---

79. Roentgen’s experiments. X-Rays, and A New Art. Wurz. Physik. Med. Gesell. Jan. ’95; Nature, Lon., Jan. ’96; The Elect., Lon. April 24, 96; Sitz. Wurz. Physik. Inst. D. Uni. Mar. 9, 96.—Uninfluenced By A Magnet In Open Air.—Although Lenard recognized several kinds of cathode rays, which differed as to penetrating and phosphorescing power, yet he always held, or inferred at least that they were deflected by a magnet, outside, as well as inside, (proved § 72a.) of the discharge tube. § 59. Prof. Wilhelm Konrad Roentgen subjected his newly discovered rays to the action of very strong magnetic fields in the open air, but no deviation was detected. This is the characteristic which more than anything else has served to distinguish X-rays from cathode rays. This property has been confirmed by others. He employed the principle of magnetic attraction of internal cathode § 59, rays to shift the phosphorescent spot, for then he noticed that the source of X-rays fluctuated also.

80. Source of X-Rays may Be At Points Within The Vacuum Space.—In one case, he employed a Lenard tube, and found that the X-rays were generated from the window which was in the path of the cathode rays. § 67. Different bodies within the discharge tube were found to have different quantitative powers of radiating X-rays when struck by the cathode rays. He stated “If for example, we let the cathode rays fall on a plate, one half consisting of a 0.3 mm. sheet of platinum and the other half a 1 mm. sheet of aluminum, the pin-hole photograph of this double plate will show that the sheet of platinum emits a far greater number of X-rays than does the aluminum, this remark applying in every case to the side upon which the cathode rays impinge.” On the reverse side, however, of the platinum, no rays were emitted, but a large amount was radiated from the reverse side of the aluminum. § 67. He admitted that the explanation was simple; but, at the same time, he pointed out that this, together with other experiments, showed that platinum is the best for generating the most powerful X-rays. One form with which he experimented is illustrated in Fig. J, in principle, being described as a bulb in which a concave cathode was opposite a sheet of platinum, placed at an angle of 45° to the axis of the curved cathode, and at the focus thereof.

81. Reflection of X-Rays.—He emphasized the knowledge that there is a certain kind and a certain amount of reflection, such as that produced upon light and, as pointed out by Lenard, upon cathode rays, by certain turbid media. The following quotation sets forth the exact experiment to show slight reflection at metal surfaces. “I exposed a plate, protected by a black paper sheet 1 to the X-rays (e.g. from bulb J) so that the glass side 2 lay next to the discharge tube. The sensitive film was partly covered with star-shaped pieces (4 slightly displaced in the Fig.) of platinum, lead, zinc and aluminum. On the developed negative the star-shaped impressions showed dark (comparatively) under platinum, lead and more markedly, under zinc; the aluminum gave no image. It seems, therefore, that the former three metals can reflect the X-rays; as, however, another explanation is possible, I repeated the experiment with only this difference, that a film of thin aluminum foil was interposed between the sensitive film and the metal stars. Such an aluminum plate is opaque to the ultra-violet rays, but transparent to X-rays. In the result the images appeared as before, this pointing still to the existence of reflection at metal surfaces.”

