■ • \ ^ Fig. i6. — Photography by x rays. in the earHer experiments, as much as one or even two hours. But with better tubes, particularly the focus tube (see page 46), the exposures have been much reduced. With an induction coil capable of giving a three-inch spark, a focus tube, and a rapid plate, distant, as before, six inches, the ex- posure for simple metallic objects may be only thirty seconds ; for a hand perhaps one or two minutes, for an arm or leg increased 42 periods according as the x rays have to pene- trate increased thickness. As to plates, any good fast plate is suit- able ; perhaps thickness of emulsion is even more important than speed, for it is obvious that the x rays pass through both the emulsion and plate. The x rays have been made to pass through a packet of 200 bromide papers, impressing the same image on the lot simultaneously. The photographic efFe(51:s are pracftically the same whether the emulsion be spread on glass, films, ferrotype, or paper. Professor Rontgen suggested that the photographic effedls might be caused either direcSlly or indire(511y. 1. Diredlly : i.e.^ the effedl of the x rays on the silver salts is similar to that of light waves. 2. Indiredlly : ix.^ the x rays cause fluor- escence in the glass, or other support of the film, and this luminosity causes the usual photographic efl'ecfl. It is evident that the fluorescence of the glass plate is unimportant, for the same effedl is produced on ferrotype, which does not fluoresce. Nevertheless the eflecfl may still be indiredf , being caused by the fluorescence of the gelatine, collodion, or the silver salts. It naturally occurred to many of us that the photographic effecft might be hastened by 43 placing a fluorescent screen in contatft with the emulsion, or by mixing fluorescent materials with the emulsion. The first method, in several hands, reduced the exposure to one-fifth the time, but gave less satisfa(5lory images. For there was a spreading of the light, a halation, as well as a granular appearance due to the particles of the fluorescent salt. If short exposures are a great desideratum the screen can be recommended. A convenient method, suitable for films is as follows : — Place the film, sensitive side downwards, on the fluor- escent screen : the x rays pass through the film and emulsion to the screen. The method is less suitable for glass plates owing to the much greater "opacity " of glass. The second method gives better results and undoubtedly hastens the exposures. In every case the development offers no special chara(5feristics ; as a rule a rather slow development seems most suitable ; and as the whole thickness of the emulsion is affecfted, development may be carried rather further than usual. It is sufficiently ob\ious that all ,i ray photography is only the photographing of shadows, though there may be, of course, "half- tone" in the varying intensities of shadows. It is desirable that the shadow images should be as sharp as possible. How are we to secure this ? 44 Fig. 17 shows the shadow cast by the ball K when the source of light is a single point S. A screen put at any distance behind K will receive a perfecSlly sharp shadow image. Fig. 18 shows the shadow cast by the ball B when the source of hght is another larger ball A. In the more deeply shaded portion the light from the luminous body is completely shut off : this portion is the umbra, or complete shadow. In the other portions there is light from some part, but not from the whole of the luminous body : these portions form the penumbra or part shadow. A screen placed at MN receives a blurred shadow, darkest in the middle, fading off towards the edges. The shadow is sharpest when the screen is as close as possible to the opaque objecff. If A were moved further from B, the umbra and penumbra would more nearly coincide, and the shadow be sharper. For sharp shadows it is, then, neces- sary: — K Fig. 17. — Shadow cast by luminous point. 