The X-Rays

Wilhelm Konrad Rontgen

I.—UPON A NEW KIND OF RAYS

1. If the discharge of a great Ruhmkorff induction coil be passed through a Hittorf vacuum tube, or a Lenard’s, Crookes’, or similar apparatus containing a sufficiently high vacuum, then, the tube being covered with a close layer of thin black pasteboard and the room darkened, a paper screen covered on one side with barium-platinum cyanide and brought near the apparatus will be seen to glow brightly and fluoresce at each discharge whichever side of the screen is toward the vacuum tube. The fluorescence is visible even when the screen is removed to a distance of 2 meters from the apparatus.

The observer may easily satisfy himself that the cause of the fluorescence is to be found at the vacuum tube and at no other part of the electrical circuit.

2. It is thus apparent that there is here an agency which is able to pass through the black pasteboard impenetrable to visible or ultra violet rays from the sun or the electric arc, and having passed through is capable of exciting a lively fluorescence, and it is natural to inquire whether other substances can be thus penetrated.

It is found that all substances transmit this agency, but in very different degree. I will mention some examples. Paper is very transmissible.

I observed fluorescence very distinctly behind a bound book of about 1,000 pages. The ink presented no appreciable obstacle. Similarly fluorescence was seen behind a double whist pack. A single card held between the fluorescent screen and the apparatus produced no visible effect. A single sheet of tin foil, too, produces hardly any obstacle, and it is only when several sheets are superposed that their shadow appears distinctly on the screen. Thick wooden blocks are transmissible. Slabs of pine 2 or 3 centimeters thick absorb only very little. A plate of aluminum about 15 millimeters thick diminished the effect very considerably, but did not cause the fluorescence to entirely disappear. Blocks of hard rubber several centimeters thick still transmitted the rays.

Glass plates of equal thickness behave very differently according to whether they contain lead (flint glass) or not. The first class are much less transmissible than the second.

If the hand is held between the vacuum tube and the screen, the dark shadow of the bones is seen upon the much lighter shadow outline of the hand. Water, carbon, bisulphide, and various other liquids investigated proved very transmissible. I could not find that hydrogen was more transmissible than air. The fluorescence was visible behind plates of copper, silver, lead, gold, and platinum, when the thickness of the plate was not too great. Platinum 0.2 millimeter thick is still transmissible, and silver and copper plates may be still thicker. Lead 1.5 millimeters thick is practically impenetrable, and advantage was frequently taken of this characteristic. A wooden stick of 20 millimeters square cross section, having one side covered with white lead, behaved differently when interposed between the vacuum tube and the screen according as the X-rays traversed the block parallel to the painted side or were compelled to pass through it. In the first ease there was no effect appreciable, while in the second a dark shadow was thrown on the screen. Salts of the metals, whether solid or in solution, are to be ranged in almost the same order as the metals themselves for transmissibility.

3. These observations and others lead to the conclusion that the transmissibility of equal thicknesses of different substances depends on their density. At least no other characteristic exerts so marked an influence as this.

The following experiment shows, however, that the density is not the sole factor. I compared the transmissibility of nearly equally thick plates of glass, aluminum, calcspar, and quartz. The density of these substances is substantially the same, and yet it was quite evident that the calcspar was considerably less transmissible than the others, which are about alike in this respect. [p.229]

4. All bodies became less transmissible with increasing thickness. For the purpose of finding a relation between transmissibility and thickness I have made photographic exposures, in which the photographic plate was partly covered with a layer of tin foil consisting of a progressively increasing number of sheets. I shall make a photometric measurement when I am in possession of a suitable photometer.

5. Sheets were rolled from platinum, lead, zinc, and aluminum of such thickness that all appeared to be equally transmissible. The following table gives the measured thickness in millimeters, the relative thickness compared with platinum, and the specific gravity:

xyz                                                             Relative      Specific
                                                  Thickness.     Thickness.     Gravity.
Platinum                                               0.018              1         21.5
Lead                                                    0.05              3         11.3
Zinc                                                    0.10              6          7.1
Aluminum                                                 3.5            200          2.6

From these values it may be seen that the transmissibility of plates of different metals so chosen that the product of the thickness and density is constant would not be equal. The transmissibility increases much faster than this product falls off.