82. Penetrating Power. The transmitted energy was tested both by a fluorescent screen and by a sensitive photographic plate. Either one was acted upon by the rays after transmission through what have ordinarily been called opaque objects. § 68. for example, 1000 pages of a book. As in Lenard’s results, so in Roentgen’s, the color of the object had no effect, even when the material was black. § 68, near beginning. A single thickness of tinfoil scarcely cast a shadow on the screen. § 66a. The same was true with reference to a pine board 2 or 3 cm. thick. They passed also through aluminum 15 mm. thick. 63b. Glass was comparatively opaque, § 66a, as compared with its power of transmitting light, but nevertheless it must be remembered that the rays passed through considerable thickness of glass. The tissues of the body, water § 68, near centre, and certain other liquids and gases were found exceedingly permeable § 67. Fluorescence could be detected through platinum 2 mm. thick and lead 1.5 mm. thick. Through air the screen was illuminated at a maximum distance of 1 m. A rod of wood painted with white lead cast a great deal more shadow than without the paint, and in general, bones, salts of the metals, whether solid or in solution, metals themselves and minerals generally were among the most resisting materials. § 155. The experiments were performed in a dark room by excluding the luminosity of the tube by a thick cloth or card board entirely surrounding the tube. He performed the wonderful experiment, so often since repeated, of holding the hand between the screen of barium platino cyanide and the discharge tube, and beholding the shadow picture of the bones. This was the accidental step which initiated the new department of photography, and which gave to the whole science of electric discharge, a new interest among scientists and electricians and which thoroughly awakened popular interest. The whole world concedes to him the honor of being the originator of the new art. In view of sciagraphs of the bones of the hand upon the screen, it occurred to him in view also of Lenard’s experiments, on the photographic plate, to produce a permanent picture of the skeleton of the hand with the flesh faintly outlined. § 84. The accompanying half tone illustration, page 37, was made by the Elect. Eng. N.Y. (June 3, ’96) by permission, and it represents the Edison X-ray exhibit at the New York Electrical Exposition of the Electric Light Association, 1896. Thousands of people, through the beneficence of Dr. Edison, were permitted to see the shadows of their bones surrounded by living flesh. The screen was made of calcic tungstate. The hand and arm were placed behind and viewed from the front. § 132, near beginning.

83. Penetrating Power and Density of Substances.—Although he found that there was some general relation between the thickness of materials and the penetrating power, yet he was satisfied that the variation of the power did not bear a direct relation to the density, (referring to solids) especially as he noticed a peculiar result when shadows were cast by Iceland spar, glass, aluminum and quartz of equal thickness. The Iceland spar cast the least shadow upon suitable fluorescent or photographic plate. The increased thickness of any one substance increased the darkness of the shadow, as exhibited by tinfoil in layers forming steps. Other metals, namely platinum, lead, zinc and aluminum foil were similarly arranged and a table of the results recorded. § 63b.

He concluded from these data that the permeability increased much more rapidly than the thickness decreased.

84. Fluorescence and Chemical Action. § 70 and 63a.—Among the substances that fluoresced were barium platino cyanide, calcium sulphide, uranium glass, Iceland spar and rock salt. In producing sciagraphs on the photographic plates, he found it entirely unnecessary to remove the usual ebonite cover, which, although black, and so opaque to light, produced scarcely any resistance to the rays. The sensitive plate, even when protected in a box, could not be kept near a discharge tube, for he noticed that it became clouded. He was not sure whether the effect upon the sensitive plate was directly due to the X-rays or to a secondary action, namely, the fluorescent light which must have been produced upon the glass plate having the film, it being well known that light of fluorescence possesses chemical power. He called attention to the fact that inasmuch as fluorescent light which can be reflected, refracted, polarized, etc., was produced by the rays; therefore, all the X-rays which fell upon a body did not leave it as such. § 67. No effect was produced upon the retina of the eye although he temporarily concluded that the rays must have struck the retina in view of the great permeability of animal tissue and liquids. § 68, at end. Conclusions of this kind not based on experiment, are never reliable, even when offered by very high authorities. Again the rays were weak. Roentgen himself admitted that the salts of metals in solution (§ 82, near centre) rendered the latter rather opaque. The eye ball is continually moistened with the solution of common salt. Further than this, Mr. Pignolet noticed in Comptes Rendus, Feb. 24, ’96, an account of an experiment of Darien and de Rochas. In anatomy it is common to experiment on fresh pig’s eyes in order to make comparisons with human eyes. The above named Frenchmen submitted the former to X-rays. The eyes were but slightly permeable thereto.