45 (1) To use a small source of illumination. (2) To put the screen receiving the shadow close to the opaque objecft. (3) To put the luminous body as far away as possible. In radiography, No. i can be effecfted by the so-called focus tube (fig. 19). The concave kathode focusses " its kathode rays on the anode A, placed at the centre of the tube. This anode is a small plate of platinum inclined at 45° to the axis M Fig. 18. — Umbra and Penumbra. of the tube, this plate reflec51:s the kathode rays downwards to the glass, whence they diverge as x rays. When the indii(5\ion coil is properly conne(5\ed, all the glass in front of the anode shines with bright green fluorescence, behind the anode it is less luminous. The concentration of the rays on the anode, and their reflecftion thence, seem to lead to a greatly increased efficiency in producing x rays. These x rays diverge as if they had started 46 from the platinum anode A, and the source of illumination" being small the shadows are consequently sharp. With other Crookes' tubes the origin of the x rays has been stated to be that part of the glass bombarded by the kathode rays ; in this tube it appears to be the bombarded piece of platinum. In every case it is probable that x rays originate from all solid bodies bombarded by kathode rays. Some experimenters have therefore suggested that they should be called anode rays : but this is very unsuitable, as the fluorescent particles of glass are not necessarily anodes : anti-kathodic rays would be better as indicating their origin from the anti-kathode^ the part opposite to the kathode. The second desideratum for sharp sha- dows can be secured by having the opaque objecSl and the photographic plate separated by as thin a substance as convenient. The plate is often put in a black paper envelope on which the objedl, or hand, is laid. I have also used a box with a light-tight lid of thinnest Fig. 19.— The Focus Tube. 47 sheet ebonite which hes in contact with the enclosed plate, the object being laid on the ebonite. The third desideratum is only limited by our desire to have short exposures: the further we put the tube away, the longer the exposure, at twice the distance, four times the exposure ; thrice the distance, nine times the exposure, and so on. For most objecfts eight to eighteen inches will be considered convenient. We need not enter into details on the ele(ftrical appliances. Any good induction coil will do the work ; one giving not less than a two-inch spark in air is desirable. It can be worked by about six primary cells, Grove's, Bunsen's, or Bi-chromate. But for frequent use, three or four portable secondary cells are far more convenient : they can usually be recharged at a small rate at any place where dynamos and electric lighting are in use. If a current from the main is obtainable suitable resistances must be inserted, for the primary of an inducftion coil is only intended to carry currents of a few amperes. 48 CHAPTER VIII. RADIOGRAPHS. Having described the methods of radio- graphy we add a few examples. Fig. 20 is the radiograph of the contents of a closed wooden box. Fig. 20.— Contents of a Box. The objedls were — a watch key, a piece of glass, a wax vesta, a pencil with platinum wire wrapped round it, a small bradawl. The metallic objecfts are most opaque, and the 49 glass (probably flint glass) is also surprisingly opaque ; the wax again is much more trans- parent than the phosphorous head of the match, and the lead is visible inside the pencil. The positions and lengths of the nails in the wood-work are very clearly seen ; the pin was hidden in the wooden lid. Fig. 21 is the radiograph of five layers of tinfoil, made by pihng up the large circle, star shape, small circle, square, and lozenge- shaped pieces. Fig. 21.— Layers of Tin-foil. The increasing opacity, with increasing thickness is very obvious. It will be ob- served that the single layer is fairly trans- parent. We might here remind the reader that gold leaf, as thin as the hundred-thousandth of an inch, is transparent to ordinary light, or at any rate to green light : for such a piece of gold appears green when looked through, lo — D 50 But the transparency of metals for x rays is far in advance of this : for x rays havj been transmitted through an inch or more of alu- minium, and even faintly through a sovereign, though gold quickly absorbs these rays. We might also add that just as a gold leaf will seledl green light for transmission and not the whole of the white light, so it is probable that there is seledlive absorption in Fig 22 —A Perch. the case of x rays, for it is improbable that they are all of one wave length. It is hoped by this selecftive method to get x rays which will more clearly distinguish the various tissues of the body, and not only indicate bones as distincft from fiesh. Fig 22 is the radiograph of a perch. The skeleton and plates of the skull 51 corne out well ; the li^ht region in the middle is the inflated air-bladder. Fig. 23 shows the bones of a child's hand and forearm. The cartilage at the joints is pracftically as transparent as the flesh, hence the apparent intervals between the bones. The child was only five years old, and the photograph beau- Fig. 23.