6. The fluorescence of barium-platinum-cyanide is not the only action by which X-rays may be recognized. It should be remarked that they cause other substances to fluoresce, as for example the photophorescent calcium compounds, uranium glass, common glass, calcspar, rock salt, etc.

It is of particular importance from many points of view that photographic dry plates are sensitive to X-rays. It thus becomes possible to fix many phenomena so that deceptions are more easily avoided; and I have where practicable checked all important observations made with a fluorescent screen by photographic exposures.

It appears questionable whether the chemical action upon the silver salts of the photographic plate is produced directly by the X-rays. It is possible that this action depends upon the fluorescent light which, as is mentioned above, may be excited in the glass plate, or perhaps in the gelatine film. "Films" may indeed be made use of as well as glass plates:

I have not as yet obtained experimental evidence that the X-rays are capable of giving heat. This characteristic might, however, be assumed as present, since in the excitation of fluorescent phenomena the capacity of the energy of the X-rays for transformation is proved, and since it is certain that of the X-rays falling upon a body not all are given up.

The retina of the eye is not sensitive to these rays. Nothing is to be noticed by bringing the eye near the vacuum tube, although according to the preceding observations the media of the eye must be sufficiently transmissible to the rays in question.

7. After I had discovered the transmissibility of various bodies of relatively great thickness I hastened to investigate whether or not the X-rays were refracted in passing through a prism. Experiments with water and carbon bisulphide in mica prisms of 30 degrees refracting angle showed no deviation either when observations were made with the fluorescent screen or with the photographic plate. For comparison, the deviation of light rays was observed under the same conditions. The refracted portion lay from 10 to 20 centimeters distant from that not refracted. With prisms of hard rubber and aluminum of about 30 degrees refracting angle I obtained exposures on a photographic plate which perhaps indicated a slight refraction. This is, however, very doubtful and the deviation is, if present, so small that the index of refraction for X-rays in these substances can not exceed 1.05 I could not observe with the fluorescent screen any deviation in these cases. Experiments with prisms of the denser metals have so far yielded no certain results on account of the slight transmissibility and the consequent decrease of the intensity of the transmitted ray.

In consideration of these results on the one hand, and on the other of the importance of the question whether or not the X-rays in passing from one medium to another undergo refraction, it is very gratifying that this question may be investigated by other means than by the help of prisms. Finely pulverized bodies in suitable layers allow but little light to pass, in consequence of refraction and reflection. If now the X-rays are transmitted equally well through powder as through the coherent substance, equal masses being presupposed, it is proved that neither refraction nor regular reflection is present in any marked degree. This experiment was performed using finely pulverized rock-salt, finely divided silver, obtained by electrolysis, and the zinc dust so frequently utilized in chemical processes. In no case was any difference in transmissibility between the powder and the coherent substance detected either by the use of the fluorescent screen or the photographic plate.

It follows of course from the results thus obtained that the X-rays can not be concentrated by the use of lenses; and, indeed, a great hard rubber lens and a glass lens actually proved without effect. The shadow of a round rod is darker in the middle than at the edges, while that of a tube which is filled with some substance more transmissible than the material of which the tube is composed is darker at the edges than at the center.

8. The question as to the reflection of X-rays is so far settled by the experiments already described that no marked regular reflection was to be found with any of the substances examined. Other experiments which I will here pass over lead to the same results.

Nevertheless an observation should be mentioned which indicated at first glance an opposite result. A photographic plate shielded from the action of light rays by a black paper was exposed to X-rays so that the glass side was toward the discharge tube. The sensitive film was partially covered with bright plates of platinum, lead, zinc, and aluminum, arranged in a star-shaped figure. Upon development it was observed that the darkening of the film under the platinum, the lead, and especially the zinc was distinctly greater than in the other parts. No such effect was produced by the aluminum. Thus it seemed as if the three metals mentioned reflected. However, there were other causes to be conceived which might have produced the increased darkening, and in order to be sure I performed a second experiment, interposing a thin sheet of aluminum foil (very transmissible to X-rays, but not to those of the ultraviolet) between the metals and the sensitive film. Since in this case again practically the same result was obtained, the fact of reflection of X-rays by the metals above mentioned is established.

Taking this result together with the observation that powder is as transmissible as coherent substance, and further, that bodies with rough surfaces behave in the transmission of X-rays and also in the experiments just described exactly like polished bodies, the conclusion is reached that there is, as before remarked, no regular reflection, but that the bodies behave toward X-rays in the same manner as a turbid medium with reference to light.