85. Non-refraction and But Little Reflection of X-rays.—He employed a very powerful refracting prism made of mica and containing carbon bi-sulphide and water. The same prism refracted light but did not refract X-rays. No one would think of making prisms for examining light, of ebonite or aluminum, but he made such a prism for testing X-rays. But if there were any refraction he concluded that the refractive index could not have been more than 1.05, which may be considered as a proof that the rays cannot be refracted. He tried heavier metals, but the difficulty of arriving at any satisfactory results was due to the resistance of such metals to the transmission of the rays. Among other tests was one consisting in passing the rays through layers of powdered materials through which the rays were transmitted in the same quantity as through the same substances not powdered. It is well known that light passed into powdered transparent materials, is enormously cut off, deviated, diffused, refracted etc., in view of the innumerable small surfaces of the particles. Hence he concluded that there was little if anything in the nature of refraction or reflection of X-rays. § 146. The powdered materials employed were rock salt, and fine electrolytic and zinc dust. The shadows, both on the screen and as recorded on the photographic plate were of substantially the same shade as given by the same materials of the same thickness in the coherent state. One of the most usual ways of testing refraction of light is by means of a lens. X-rays could not be brought to a focus with the lens of what ever material it was made. Among the substances tried were ebonite and glass. As expected, therefore, the sciagraph of a round rod was darker in the middle than at the edges; and a hollow cylinder filled with a more transparent liquid showed the centre portion brighter than its edges. If one considers this observation in connection with others, namely the transparency of powders, and the state of the surface not being effective in altering the passage of the X-rays through a body, it leads to the probable conclusion that regular reflection does not exist, but that bodies behave to the X-rays as turbid media to light, § 69.

86. Velocity of X-Rays In Different Bodies. p. 46.—Although he performed no direct experiment in this direction yet he inferred in view of the absence of refraction at the surfaces of different media, that the rays travel with equal velocities in all bodies.

87. Double Refraction and Polarization.—Neither could he detect any action upon the rays by way of refraction by Iceland spar at whatever angle the crystal was placed. As to this property of light see Huygen’s Works of 1690 and Malus’ Works of 1810. quartz also gave negative results. Prof. Mayer of Stevens Institute submitted to Sci., Mar. 27, ’96, the report of a crucial test for showing the non-polarization of X-rays. On six discs of glass, 0.15 mm. thick and 25 mm. in diameter, were placed very thin plates of Herapath’s iodo-sulphate of quinine. The axes of these crystals crossed one another at various angles. When the axes of two plates were crossed at right angles no light was transmitted; the overlapping surfaces of the plates appearing black. If the Roentgen rays be polarizable, the Herapath crystals, crossed at right angles, should act as lead and not allow any of the Roentgen rays to be transmitted. Prof. Mayer is well known as exceedingly expert in connection with minute measurements and in the manipulation of scientific experiments. Dr. Morton, Pres. Stevens Inst., attested the results as an absolute demonstration that X-rays are incapable of polarization. Stevens Indicator, Jan., ’96.

88. The Propagation of X-Rays Rectilinear.—There would be no difficulty in producing photographs of the bones of the hand with the rays of light, if it were not for the tremendous amount of reflection and refraction causing so much diffusion that no sharply defined shadow of the bones would be produced. By means of a powerful lens and a funnel pointed into a dark room, the author noticed that the condensed light thereby obtained when passed through the hand, and when the incident rays were parallel, came out so diffused that one would think that the light went through bones as easily as any part of the hand. An experiment of this kind serves to emphasize that the success of sciagraphy by X-rays is due not only to the great penetrating power, but to practically no refraction nor reflection. In view of the sharp shadows cast of objects even when located in vegetable or animal media, Roentgen was justified in giving the name of ray to the energy. He tested the sharpness of the shadow by making sciagraphs and fluorescent pictures not only of the bones of the hand, but of a wire wound upon a bobbin, of a set of weights in a box, of a compass, card and needle, conveniently closed in a metal case, and of the elements of a non-homogeneous metal. To prove the rectilinear propagation further, he received the image of the discharge tube upon a photographic plate by means of a pinhole camera. The picture was faint but unmistakable.

89. Interference. The rays of light may be caused to interfere with each other. See Newton’s Principia, Vol. III.; Young’s Works, Vol. I.—Theory points out that waves of ether of two pencils of light, when caused to be propagated at certain relative phases partially or wholly neutralize or strengthen each other. Roentgen could obtain no interference effects of the X-rays, but did not conclude that the interference property was absent. He was not satisfied with the intensity of the rays and therefore could not test the matter severely.