— Hand of Child. tifully illustrates the incomplete ossification : the heads and bases of the phalanges in the fingers are not yet united to the shafts, and the wrist mainly consists of cartilaginous material rather than of bones. Fig. 24 shows the contrast with the bones of an adult. 52 This radiograph was obtained from the hand of a mummy, probably a lady's hand, certainly over 3,000 years old. The thumb and httle finger have sustained damage, but the bones are still astonishingly clear. The bones have retained their original charadler- Fig. 24.— Hand of Mummy. istics, though the flesh is black and shrivelled. Parts of the cloth binding are visible between the fingers. The whole represents a curious intermingling of the science of ancient Egypt and of modern times. 53 CHAPTER IX. USES OF RADIOGRAPHY. It would be quite premature to attempt a complete list of the uses of radiography — in many respe(5ls the subjecfl: is only passing through its initial trials. We shall therefore give only a few of the results obtained. First we give one of Prof. Rontgen's tables indicating the varying opacity of metals. The sheets were made of such thickness that they cast equally dense shadow^s ; for comparison the platinum is said to be of unit thickness, the aluminium of unit opacity. Thickness. Opacity. Density. Platinum i 200 .. 21-5 Lead 3 . . 66-6 . . 11-3 Zinc 6 . . 33 3 . . 71 Aluminium 200 . . i . . 2 6 Obviously opacity depends on density, though no definite law has been discovered. Opacity does not vary direcflly as density, for platinum is 200 times as opaque as aluminium, though only a little more than 8 times as dense. The salts and compounds of the metals also, as a rule, increase in density and opacity 54 together ; their solutions behave similarly. The powdered material seems to have the same opacity as a corresponding thickness of the solid material. Powders, even of trans- parent materials, are very opaque to ordinary light, owing to the great amount of light which is scattered by refledlion at the innumer- able surfaces of the particles. Obviously x rays are not freely scattered — in fadl, they are refle(5led to a very small extent, even by powders. For the same reason the opacity of a material does not depend on the roughness of, its surface. Several experimenters consider that opacity depends on molecular weight rather than density. For example, mercury is less opaque than lead, their molecular weights being as 200 to 206, their densities as 13*4 to 11*3. However, there is some hesitation in accepting these comparisons of densities, so long as the measurements depend on estima- tion of the depths of shadows cast by a variable source like a Crookes' tube. It may however be safely assumed that the denser substances are more opaque, and as of course weight depends on density there seems some possible connecftion between x rays and the force of gravity, a connedlion which may perhaps lead to a solution of the great physical problem, gravitational attrac- tion. 55 The X rays do not dire(511y affecl the eyes, possibly they do not reach the retina, for the lens and humours of the eye, though not very dense, are found to be very opaque. The bones owe their greater opacity as compared with flesh to the presence of mineral matter, mainly calcium phosphate. If this be dissolved out by acids the remain- ing gelaiiuous substance is about as trans- parent as flesh. Metals and glass, particularly English glass containing lead, are more opaque than bones. The medical profession will decide for ilself how far radiography may be of service. 1^'or the detection of foreign bodies, such as needles, glass and even swallowed coins, its use is obvious and now frequent. Many photographs of bone have been taken to dis- tinguish dislocation from fracfture at a much swollen jomt, ankylosed joints, tubercular disease of bune, good or bad union after fracture, the exac5t nature of abnormalities such as a supernumerary phalanx. Bandages and splints are generally so transparent that they need not be removed except to improve the sharpness of the negative. The use of x rays for the softer tissues has been limited, but photographs have been taken of the cardiac region, of the neck show- ing larynx and trachea, and even with some success of the