As I have not been able to discover any refraction in the passage from one medium to another, it appears as if the X-rays travel with equal velocity in all bodies, and hence in a medium which is everywhere present and in which the particles of the bodies are embedded. These latter act as a hindrance to the propagation of the X-rays, which is in general greater the greater the density of the body in question.

9. In accordance with this supposition it might be possible that the arrangement of the molecules of the body would exert an influence on its transmissibility, and that, for example, a piece of calcspar would be unequally transmissible for equal thicknesses when the rays passed along or at right angles to the axis. Experiments with calcspar and quartz gave, however, a negative result.

10. It will be recalled that Lenard, in his beautiful experiments on the transmission of the Hittorf cathode rays through thin aluminum foil, obtained the result that these rays are disturbances in the ether, and that they diffuse themselves in all bodies. We may make a similar statement with regard to our rays.

In his last research Lenard has determined the relative absorption of different substances for the cathode rays, and in determining the same for air at atmospheric pressure has given the values 4.10, 3.40, 3.10 as referred to 1 centimeter thickness according to the density of the gas in the discharge tube. Judging from the length of spark observed, I have, in my researches, generally employed tubes of about equal exhaustion and only seldom those of much greater or less density. Using the photometer of L. Weber, the best at my command, I compared the intensity of fluorescence on the screen in two positions distant 100 and 200 millimeters, respectively, from the discharge tube. From the results of these experiments, agreeing well with each other, it appeared that the intensity varies inversely as the square of the distance. Hence the air absorbs a much smaller portion of the X-rays passing through it than of cathode rays. This result is in accord with the observation above mentioned, that it is possible to distinguish fluorescence at 2 meters distance from the discharge tube.

Most other substances are, like the air, more transmissible for X-rays than for the cathode rays.

11. Another very noteworthy difference between the behavior of the cathode rays and the X-rays was exhibited in that I was unable to produce any deviation of the latter by the action of the most powerful magnetic fields. The property of being subject to deviation by magnets is, on the other hand, very characteristic of the cathode rays. Hertz and Lenard have observed various kinds of cathode rays which "are to be distinguished by their differences in their capacities for exciting phosphorescence in their absorbability and in their deviation by the magnet," but a considerable magnetic deviation was to be observed with all of them, and I do not believe that this characteristic would be given up except for the most urgent reasons. [p.233]

12. According to the results of experiments particularly directed to discover the source of the X-rays, it is certain that the part of the wall of the discharge tube which most strongly fluoresces is the principal starting point. The X-rays therefore radiate from the place where, according to various observers, the cathode rays meet the glass wall. If one diverts the cathode rays within the tube by a magnet, the source of the X-ray is also seen to change its position so that these radiations still proceed from the end points of the cathode rays. The X-rays being undeviated by magnets cannot, however, be simply cathode rays passing unchanged through the glass wall. The greater density of the gas outside of the discharge tube cannot, according to Lenard, be made answerable for the great difference of the deviation.

I come therefore to the results that the X-rays are not identical with the cathode rays, but that they are excited by the cathode rays in the glass wall of the vacuum tube.

13. This generating action takes place not only in glass, but as I observed it in apparatus with aluminum walls 2 millimeters thick, exists also for this metal. Other substances will be investigated.

14. The warrant for giving the title "rays" to the agent which proceeds from the wall of the discharge tube arose in part from the quite regular formation of shadows appearing when more or less transmissible substances are interposed between the generating apparatus and a phosphorescent screen or photographic plate. I have many times observed and sometimes photographed such shadow forms, in whose production there lies a particular charm. I have, for example, photographs of the shadow of the profile of a door which separates the two rooms, in one of which was the discharge apparatus, in the other the photographic plate; of the shadow of the hand bones; of the shadow of a wooden spool wound with wire; of a set of weights in a box; of a compass in which the magnetic needle is quite inclosed in metal; of a piece of metal which is shown to lack homogeneity by the use of X-rays, etc.

The propagation of the X-rays in right lines is shown by pin-hole photography, which I have been able to do with the discharge apparatus covered with black paper. The picture is weak, but unmistakably correct.

15. I have much sought to obtain interference phenomena with X-rays, but unfortunately—perhaps on account of their slight intensity—without result.