90. Electrified Bodies Discharged by X-Rays. p. 47.—After Roentgen’s first announcement, others, and probably J. J. Thomson as the first, found that the X-rays would discharge both negatively and positively electrified bodies. Roentgen, in his second announcement, stated that he had already made such a discovery, but had not carried the investigation far enough to report satisfactorily on the details. At last he put forth an account of the whole phenomena and stated that the discharge varied somewhat with the intensity of the rays, which was tested in each instance by the relative luminosity of the fluorescent screen, and by the relative darkness produced upon the photographic plate in several instances. Electrified bodies, whether conductors or insulators, were discharged when placed in the path of the rays. All bodies whatsoever behaved in the same manner when charged. They were all discharged equally by the X-rays. He noticed that “If an electrical conductor is surrounded by a solid insulator such as paraffin instead of by air, the radiation acts as if the insulating envelope were swept by a flame connected to earth.” Upon surrounding said paraffin by a conductor connected to earth, the radiation no longer acted on the inner electrified conductor. The above observations led him to believe that the action was indirect and had something to do with the air through which the X-rays passed. In order to prove this, it was necessary for him to show that air ought to be able to discharge the bodies if first subjected to the rays, and then passed over the bodies. The apparatus for performing an experiment to test this prediction is shown in Fig. L, which serves to illustrate also the manner in which he prevented electro-static influences of the discharge tube, leading in wires and induction coil. § 71, near centre. For this purpose he built a large room in which the walls were of zinc covered with lead. The door for his entrance and exit was arranged to be closed in an air-tight manner. In the side wall opposite the door there was a slit 4 cm. wide, covered hermetically with a thin sheet of aluminum for the entrance of X-rays from the vacuum tube outside of the room. All the electrical apparatus connected with the generation of the X-rays was outside of the room. No force whatever came into the room, therefore, except the X-rays through the aluminum. § 71. In order to show that air which had been subjected to the X-rays would discharge a body immediately afterwards upon coming in contact therewith, he arranged matters so that the air was propelled by an aspirator. He passed air along a tube made of thick metal so that the rays could enter only through a small aluminum window near the open end. At over a distance of 20 cm. from the window was an insulated ball charged with electricity, and connected to any electroscope or electrometer. The professor used a Hankel electroscope. No published sketch was made by Roentgen; therefore, that shown in the figure was produced by inference from the description. The operation was as follows: The X-rays passed into the room through the aluminum window, and then into the metal tube through its aluminum window. When the air was at rest, the ball was not discharged. When the aspirator was at work, however, so that the air moved past the aluminum window and past the ball, the latter was discharged whether electrified positively or negatively. He modified the operation by maintaining the ball at a constant potential by means of accumulators, while the air which had been treated by X-rays was passed by the ball. “An electric current was started as if the ball had been connected with the wall of the tube by a bad conductor.” He was not sure whether the air would retain its power to discharge bodies as long as it remained out of contact with any bodies. He determined, however, that any slight “disturbance” of the air by a body having a large surface and not electrified, rendered the air inoperative. He illustrated this by saying that “If one pushes, for example, a sufficiently thick plug of cotton-wool so far into the tube that the air which has been traversed by the rays must stream through the cotton-wool before it reaches the ball, the charge of the ball remains unchanged when suction is commenced.” With the cotton-wool immediately in front of the window, it had no effect, showing, therefore, that dust particles in the air are not the cause of the communication of the force of the discharge from the X-rays to the electrified body. Very fine wire gauze in several thicknesses also prevented the air from discharging the body when placed between the aluminum window and the ball within the thick metal tube, as in the case of the cotton plug. Similar experiments were instituted with dry hydrogen instead of air, and, as far as he could discern, the bodies were equally well discharged, except possibly a little slower in hydrogen. He experienced difficulty in obtaining equally powerful X-rays at different times. All experimenters are acquainted with this difficulty. Further, he called attention also to the thin layer of air which clings to the surface of the bodies, and which, therefore, plays an appreciable part in connection with the discharge. § 16, near end. In order to test the matter further as to discharge of electrified bodies, he placed the same in a highly exhausted bulb and found that the discharge was in one case, for example, only 1/70 as rapid as in air and hydrogen at ordinary pressure, thereby serving as another proof that gas was the intermediate agency. Allowance should be made in all experiments in connection with the discharging quality of X-rays. The surrounding gas should be taken into account.