abdominal viscera. In par- ticular, diagnosis has been couflrmed and 56 assisted in cases of gall stones and renal calculi. The surgical uses will extend as tubes improve, and especially when the hetero- geneous X rays have been so sifted as to find those which will more clearly distinguish the softer tissues. More attention has been devoted to find- ing surgical uses than for other purposes. We have already mentioned how x rays, by the fluorescent screen, as well as by photo- graphy, can discover coins and other illicit matter in postal cases. They might be used on dynamite bombs or dangerous parcels ; they distinguish diamonds from the much more opaque paste. They show the charadler of a welding when the metal is thin enough. The engineer hopes for the time when cavities and flaws in much thicker pieces of metal will be made apparent. Of the many other uses mentioned in print the reader can doubtless discriminate between the genuine and the grossly exag- gerated accounts. 57 CHAPTER X. THE NATURE OF X RAYS. Most of the work on radiography has been in confirmation and extension of Prof. Rontgen's original results. A new discox^ery was made by Prof. J.J. Thomson in England, and other workers on the continent, who tried the effedl of x rays on elecftrified bodies. Prof. Thomson enclosed his Crookes' tube and indu(51:ion coil in a metal-lined case, so as to completely shut in all direcfl ele(5lro- static a(51ion. The x rays from the tube passed out of the box through an aluminium window, and then fell on the electrified body, which was in connection with a gold leaf elecftroscope or other instrument for indicating ele(5lric charges. As soon as the x rays fall on the body they begin to discharge it, as shown by the falling together of the previously divergent gold leaves. The discharge is effected whether the original charge be positive or negative — even if the elecflrified body be buried in insulating material like wax. The experiment may be worked by the apparatus shown in fig. 25. 58 The large metal sheet screens off dire(fl: elecStric acftion ; the x rays pass through the aluminium window, and fall on the copper plate buried in wax. The copper plate is connedled by a wire with the charged gold leaf elecftroscope : the diverging leaves soon fall when the x rays reach the copper plate. The wax is no longer able to insulate the ele(51:ric charge, which consequently passes away. All insulators through which x rays are passing become, for the time being, con- ductors of electricity. Fig. 25.— Ele(5lrified Plate Discharged by x Rays. This method has been used by Prof. Thomson for accurate quantitative measure- ments, the time required for complete dis- charge being a measure of the intensity of the X rays. It was previously known that ultra-violet light had a similar power of discharging elecSlrified bodies, but only if the original chiirge be negative. Indeed, the analogies between x rays and light have now become so close, that the only 59 theory which gains much credence is that which regards x rays as due to transverse ether vibrations, similar to, but much more rapid than, ordinary Hght vibrations. It is of interest to see why Prof. Rontgen was originally led to consider them as longi- tudinal vibrations. His argument was based on the behaviour of crystals towards x rays. Light vibrations, passing through crystals, undergo various modifications on account of their transverseness. For example, Iceland spar breaks an ordinary beam of light into two; the light is doubly refracted into two sHghtly divergent direcftions, and double images are produced. But X rays pass through this crystal without double refraction, producing oniy one image, as they would do if due to longitudinal vibrations. But it is unsafe to conclude that they are necessarily longitudinal, for they are not refrac^ted at all, but pass straight through without deviation. There is a natural desire to explain the new phenomenon without attributing new and unusual properties to the ether ; and scientists do not see the necessity for longi- tudinal ether vibrations, if x rays can be accounted for by transverse ether vibrations. Now the formulcU deduced by mathema- ticians indicate that the refradfive indices for all substances transmitting transverse vibra- tions approach unity, when the wave lengths 6o are so small as to be comparable with the size of the molecules. If the waves are sufficiently small they will be neither singly nor doubly refracSted, but pass straight through any sub- stance, just as X waves do. Again, transverse light vibrations which pass through a tourmaline crystal are polarised — ix.