16. Experiments have been begun to see if electrostatic forces can in any way influence X-rays, but these are not yet finished. [p.234]

17. If the question is asked what the X-rays—which certainly are not cathode rays—really are, one might at first, on account of their lively fluorescent and chemical action, compare them to ultra-violet light. But here one falls upon serious difficulties. Thus, if the X-rays were ultra-violet light, then this light must possess the following characteristics:

(a) That in passing from air into water, carbon bisulphide, aluminum, rock salt, glass, zinc, etc., it experiences no notable refraction.

(b) That it is not regularly reflected by these substances.

(c) That it cannot be polarized by the usual materials.

(d) That its absorption by substances is influenced by nothing so much as by their density.

In other words, one must assume that these ultra-violet radiations comport themselves quite differently from all previously known infra red, visible, and ultra-violet rays.

I have not been able to admit this, and have sought some other explanations.

A kind of relation seems to subsist between the new radiation and light radiation, or at least the shadow formation, the fluorescence, and the chemical action, which are common phenomena of these two kinds of radiation, point in this direction. It has been long known that longitudinal as well as transverse vibrations are possible in the ether, and according to various physicists must exist. To be sure, their existence has not, up to the present time, been proved, and hence their characteristics have not thus far been experimentally investigated.

Should not the new radiations be ascribed to longitudinal vibrations in the ether? I may say that in the course of the investigation this hypothesis has impressed itself more and more favorably with me, and I venture to propose it, although well aware that it requires much further examination.

(WUERZBURG, PHYSIK. INSTITUT D. UNIV., December, 1895.)

II.—UPON A NEW KIND OF RAYS (ABSTRACT.)

As my work must be interrupted for several weeks, I take the opportunity of presenting in the following some new results:

18. At the time of my first publication I was aware that the X-rays have the property of discharging electrified bodies, and I intimated that it was the X-rays and not the cathode rays passing unchanged through the aluminum window of his apparatus which produced the effect described by Lenard on electrified bodies at a distance. I have, however, delayed publication of my experiments until I could present conclusive results.

These can be obtained only when the observations are carried on in a room which is not only completely insulated from the electrostatic forces emanating from the vacuum tube, the conducting wires, the induction apparatus, etc., but is also closed to the air which comes in the neighborhood of the discharge apparatus.

For this purpose I had a box constructed by soldering together zinc sheets, and this box was large enough to contain me and the necessary apparatus, and was air-tight with the exception of an opening which could be closed by a zinc door. The side opposite to the door was mostly lined with lead, and immediately adjacent to the discharge tube an opening 4 centimeters wide was cut in the lead and zinc wall, and its place filled up air-tight with aluminum foil. Through this window passed the X-rays to be investigated. I have with this apparatus verified the following results:

(a) Positively or negatively electrified bodies placed in air are discharged when immersed in X-rays, and the action is the more rapid the more intense the radiations. The intensity of the rays is determined by their action upon a fluorescent screen or a photographic plate.

It is in general immaterial whether the electrified substance is a conductor or non-conductor. Thus far I have discovered no difference in the behavior of different bodies relative to the rapidity of their discharge, or between positive or negative charges. These points are, however, open to further investigation.

(b) When an electrified conductor is surrounded by a solid insulator, as for example, paraffine, the radiation produces the same effect as would the flashing of the insulating shell by a flame placed in contact with the ground.

(c) If this insulator be in its turn closely surrounded by a grounded conductor and both itself and this outer conductor be transmissible to X-rays, the action of the X-radiations upon the inner conductor is unnoticeable with the apparatus at my command.

(d) The observations recorded under (a), (b), and (c) indicate that the air through which X-rays pass possesses the property of discharging any electrified bodies with which it comes in contact.

(e) If this be indeed the case, and if the air retains for some considerable time this property imparted to it by the X-rays, it must be [p.236] possible to discharge electrified bodies not themselves under the influence of X-rays by bringing to them air which has been subject to these radiations.

One may satisfy himself in various ways that this is the case. The following, though perhaps not the simplest method, may be mentioned:

I employed a brass tube 3 centimeters wide and 45 centimeters long. At 1 centimeter’s distance from one end a portion of the tube was cut away and replaced by a thin sheet of aluminum. At the other end there was introduced a brass ball, which was supported by a metal support, and this end was closed air-tight. Between the brass ball and the closed end of the tube a side tube was soldered in, which was connected with an air-pump. By this means a current of air was made to flow by the brass ball, after having passed the aluminum window. The distance from the ball to the window was 20 centimeters.