90a. Application of Principle of Discharge by X-Rays.—Professor Robb, of Trinity College, (Science, Apr. 10, ’96), proposed and explained and practically tested the principle of the discharge of X-rays to determine the relative transparencies of substances to X-rays. He plotted a curve in which the co-ordinate represented the charge of the condenser in micro-coulombs, and the abscissÆ the time between charging and discharging the condenser. The same plan could be adopted, he suggested, for making quantitative measurements of the intensity of X-rays from different tubes or the same discharge tube at different times. J. J. Borgmann, of St. Petersburg, probably was the first to show that X-rays charged as well as discharged bodies. See The Elect., Lon., Feb. 14, ’96, p. 501. Soon, a similar announcement was made by Prof. Righi, of Bologna. § 90.

90A. Borgmann and Gerchun’s Experiments. Action of the X-Rays on Electro-static Charges and (La Distance Explosive.) Comptes Rendus, Feb. 17, ’96; from Trans., by Louis M. Pignolet.—A positively charged zinc disk connected to an electroscope lost its charge almost instantly and acquired a negative charge. When the charge on the zinc disk was negative, the loss was much slower and was not complete, a certain charge remaining. When the rays fell upon two small platinum balls connected to the terminals of an induction coil but separated beyond its sparking distance, sparking took place between them, showing that X-rays, like ultra-violet rays, increase the sparking distance of static charges.

90b. Righi’s Experiments. Bodies In The Neutral or Negative State, Positively Electrified By X-Rays. Comptes Rendus, Feb. 17, 1896. From Trans. by Louis M. Pignolet.—The measurements were made by this eminent Italian physicist, with a Mascart electrometer connected with the bodies upon which the X-rays impinged and enclosed in a grounded metallic case (Faraday cylinder) provided with an aluminum window for the entrance of the rays. A metallic disk connected with the electrometer lost its charge rapidly whether positive or negative.

§ 99S. Initial positive charges were not completely dissipated; negative charges were not only completely dissipated but the bodies acquired positive charges. Disks in the neutral state were charged positively by the X-rays the same as takes place with ultra-violet rays. The final positive potential was greater for copper than for zinc and still greater for retort carbon (“le carbon de cornue”) 90c. at end. The various results are not conflicting if the particular materials are taken into accounts. 90c at end.

90c. The experiments of Prof. Minchin, an expert in such measurements, are properly described here, in that they seem to clear up the superficial ambiguity. He formulated the conclusion (The Elect., Lon., Mar. 27, ’96, p. 736) thus:—“The X-rays charge some bodies positively and some negatively, and whatever charge a body may receive by other means, the X-rays change it, both in magnitude and sign, to the charge which they independently give to the body.” Thus, in the case of magnesium, if the same is first positively charged by any suitable means, then will the X-rays not only discharge it, but electrify it negatively, while if this metal is first negatively charged, the X-rays either diminish or increase the discharge. It must be remembered, however, that this is not true with all metals, for he found that gold, silver, copper, platinum, iron, aluminum, bismuth, steel and antimony, are all positively electrified.

90d. Benoist & Hermuzescu’s Experiment. Negative Charges Dissipated Faster Than Positive By X-Rays. Rate Depends Upon Absorption. Law Formulated. Comptes Rendus, Feb. 3, Mar. 17 and April 27, ’96. They observed that the rays dissipated entirely the charge of electrified bodies in their path, and that negative charges were dissipated more rapidly than positive. § 99Q. They also noticed the discharge augments with the opaqueness of the body and that the effect is more considerable with two thin superposed sheets than with one. In experimenting upon the influence of the discharge of the gaseous dielectric in which the bodies were located, they formulated the following law. The rapidity of the dissipation of the electric charge of an electrified body under the same condition varies as the square root of the density of the gas surrounding the body. The dissipation of the electric charge depends upon the nature of the electrified body, due to a sort of absorbing power (§ 99M) connected with the opaqueness of the body and upon the nature of the surrounding gas, due to the density of the gas or when passing from one gas to another. (From trans. by Louis M. Pignolet.)