^ the vibrations are all made perpendicular to a particular plane dependent on the crystal. Such polarised light can pass through a second tourmahne placed '-^ pavalUV — i,e.^ in a position similar to the first ; but cannot pass if the second tourmaline be crossed " — i.e., turned through an angle of 90*^ compared to the first. Longitudinal vibrations cannot be polarised. Can X rays be polarised ? Prince Galitzine and M. de Karnojitsky have got more x rays transmitted through parallel than through crossed tourmalines. If this is confirmed, the polarisation, and there- fore transverseness of x rays are undoubtedly established. But unfortunately this experi- ment is hardly yet decisive, for other experi- menters have not obtained the same result, which may depend to some extent on the exadl composition of the crystals. Though Prof. Rontgen got no good evidence of reflecftion, other experimenters have. They have placed the photographic plate at the extremity of a tube made of thick 6i metal, impermeable to x rays. The x rays can only enter the tube at the open end where a polished metallic reflecflor is placed. In this way some small percentage of the x rays has been reflecfted down the tube so as to produce the usual photographic effecSl. The percentage is never very high, perhaps not over two per cent. ; it is greatest when the incident and reflected radiation are nearly parallel to the surface. The difficulty in reflecflion is due to the surface being insufficiently smooth. The reflecting surface must have its roughness, its hills and valleys, small in comparison with the wave length of the reflecfled radiation. Rough rocks reflect sound, polished mirrors reflect light, a still higher degree of smoothness is probably required for the minute x waves. Recently, another remarkable connection between light and x rays has been discovered by Mons. Becquerel. He has, in fac^t, found a missing link between ultra-violet light and X rays, filling up one of the gaps in the series of ether vibrations. When sunlight falls on the double sul- phate of potassium and uraniun^ the salt fluoresces. On removal from sunlight the visible fluovesccnce lasts for an exceedingly short time, not more than ^^-^ of a second. But an invisible fluorescence , if one may use the term, continues for many hours and days afterwards. 62 In later experiments, Mons. Becquerel used the metal uranium, obtaining better results than with its salts. These fliwrescent radiations have some of the properties of x rays, and some of the properties of ordinary light. They can pene- trate " opaque " substances, such as black paper, thin aluminium and copper, and pro- duce the now familiar shadow photographs. They can discharge ele(51:rified bodies, whether the charge be positive or negative. In all these respedls, however, their effecSls are much feebler than those produced by x rays. The fluorescent rays differ from x rays in- asmuch as they can be refleCled and polarised like ordinary light. They are, therefore, certainly transverse vibrations, and add to our belief that Rontgen vibrations are also transverse. They are intermediate between light and x rays, and would appear ia the gap between ultra-violet and Rontgen vibrations in the table given on page 15. This table is then a record of some of the work on ether produced during the present century by scientists of the three great European nations, the English, French and German. A friendly rivalry during the next century will doubtless extend this work, with results, which, in their pra(5lical applica- tions, will be beneficial to all mankind. Scientific discoveries are always unfold- ing the wonders of the universe, and in no 63 direcflion is the work more hopeful than in this conne(51ion between elecflricity and ether vibrations. Even before Prof. Rontgen's dis- covery, Sir Douglas Galton could make use of such words as these: — It is only within the last few years that we have begun to realise the close connecflion between elecl:ricity and those vibrations which cause heat and light, and which seem to pervade all space — vibra- tions which may be termed the voice of the Creator calling to each atom and to each cell of protoplasm to fall into its ordained position, each as it were a musical note in the harmonious symphony which we call the Universe." Advts.] THE Bull sEye Cahera 1896 Model. A Camera for film, which is unloaded and reloaded in daylight. Noteworthy Features: COMPACTNESS, LIGHTNESS, SIMPLICITY. 