I mounted this tube in the zinc box in such a manner that the X-rays entered the tube at right angles to its axis, and the insulated ball lay outside the reach of these rays, in the shadow. The tube and zinc box were placed in contact and the ball was connected with a Hankel electroscope.

It was shown that a charge on the ball, whether positive or negative, was not influenced by X-rays so long as the air remained quiet in the tube, but that a marked diminution of the charge was produced by sucking a strong current of air through. If the ball was kept at constant potential by connecting it with accumulators, and a continuous current of air was kept flowing in the tube, an electrical current was set up just as if the ball was connected with the walls of the tube by a conductor of high resistance.

20. In section 13 of my first article it was stated that the X-rays may be generated not only in glass but in aluminum. In conducting experiments in this direction no solid bodies were found which were not capable of producing X-rays when under the influence of cathode rays. I know no reason to suppose that liquids and gases also do not act similarly.

Different substances, however, possess this property in different degrees. For example, if cathode rays are caused to fall upon a plate of which one-half is composed of platinum foil 0.3 millimeter thick and the other half of aluminum 1 millimeter thick, one may observe in the photographic image taken with the pinhole camera that the platinum foil sends out many more X-rays from the side bombarded by the cathode rays than does the aluminum on the same side. But from the back side of the plate there go out almost no X-rays from the platinum, while the aluminum sends out a relatively large number. These latter rays are generated at the front layers of the aluminum and pass through the plate.

It should be remarked that these observations have a practical significance. For the generation of X-rays of the greatest possible intensity my experience recommends the employment of platinum. I have used for some weeks with advantage a discharge apparatus having a concave mirror of aluminum as cathode, and as annode a platinum plate placed in the center of curvature, and at an angle of 45 degrees with the axis.

21. The X-rays proceed from the annode with this apparatus. As I have concluded from experiments with apparatus of various forms, it is immaterial with regard to the intensity of the X-rays whether they proceed from the annode or not.

(WUERZBURG, PHYSIK. INSTITUT D. UNIVERSITAET, March 9, 1896.)

III.—FURTHER OBSERVATIONS ON THE PROPERTIES OF X-RAYS

With reference to practical applications, the observation of the distribution of intensity of the rays proceeding from the platinum plate has some value in connection with the formation of shadow pictures by means of X-rays. In accordance with the observations above recorded it is to be recommended that the discharge tube be so arranged that the rays employed for formation of pictures be those making a large angle, though not much exceeding 80 degrees, with the platinum plate. In this way the sharpest possible delineation will be obtained, and if the platinum plate is flat and the construction of the tube such that the rays proceeding obliquely pass through not much greater thickness of glass than those going out at right angles to the platinum plate, then no material loss in intensity will be experienced in this arrangement.

5. If two plates of different substances are equally transmissible this equality will not in general be retained for another pair of plates of the same substances with thicknesses altered in the same ratio. This fact may be shown very easily by the use of thin sheets, as, for example, of platinum and aluminum. I used for this purpose platinum foil 0.0026 millimeter thick and aluminum foil 0.0299 millimeter thick. I found in one instance that one sheet of platinum was equally transmissible with six sheets of aluminum; but the transmissibility of two sheets of platinum was less than that of twelve sheets of aluminum and about equal to that of sixteen sheets of the latter metal. Using another discharge tube, I found 1 platinum equal 8 aluminum, but 8 platinum equal 90 aluminum. From these experiments it follows that the ratio of thicknesses of platinum and aluminum of equal transmissibility is less the thicker the sheets under examination.

6. The ratio of the thicknesses of two equally transmissible plates of different material is dependent on the thickness and the material of the body, as, for instance, the glass wall of the discharge tube, through which the rays have to pass before they reach the plates investigated.

7. The experiments described in sections 4, 5, and 6 relate to the alterations which the X-rays proceeding from a discharge tube experience in their transmission through different substances. It will now be shown that one and the same body may for the same thickness be unequally transmissible for rays emitted from different discharge tubes.