91. Before Roentgen published in his second paper of Mar. 9, ’96, an account of his focus tube, the Kings College published a description of an exactly similar one, represented in the cut. See Elec. Rev., Lon., Mar. 13. ’96, p. 340. The cathode is concave and the anode is formed of platinum and is plane and at such an angle that the X-rays generated, § 63b., on diffusion of internal cathode rays, will be thrown out through the thin walls of the bulb. § 55 and 57. As the rays emanate from a point, the shadows are much clearer, especially in conjunction with powerful rays permitting several feet between the object and the tube. Mr. Shallenberger was among the first, and was the first as far as the author knows (Elect. World, Mar. 7, ’96, see cut reproduced) to originate the use of an X-ray focus tube.

91a. Apparatus Employed.—Prof. Roentgen paid tribute to Tesla, by alluding to the advantages resulting from the use of the Tesla condenser and transformer. In the first place, he noticed that the discharge apparatus became less hot, and that there was less probability of its being pierced. Again the vacuum lasted longer, at least in the case of his particular apparatus. Above all, stronger X-rays were produced. Again careful adjustment of the vacuum was not as necessary as with the Ruhmkorff coil.

92. X-Rays and Longitudinal Vibrations.—Prof. Roentgen did not consider X-rays and ultra-violet rays to be of the same nature, although they produced many common effects. The X-rays, as he found, by the above related experiments, behaved quite differently from the ultra-violet rays, which are highly refrangible, practically all subject to reflection, capable of being polarized, and absorbed according to the density of the absorbents. For valid reasons, the X-rays cannot be infra-red rays. While he does not affirm any theory, yet he suggests the theory of longitudinal waves for explaining the properties of X-rays. (This was not suggested again in his second announcement.) He stated that the hypothesis needs a more solid foundation before acceptance. The reason why Roentgen termed the energy X-rays is simply because X in algebra represents an unknown quantity.

93. At the Johns Hopkins University, U. S., in 1884, Sir William Thomson, (Kelvin) delivered a lecture in which he argued that the production of longitudinal vibrations, by electrical means, is reasonable and possible of occurrence. J. T. Bottomly, in Nature, Lon. Feb., (see also Elect. Eng., N.Y., Feb. 19, ’96, p. 187) called attention to this lecture as being of interest in view of Roentgen’s suggestion about longitudinal vibrations. Lord Kelvin called attention to what had been developed in connection with the electro-magnetic theory of light and referred to his own work in 1854, in connection with the propagation of electric impulses along an insulated wire surrounded by gutta percha, but he said that at that time no one knew the relation between electro-static and electro-magnetic units. The part of the lecture referring particularly to the possibility of longitudinal waves in luminiferous ether by electrical means reads as follows. “Suppose that we have at any place in air, or in luminiferous ether (I cannot now distinguish between the two ideas) a body that, through some action we need not describe, but which is conceivable, is alternately, positively and negatively electrified; may it not be that this will give rise to condensational waves? Suppose, for example, that we have two spherical conductors united by a fine wire, and that an alternating E. M. F. is produced in that fine wire, for instance, by an alternate current dynamo-electric machine, and suppose that sort of thing goes on away from all other disturbance—at a great distance up in the air, for example. The result of the action of the dynamo-electric machine will be that one conductor will be alternately, positively and negatively electrified, and the other conductor negatively and positively electrified. It is perfectly certain, if we turn the machine slowly, that in the air in the neighborhood of the conductors, we shall have alternately, positively and negatively directed electric force with reversals of, for example, two or three hundred per second of time, with a gradual transition from negative, through zero to positive, and so on; and the same thing all through space; and we can tell exactly what the potential and what the electric force are at each instant at any point. Now, does any one believe that, if that revolution were made fast enough, the electro-static law of force, pure and simple, would apply to the air at different distances from each globe? Every one believes that if the process can be conducted fast enough, several million times, or millions of millions times per second, we should have large deviations from the electro-static law in the distribution of electric force through the air in the neighborhood. It seems absolutely certain that such an action as that going on would give rise to electrical waves. Now, it does seem to me probable that these electrical waves are condensational waves in luminiferous ether; and probably it would be that the propagation of these waves would be enormously faster than the propagation of ordinary light waves.” Notice that the above was written twelve years prior to Roentgen’s discovery.