5ize:— Only 4.} x 4^ x 5] inches. Weig^ht: -lyOaded for 12 pictures, only 21 ounces. Size of Picture:— 3} x 3} inches. Lenses:— The lenses are of the finest quality; strictly achromatic, and perfect in definition. 5tops:— The Bull's-Eye Camera is provided with a .set of three stops. The Shntter is of an improved rotary form, and is always set. It is actuated simply by pushing: a lever alternately to the right or left. Arranged for both time and instantaneous exposures. The stops and time movement arc regulated by two slides which are near the exposure lever. The number of each exposure is always distinctly visible. The Bull's-Eye Camera is provided with a brilliant view finder, showing the exact scope of view taken in by the lens. The Camera may be used in the hand or on a stand. The Bull's-Eye is covered with black leather, is handsomely finished, and is a piece of high-class workmanship throughout. Price, including a roll of film for 12 exposures, £l 13s. Afade solely by F A M A ISJ Photographic 1 iTirVi^ Materials Co. Ltd. Manufacturers of the famous KODAK. 1 15- 1 17 Oxford St., London, W. Paris: 4 Place Vendomc. Rochkstkk, N.Y., U.S.A., Eastman Kodak Co. TO E [Advts. Why not read, think and keep pace with evolution? Why not know something of photography other than rule of thumb? Why not consider the work and writings of others ? Why not avoid as much as possible the expensive school of experience ? Why not read The Practical Photographer? Monthly. Illustrated. Threepence. Pith of Matters Photographic. Percy Lund & Co., Ltd., The Country Press, Bradford; and London. Advts.] Established 1830. DOSS & CO., Manufacturers of Celebrated ROSS' ZEISS and GOERZ Photographic Lenses. (More \ ai iety tlian any other I a u M.r.i , Mctiirer in the World.) Superior Cameras — for - The Field The Studio Process Wori< Etc., Etc. Several very tine New HAND CAMERAS for Season 18S6. Lanterns. Science, Enlarging and Ordinary Projection. New Models, superior to all others, also a New Arc Lamp, Etc., Etc. CATALOGUES FREE. ROSS & CO., m, New Bond St., London, W. [Advts. THE GREAT CITY DEPOT for Lancaster's and Underwood's Cameras ALL PHOTOGRAPHIC ACCESSORIES. Catalogue, ""."'?i?uV/aiions. free. Benetfink & Co., 89, 90, 107 & 108, Cheapslde, LONDON. Advts.] Jas. Lancaster &Son,^ Opticians, BIRMINGHAM. The Largest Makers of Photographic Apparatus in the World. Upwards of 160,000 Cameras Sold. Lancaster's "Le Meritoire." (Fat.) Jpl.,31/6; Apl.,63/- 1-1 pi., 90'-. Complete Catalogue, 4d. Abridged List Free. Lancaster's *■ Instantograph.' (Pat.) i plate . . 42'- A plate . . 84/- 1-1 plate . . 136/- Lancaster's Hand-Camera, The *RoYer' (Pat.) Jpl, 63/- Only one plate in camera during exposure. Simplest ever made. Slmtter always set, any speed. Lancaster's Amateur Enlarging and Home Lantern. With 4 in. Condensers, Achromatic Front, and 8 wick Lamp, 21/0. Ditto, with rack front, 2S/0 [Advts, The Amateur's Magazine. Conduded by Matthew Surface. Editor of "The Pradlical Photographer ' 15th of Month. 3d. Post=free, 4Ad. Specimen of Illustrations. Useful Articles. Splendid Illustrations. Photographs Criticised. OF ALL DEALERS. Advts. Archer's Latest Hand=Caniera. The "Tourist' Recommended by the i:ditor of •' The Praaical Pliotographer." This Camera is made to give twelve good negatives from twelve plates with certainty. Dark Slid(!S being the most reliable, we have designed this Camera to carry Six Double Slides holding Twelve Plates, all of whicii iit into the back j'ortion of the Camera Case for convenience. The Camera is fitted with a Hellows Front for focussing, worked by a Rack from the outside, and has a correct Scale Platt^ of distances. Rising hront. Thornton-Plckard instantaneous Shutter (various speeds). Two Finders. Focussing; Screen with door behind. First Quality Rapid Rectilinear Lens witii Improved "Iris" Diaphrasrms. The Case is Mahogany, covered with Morocco Leather, and the whole instrument beautifully made and finished. Price complete as described, f ^ - dLJ ,0.0, with six Double Slides, ^-plate Can be uscnl on a Stand ^'Tourist^' No. 2, ^5 5 0. More Portable, holding one slide at a tiine. Archer's Celebrated "Express'* Hand^Camera, Price £2. 