In the following table are given the values of the transmissibility of an aluminum plate 2 millimeters thick for the rays given out by different tubes:

xyz                                                               Tube.
                                                  1     2     3     4     2     5
Transmissibility for vertically incident rays of
a 2-millimeter thick aluminum plate            0.0044   0.22   0.30   0.39   0.50   0.59

The discharge tubes were not materially different in their construction or in the thickness of their glass wall, but varied in the density of the gas within them, and hence in the potential required to produce discharge. Tube 1 required the least and tube 5 the greatest potential, or, as we may say for short, the tube 1 is the "softest" and tube 5 the "hardest." The same Ruhmkorff in direct connection with the tubes, the same circuit breaker, and the same current strength in the primary circuit were used in all cases.

Various other substances which I have investigated behaved similarly to aluminum. All are more transmissible to rays from harder tubes. This fact seemed to me particularly worthy of attention.

The relative transmissibility of plates of different substances proved also to be dependent on the hardness of the discharge tube employed. The ratio of the thickness of platinum and aluminum plates of equal transmissibility becomes less the harder the tubes from which the rays proceed, or, referring to the results just given, the less the rays are absorbed.

The different behavior of rays excited in tubes of different hardness is also made apparent in the well known shadow picturing of hands, etc. With a soft tube a dark shadow is obtained, in which the bones are little prominent; when a harder tube is used the bones are very distinct and visible in all their details, whereas the softer portions are less marked, and with very hard tubes even the bones themselves become only weak shadows. From these considerations it appears that the choice of the tube must be governed by the character of the objects which it is desired to portray.

It remains to remark that the quality of rays proceeding from one and the same tube depends on various conditions. Of these the most important are the following: (1) The action of the interrupter, or, in other words, the course of the primary current. In this connection should be mentioned the phenomena frequently observed that particular ones of the rapidly succeeding discharges excite X-rays which are not only more intense, but which also differ from the others in their absorption. (2) The character of the sparks which appear in the secondary circuit of the apparatus. (3) The employment of a Tesla transformer. (4) The degree of evacuation of the discharge tube (as already stated). (5) The varying, but as yet not satisfactorily known, procedure within the discharge tube. Separate ones among these conditions require further comment.

The hardness of a tube had been considered to be brought about solely by the continuation of the evacuation by means of the pump; but this characteristic is affected in other ways. Thus a sealed tube of medium hardness becomes gradually harder by itself—unfortunately to the shortening of the period of its usefulness when used in a suitable manner for the production of X-rays, that is to say, when discharges which do not cause the platinum to glow or at least to glow only weakly are passed through. A gradual self-evacuation is thus effected.

With a tube thus become very hard I took a very fine photograph of a double-barreled gun with inserted cartridges, which showed all the details of the cartridges, the inner faults of the Damascus barrels, etc., very sharply and distinctly. The distance from the platinum plate of the discharge tube to the photographic plate was 15 centimeters and the exposure twelve minutes—comparatively long in consequence of the small photographic action of the very slightly absorbable rays (see below). The Duprez interrupter had to be replaced by the Foucault form. It would be of interest to construct tubes which would make it possible to use still higher potentials than before.

Self-evacuation has been above assigned as the cause of the growing hardness of sealed tubes, but this is not the only cause. There are changes in the electrodes which produce this effect. I do not know the nature of these changes.

The observations recorded in these paragraphs and others not given have led me to the view that the composition of the rays proceeding from a platinum anode of a discharge tube depends upon the frequency and form of the discharge current. The degree of tenuity, the hardness, is important only because the form of the discharge is thereby influenced. If it were possible to produce the proper form of discharge for the generation of X-rays in any other way, the X-rays might be obtained with relatively high pressures.

9. The results appearing in the five preceding paragraphs have been those most evidently to be derived from the accompanying experiments. Summing up these separate results, and being guided in part by and X-rays, one arrives at the following conclusions:

(a) The radiations emitted by a discharge tube consist of a mixture of rays of different absorbability and intensity.

(b) The composition of this mixture is in a marked degree dependent on the frequency and form of the discharge current.

(c) The rays receiving preference in absorption vary with different bodies.

