On a New Kind of Ray, A Preliminary Communication

Dr. W. C. Roentgen


Editor's Introduction

The results of Dr. Roentgen's first experiments to determine the properties of the new radiation were described in this manuscript which he submitted to the Wurzburg Physico-Medical Society on December 28, 1895.

This was the first public disclosure, although we began to observe Dr. Roentgen's investigation in the last edition of The X-ray Century.

I have added annotations to emphasize the extensive scope of his investigation and the questions which he was attempting to answer.

The new kind of radiation can be produced by using several types of tubes which are available in many physics laboratories.

1. If the discharge of a fairly large induction-coil be made to pass through a Hittorf vacuum-tube, or through a Lenard tube, a Crookes tube or other similar apparatus, which has been sufficiently exhausted, the tube being covered with thin, black cardboard which fits it with tolerable closeness, and if the whole apparatus be placed in a completely darkened room, there is observed at each discharge a bright illumination of a paper screen covered with barium platinocyanide, placed in the vicinity of the induction-coil, the fluorescence thus produced being entirely independent of the fact whether the coated or the plain surface is turned towards the discharge tube. This fluorescence is visible even when the paper screen is at a distance of 2 meters from the apparatus.

It is easy to prove that the cause of fluorescence proceeds from the discharge-apparatus, and not from any other point in the conducting circuit.

Is there anything that is opaque to this radiation?

2. The most striking feature of this is the fact that an active agent here passes through a black cardboard envelope, which is opaque to the visible and ultraviolet rays of the sun or of the electric arc; an agent, too, which has the power of producing active fluorescence. Hence we may first investigate the question whether other bodies also possess this property.

We soon discover that all bodies are transparent to this agent, though in very different degrees. I proceed to give a few examples: Paper is very transparent. By "transparency" of a body I denote the relative brightness of a fluorescent screen placed close behind the body, referred to the brightness which the screen shows under the same circumstances, though without the interposition of the body. Behind a bound book of about 1000 pages, I saw the fluorescent screen light up brightly, the printer's ink offering scarcely a noticeable hindrance. In the same way the fluorescence appeared behind a double pack of cards ; a single card held between the apparatus and the screen being almost unnoticeable to the eye. A single sheet of tin-foil is also scarcely perceptible; it is only after several layers have been placed over another that their shadow is distinctly seen on the screen. Thick blocks of wood are also transparent, pine boards two or three centimeters thick absorbing only slightly. A plate of aluminum about 15 mm thick, though it enfeebled the action seriously, did not cause the fluorescence to disappear entirely. Sheets of hard rubber several centimeters thick still permit the rays to pass through them. For brevity's sake I shall use the expression "rays" and to distinguish them from others of this name I shall call them "X-rays". Glass plates of equal thickness behave quite differently, according as they contain lead (flint-glass) or not; the former are much less transparent than the latter. If the hand be held between the discharge-tube and the screen, the darker shadow of the bones is seen within the slightly dark shadow-image of the hand itself. Water, carbon disulphide, and various other liquids, when they are examined in mica vessels, are seen also to be transparent. That hydrogen is to any considerable degree more transparent than air than I have not been able to discover. Behind plates of copper, silver, lead, gold, and platinum the fluorescence may still be recognized, though only if the thickness of the plate is not too great. Platinum of a thickness of 0.2 mm is still transparent; the silver and copper plates may be even thicker. Lead of a thickness of 1.5 mm is practically opaque; and on account of this property this metal is frequently most useful. A rod of wood with a square cross-section (20 x 20 mm), one of whose sides is painted white with lead paint, behaves differently according to how it is held between the apparatus and the screen. It is almost entirely without action when the X-rays pass through it parallel to the painted side; whereas the stick throws a dark shadow when the rays are made to traverse it perpendicular to the painted side. In a series similar to that of the metals themselves their salts can be arranged with reference to their transparency, either in solid form or in solution.

Density is a major factor affecting the transparency of materials.

3. The experimental results which have now been given, as well as others, lead to the conclusion that the transparency of different substances, assumed to be of equal thickness, is essentially conditioned upon their density: no other property makes itself felt like this, certainly to so high a degree.

The following experiments show, however, that the density is not the only cause acting. I have examined, with reference to their transparency, plates of glass, aluminum calcite, and quartz, of nearly the same thickness; and while these substances are almost equal in density, yet, it was quite evident that the calcite was sensibly less transparent than the other substances, which appeared almost exactly alike. No particularly strong fluorescence (see below) of calcite, especially by comparison with glass, has been noticed.

Increasing thickness reduces transparency.

4. All substances with increase in thickness become less transparent. In order to find a possible relation between transparency and thickness, I have been made photographs (see below) in which portions of the photographic plate were covered with layers of tin-foil, varying in the number of sheets superimposed. Photometric measurements of these will be made when I am in possession of a suitable photometer.

There is some characteristic of materials, other than density and thickness, that has an effect on transparency.

5. Sheets of platinum, lead, zinc and aluminum were rolled of such thickness that all appeared nearly equally transparent. The following table contains the absolute thickness of these sheets measured in millimeters, the relative thickness referred to that of the platinum sheet, and their densities:

Element Thickness
(mm)
Relative
Thickness
Density
Pt 0.018 1 21.5
Pb 0.05 3 11.3
Zn 0.10 6 7.1
Al 3.50 200 2.6

We may conclude from these values that different metals possess transparencies which are by no means equal, even when the product of thickness and density are the same. The transparency increases much more rapidly than this product decreases.