2. o. 20 different cameras, all kinds, all prices. List 132 pages, post-free, i^d. ARCHER & SONS, I>hotograp!iic Stores, 43 to 49, Lord St., LIVERPOOL. ( E<.td. 1S48 J [Advts. HORNTON-0ICKARD Patent Time and Instantaneous SHUTTER Is both theoretically and pradtically the most efficient in the market. It gives any exposure, from fracStions of a second up to minutes or hours, without vibration. See Catalogue for ull particulars. Largest Sale in the World. Price from 18/6. Speed Indicator (recommended) 3/6 extra. Snap-Shot Shutter from 10/-. Catalogue free. The THORNTON-PICKARD MANUFACTURING CO., ALTRINCHAM, Nr. Manchester. SOME OF PERCY LUND & CO.'S ALBUMS " HAUNTS OF MEMORY." A bijou album, holding twelve I plate unmounted prints, and one in the cover. Handy for the pocket, or as a present to a friend. Remarkably cheap. Price 6d. ; post-free, 7d. "SUNSCAPES" ALBUM. An elegant pattern, handsomely bound in leather, with gilt edges, and highly finished in every respe(5t. Holds one photo- graph on a page, which is "slipped in." Leaves of antique rough cardboard. I plate, 4/- ; Cabinet, 5/6; J plate, 6/- ; Whole plate, 10/-. The same Album, in cloth binding, sprinkled edges : — i plate, 2/6 ; ^ plate, 6/- ; Whole plate, 5/. The Country Press, BRADFORD; and LONDON. Advts.] Apparatus for X Ray Photography Supplied by ... . W. BUTCHER&SON BLACKHEATH, S.E. Price List on application. [Advts. VflCUUlVl TU BES for producing 'X' Rays. A. C. Cossor, original maker in England of above, supplies tubes, each of which is guaranteed. Dr. Hall Edwards writes: — "I find your tubes far superior to any others I have tried." J. W. GiFFORD, Esq, writes: — "All the tubes you have sent me latterly have been unmatched by those of any other maker." A. C. C0550R, Experimental Glass Blower, Manufacturer of Incandescent Electric Lamps^ 67, Farringdon Road, E.C. Advts.J R, &J. BECK. 68 COPNHILL LONDON. FRENA HAND CAMERA is the caiiu l a of the day, it carries FORTY FLAT FILMS " LIKE a PACK of CARDS." It has a Beck Lens. Equally serviceable for the expert or the complete novice. The Frena Film Holder can be fitted to any camera. Quarter plate and Half plate sizes ready. It carries Twenty Films " Like a Pack of Cards." R.&J. BECK, Ltd., 68, Cornhill, London, E.G. West Vlnd Agents — London Stereoscopic Co., 1C6 & 108, Recent St., W. [Advts. The ''LUND" LIBRARY of PHOTOGRAPHY IN TWO-SHILLING VOLUMES, NET. A series of text books devoted to the branches and applications of photography Plain wording and explicit teaching is aimed at as far as possible. The Stereoscope and Stereoscopic Photography. Translated from the French of F. Drouin by Matthew Sur- face. 180 pages. More than 100 illustrations. The Half=Tone Process. By Julius Verfasser. A Practical Manual of Photo-Engraving in Half-Tone on Zinc and Copper. 172 pages and 75 illustrations, with four engravings in half-tone by the Author. Second Edition, in great part re-written, and revised up to date. Photographic Lenses: How to Choose and How to Use. By John A. Hodges. An Elementary and Prac- tical Guide to the selection and use of Photographic Objectives. 148 pages and 36 original illustrations, including eight half-tone engravings. Half=Tone, on the American Basis. From the Personal Experience of Wilhelm Cronen- BERG, Translated by William Gamble. 164 pages, with 56 illustrations in the text, and twelve full-page illustrations. Photography for Artists. By Hector Maclean, F.C.S., F.R.P.S. Brief and ■useful information respecting the many uses of photo- graphy in various walks of the pictorial and allied arts.. 152 pages, 19 diagrams and illustrations, and an appen- dix containing 18 supplementary illustrations. The Following is in Active Pyeparation : Plates and Papers: How Made and Used. By Dr. H. 0. Stiefel. PERCY LUND & CO., LTD., The Country Press, BRADFORD; and London. Advts.] Hughes' Patent Bijou ENLARGING Portability, Perfection, and Rapidity. LANTERN Tlie only Perfedt Enlarging Lantern for results. Fitted with Hughes' new pattern Pamphengos Lamp, arranged to suit any size Enlargement. The boay and front lens extend by means of solid brass tube runners, very rigid and firm. No Photographer should be without one. PAKTTCin.ARS KRKK. Instead of Circular Condensers, Kectangular or Sqnar^>. reduces the lantern to half its si/e, and gives finer definition than any other. Perfe(5t combustion and a pure white light. Tliis instrument is scientifically considered, and not the common commercial article sold for cheaprevs Prices- 8J X 6.