(d) Since the X-rays are generated by the cathode rays and have in common with them various characteristics—as the exciting of fluorescence, photographic and electrical actions, an absorbability depending in a marked degree on the density of the medium traversed, etc.—the conjecture is prompted that both phenomena are processes of the same nature. Without committing myself unconditionally to this view, I may remark that the results of the last paragraphs are calculated to raise a difficulty in the way of this hypothesis. This difficulty consists in the great difference between the absorption of the cathode rays investigated by Lenard and the X-rays, and second, that the transmissibility of bodies for the cathode rays is related to their density by Other laws than those which govern their transmissibility for X-rays.

With regard to the first point, considerations present themselves under two heads: (1) As we have seen in section 7, there are X-rays of different absorbability, and the investigations of Hertz and Lenard show that the cathode rays are similarly to be discriminated. While the "softest" tubes investigated generated rays much less subject to absorption than any cathode rays investigated by Lenard, yet there is no reason to doubt the possibility of X-rays of greater absorbability, and cathode rays of less. It therefore appears probable that in future investigations rays will be found bridging over the gap between X-rays and cathode rays, so far as their absorption is concerned. (2) We found in section 4 that the specific transmissibility of a body becomes less the thinner the plate passed through. Consequently, had we made use in our experiments of plates as thin as those employed by Lenard it would have been found that the X-rays were more nearly like those of Lenard in their absorbability.

10. Besides the fluorescent phenomena, there may be excited by X-rays photographic, electric, and other actions, and it is of interest to know how far these various manifestations vary in similar ratio when the source of the rays is altered. I must restrict myself to a comparison of the first two phenomena.

A hard and a soft tube were so adjusted as to give equally bright fluorescence as compared by means of the photometer described in section 2. Upon substituting a photographic plate in the place of the fluorescent screen it was found, on development, that the portion subject to the rays from the hard tube was blackened to a less degree than the other. The rays, though producing equal fluorescence, were thus for photographic purposes unequally active.

The great sensitiveness of a photographic plate even for rays from tubes of medium hardness is illustrated by an experiment in which 96 films were superposed, placed at a distance of 25 centimeters from the discharge tube, and exposed five minutes with due precautions to protect the films from the radiations of the air. A photographic action was apparent on the last film, although the first was scarcely over-exposed.

If the intensity of the radiations is augmented by increasing the strength of the primary current, the photographic action increases in the same measure as the intensity of the fluorescence. In this case, as in the case where the intensity of the radiation was increased by an alteration of the distance of the fluorescent screen, the brightness of the fluorescence is at least approximately proportional to the intensity of the radiation. This rule should not, however, be too generally applied. [p.242]

11. In conclusion, mention should be made of the following particulars:

With a discharge tube of proper construction, and not too soft, the X-rays are chiefly generated in a spot of not more than 1 or 2 millimeters diameter where the cathode rays meet the platinum plate. This, however, is not the sole source. The whole plate and a part of the tube walls emit X-rays, though in less intensity. Cathode rays proceed in all directions, but their intensity is considerable only near the axis of the concave cathode mirror, and, consequently, the X-rays are strongly emitted only near the point where this axis meets the platinum plate. When the tube is very hard and the platinum thin, many rays proceed also from the rear surface of the platinum plate, but, as may be shown by the pinhole camera, chiefly from the spot lying on the axis of the mirror.

I can confirm the observation of G. Brandes that the X-rays are able to produce a sensation of light upon the retina of the eye. In my record book appears a notice entered in the early part of November, 1895, to the effect that when in a darkened chamber, near a wooden door, I perceived a weak appearance of light when a Hittorf tube upon the other side of the door was put in operation. Since this appearance was only once observed, I regarded it as a subjective, and the reason that it was not then repeatedly observed lay in the fact that other tubes were substituted for the Hittorf tube which were less completely evacuated and not provided with platinum anodes. The Hittorf tube furnishes rays of slight absorbability on account of its high vacuum, and, at the same time, of great intensity on account of the employment of a platinum anode for the reception of the cathode rays.

With the tubes now in use I can easily repeat the Brandes experiment.

Since the beginning of my investigation of X-rays I have repeatedly endeavored to produce diffraction phenomena with them. I obtained at various times, when using narrow slits, appearances similar to diffraction effects, but when modifications were made in the conditions for the purpose of thoroughly proving the accuracy of this explanation of the phenomena it was found in each case that the appearances were produced in other ways than by diffraction. I know of no experiment which gives satisfactory evidence of the existence of diffraction with the X-rays.