94. Prof. Schuster, in Nature, Lon., Jan. ’96, stated that the great argument against the supposition of waves of very small length lies in the absence of refraction, but questioned whether this objection is conclusive. He further stated: “The properties of the ether may remain unaltered within the greater part of the sphere of action of a molecule. The number of molecules lying within a wave length of ordinary light is not greater than the number of motes which lie within a sound wave, but, as far as I know, the velocity of sound is not materially affected by the presence of dust in the air. Hence there seems nothing impossible in the supposition that light waves, smaller than those we know of, may traverse solids with the same velocity as a vacuum. We know that absorption bands greatly affect the refractive index in neighboring regions; and as probably the whole question of refraction resolves itself into one of resonance effects, the rate of propagation of waves of very small lengths does not seem to me to be prejudged by our present knowledge. If Roentgen rays contain waves of very small length, the vibrations in the molecule which respond to them, would seem to be of a different order of magnitude from those so far known. Possibly, we have here the vibration of the electron with the molecule, instead of the molecule carrying with it that of the electron.”

95. Prof. J. J. Thomson showed how it was possible that “longitudinal waves can exist in a medium containing moving charged ions, and in any medium, provided the wave length is so small as to be compared with molecular dimensions, and provided the ether in the medium is in motion. It follows from the equation of the electro-magnetic field that the ether is set in motion in a varying electric field. These short waves would not be refracted, but in this respect they do not differ from transverse waves, which on the electro-magnetic theory would not be refracted if the wave length were comparable with molecular distances.” From Elect. Eng., N.Y., Mar. 18, ’96, p. 286, in reference to a paper before the Cam. Phil. So.

96. One of the very first questions asked in reference to a discovery is as to its practical utility. Already, we have important applications in one of the most humane directions, and that is in connection with diagnosis. Sciagraphs can also be employed in schools for the purpose of education, in some departments of anatomy, etc. The interest that it excites and the amusement that it affords are not to be overlooked, for anything in the nature of recreation possesses utility. However, we may greatly thank all experimenters who have investigated the subject, and who have not left its development alone to Roentgen; for predictions as to the utility of a discovery, however, apparently exaggerated, are very often proved, by subsequent developments, to have been underrated. Upon this point Prof. Boltzmann, in Zeit. Elect., Jan. 15, ’96, see also, The Elec., Lon., Jan. 31, ’96, p. 447, stated, “If we remember to what discoveries the most insignificant new natural phenomenon, such as the attraction of small objects by rubbed amber, of iron by the lode-stone, the convulsive twitches of a frog’s leg due to electric discharges, the influence of the electric current upon the magnetic needle, electro-magnetic induction etc., has led us, one can imagine to what applications an agent will be turned, which a few weeks after its discovery has given rise to such surprising results.”

97. Soon after hearing, (about the first of Feb. ’96,) of the Roentgen discovery, it occurred to the author to carry on experiments with fluorescence, but finding that it was inconvenient to work in a perfectly dark room, and, recognizing that black cardboard had practically no effect upon absorbing the X-rays, he devised a sciascope (daily papers, Feb. 13, and Elect. Eng., Feb. 19) which he afterwards learned was independently invented and used at about the same time by Prof. William F. Magie, of Princeton University, (see Amer. Jour. Med. Sci., Feb. 7, ’96 and Feb. 15, ’96) and by Prof. E. Salvioni, of Italy under the name of cryptoscope, (see Med. Sur. Acad. of Perugia, Italy, Feb. 8, ’96.) In about a month afterwards (Elect. Eng., N.Y., Apr. 1, ’96, p. 340) the instrument (with phosphorescent calcic tungstate § 13. in place of fluorescent barium platino cyanide) was again published under the name of the Edison fluoroscope. There are probably many other claimants—some professor in London—name forgotten. They all consist of a tapering tube with a sight hole at one end and a fluorescent screen in the other, which is closed by opaque card board. (Frontispiece at Chap. X). For the sake of conformity, the words sciagraph and sciagraphy and similar derivatives, and in view of the meaning of the radical definitions, have been employed throughout the book. The objection to the word fluoroscope is that the instrument is practically universally employed in seeing the shadows of objects, otherwise invisible to the naked eye, rather than to test fluorescence. The name sciascope was early suggested by Prof. Magie. For those who wish to make a screen, the author may state that he obtained a good one by mixing pulverized barium platino cyanide with varnish and spreading the mixture over a sheet of tracing cloth.