^, £2t 10s.: f>h ^ £14 14s.; £7 185. SPECIAL BIJOU- 3x4. £8 13s. Hughes' Patent Rectangular Condensers (Tsod 1)V Van 'Lt \\ vM\e, Ksq.) 8§x6i, £7 lOs ; 6^ x 4:^. £3 5s.: 4i ^ 3.i. *2 3s. Before piirchasinp, see Riandly li.r.usrRATKn C»talo<;i:k, ov< r 160 Encrav- infijB, not made up with Trade Hlocks borrowed from wholosale houBes, but oiiginal prodnctionH nf Macir LantornH and Dis.solvinR ViewH. the finest and cheapest in the world. TriploR. niimials. Pamplientjos, Nov* IticB and KtTects, etc. Price fid., postage. 'Mi.; ditto of (".0,000 slides, 4d.. postage 2d. Pamphlets free. THE MARVELLOTTS D A mi DU C M/^/^O The Most Powerful Oil r MmrriClll^awO- Llghtin theMarket. Over 3000 sold. SIMPLE. RELIABLE. SAFE. It has challentK'd compariHon for over 1 1 yoais. No Smell. No Smoke. No Broken Glasses. Prices— £6 6s., £4 4s., £2 10s. The Docwra Triple Prize Medal, hijihest award. Supplied to Dr. H. Grattan Guinness, Madame Adelina Patti, and the Royal Polytechnic, etc., etc., B. J. Maiden, Esq., Capt. Chas. Reade, R.N., and principal exhibitors. The Universal Lantern, four wicks, four inch condensers, £1 Is. Od.; three wicks, 19/6; elej^ant mahogany biunials, brass fronts, £6 10s.; blow-throneh jets, 8/6; mixed, 12/0; life models, sub- jedls coloured, 1/3 each ; Scripture, hymns, temperance tales, etc. The Cheapest atid Best Hotise for complete Lantern Outfits. WC HIinHFC Patentee. BREWSTER HOUSE. . IIUUIICO, MortimerRd.,King8land, LONDON, N. [Mvts. A Selection from . . . PERCY LUND & CO.'S General Photographic Books Burton's Manual of Photography. By W. K. Burton, C.E. A practical handbook for all who are taking up photography. An explicit guide to all ordinary photographic manipulations. The latest information. With examples of the author's own work. 184 pages, well illustrated. Paper covers, 1/0 net. Talks on Pen and Ink. By Elizabeth M. Hallowell. Many photo- graphers would like to be able to sketch, so as to dot down ideas or little bits too small to spare a plate for, or too inconveniently situated to enable the camera to perform its work properly. Besides, how nice it is to send a manuscript to some magazine, illustrated not only with photographs, but with little pen and ink sketches. When a good elementary book on the subject of drawing can be purchased for a shilling, and when a few hours' practice will confer a moderate degree of proficiency, there is no need to hesitate. Fifty-two pages. Crown 4:to. 1/0 net. Lund's Directory of Photographers. Contains over 15,000 entries of Photographers, Photographic Material Manufacturers, Merchants, Dealers and Shippers in the British Isles, the British Colonies, etc. Strongly bound in cloth. 196 pages. Boyal 8vo. 7/6 net. The Elements of a Pictorial Photograph. By H. P. Robinson. Synopsis of Chapters; Introduction — Imitation — The Study of Nature — The Use of Nature — Some Points of a Picture — Selection and Suppression — Composition — Expression in Land- scape — Idealism, Realism and Expressionism — Limita- tions. The Nude — False Purity — The Question of Focus — Models — Foregrounds — The Sky — The Sea — Rural Subjects — Lessons from Birket Foster — Winter Photo- graphy — Individuality — Conclusion. Demy 8vo., haif bound. 1 68 pages, with frontispiece and 36 pictures in the text. 3/6 net. PERCY LUND & CO., LTD., The Country Press, BRADFORD; and London. Advts.] Apparatus for "X WS " PriOTOCRAPtiV. FOCUS TUBES, 25/- each. Each tube is tested before being sent out, and will produce brilliant negatives with the shortest exposures. INDUCTION COILS, From £8 los. FLUORESCENT SCREENS, Clearly showing the l^ones of Hand, Arm, etc. BICHROMATE BATTERIES, ACCUMULATORS. PRICE LIST POST-FREE ON APPLICATION. The Apparatus can be Seen and Tested at our Show Rooms. G. HOUGHTON & SON, 89, HIGH HOLBORN, W.C. Telegrams: "Bromide, London." [Advts. RONTGEN'S PHOTOGRAPHY Fig. I. Sets of Superior *X' RAY Apparatus, as used at the Leeds General Infirmary, Hull Royal Infirmary, West Riding Asylum (Wakt field), and other Institutiops. COMPLETE SETS, includin? in. Coil, w^.^ Battery, and Focus Tube, etc., from— Catalogue of ^X' RAY APPARATUS free on application Fig. 2. REYNOLDS and BRANSON'S 'Phoenix' Lamp. Fig 2. This Lamp is a boon to all photographers who have gas at their disposal. The lamp is iii ins. high and 7f ins. wide, has double casing, which secures a cool ex- terior, good ventila- tion, and absence of smell. PRICE 15/=. PORTABLE HYDROKINONE DEVELOPER, In two tubes, only requires the addition of water. Price: |pint,6d., post, 2d.; Pint,8d., post, 2d.; Quart, i/=, post, 3d. ILLUSTRATED CATALOGUES GRATIS. REYNOLDS & BRANSON, '