Fluorescence is produced in many materials by the radiation.

6. The fluorescence of barium platinocyanide is not the only recognizable effect of the X-rays. It should be mentioned that other bodies also fluorescence; such, for instance, as the phosphorescent calcium compounds, then uranium glass, ordinary glass, calcite, rock-salt, and so on.

Photographic plates respond to exposure by the radiation.

Of special significance in many respects is the fact that photographic dry plates are sensitive to X-rays. We are, therefore, in a condition to determine more definitely many phenomena; and so the more easily to avoid deception, wherever it has been possible, therefore, I have controlled, by means of photography, every important observations which I have made with the eye by means of the fluorescent screen.

In these experiments the property of the rays to pass almost unhindered through thin sheets of wood, paper, and tin-foil is most important. The photographic plates can be obtained in a non-darkened room with the photographic plates either in holders or wrapped up in paper. On the other hand, from this property, it results as a consequence that undeveloped plates cannot be left for a long time in the neighborhood of the discharge-tubes, if they are protected merely by the usual covering of pasteboard and paper.

It appears questionable, however, whether the chemical action on the silver salts of the photographic plates is directly caused by the X-rays. It is possible that the action proceeds from the fluorescent light which, as noted above, is produced in the glass plate itself or perhaps in the layer of gelatin. "Films" can be used just as well as glass plates.

Does the absorption of x-rays produce heat?

I have not yet been able to prove experimentally that the X-rays are also able to produce a heating action; yet we may well assume that this effect if present, since the capability of the X- rays to be transformed is proved by means of the observed fluorescent phenomena. It is certain, therefore, that all the X-rays which fall upon a substance do not leave it again as such.

Can we see x-rays with the eye?

The retina of the eye is not sensitive to these rays. Even if the eye is brought close to the discharge-tube, it observes nothing, although, as experiment has proved, the media contained in the eye must be sufficiently transparent to transmit the rays.

Can x-rays be refracted like light?

7. After I had recognized the transparency of various substances of relatively considerable thickness, I hastened to see how the X-rays behaved on passing through a prism and to find whether they were thereby deviated or not.

Experiments with water and with carbon disulphide enclosed in mica prisms of about 30 degrees refracting angle showed no deviation, either with the fluorescent screen or on the photographic plate. For purposes of comparison the deviation of rays of ordinary light under the same conditions was observed; and it was noted that in this case the deviated images fell on the plate about 10 or 20 mm distant from the direct image. By means of prisms made of hard rubbers and of aluminum, also of about 30 degree refracting angle, I have obtained images on the photographic plate in which some deviation may perhaps be recognized. However, the fact is quite uncertain; the deviation, if it does exist, being so small that in any case the refractive index of X-rays in the substances named cannot be more than 1.05 at the most. With a fluorescent screen I was unable to observe any deviation.

Up to the present time experiments with prisms of denser metals have given no definite results, owing to their feeble transparency and consequently diminished intensity of the transmitted rays.

With reference to the general conditions here involved on the other hand, and on the other to the importance of the question whether the X-rays can be refracted or not on passing from one medium into another, it is most fortunate that this subject may be investigated in still another way than with the aid of prisms. Finely divided bodies in sufficiently thick layers scatter the incident light and allow only a little of it to pass, owing to reflection and refraction; so that if powders are as transparent to X-rays as the same substances are in mass--equal amounts of material being presupposed--it follows at once that neither refraction nor regular reflection takes place to any sensible degree. Experiments were tried with finely powdered rock salt, with finely electrolytic silver-powder, and with zinc-dust, such as is used in chemical investigations. In all these cases no difference was detected between the transparency of the powder and that of the substance in mass, either by observation with the fluorescent screen or with the photographic plate.

From what has now been said it is obvious that X-rays cannot be concentrated by lenses; neither a large lens of hard rubber nor a glass lens having any influence upon them. The shadow- picture of a round rod is darker in the middle than at the edge; while the image of a tube which is filled with a substance more transparent than its own material is lighter at the middle than at the edge.

Can x-rays be reflected like light?

8. The question as to the reflection of the X-ray may be regarded as settled, by the experiments mentioned in the preceding paragraph, in favor of the view that no noticeable regular reflection of the rays takes place from any of the substances examined. Other experiments, which I here omit, lead to the same conclusion.

One observation in this connection should, however, be mentioned, as at first sight it seems to prove the opposite. I exposed to the X-rays a photographic plate which was protected from the light by black paper, and the glass side of which was turned towards the discharge-tube giving the X-rays. The sensitive film was covered, for the most part, with polished plates of platinum, lead, zinc, and aluminum, arranged in the form of a star. On the developing negative it was seen plainly that the darkening under the platinum, the lead and particularly the zinc, was stronger than under the other plates, the aluminum having exerted no action at all. It appears, therefore, that these metals reflect the rays. Since, however, other explanations of a stronger darkening are conceivable, in a second experiment, in order to be sure, I placed between the sensitive film and the metal plates a piece of thin aluminum-foil, which is opaque to ultraviolet rays, but it is very transparent to the X-rays. Since the same result substantially was again obtained, the reflection of the X-rays from the metals above named is proved.

If we compare this fact with the observation already mentioned that powders are as transparent as coherent masses, and with the further fact that bodies with rough surfaces behave like polished bodies with reference to the passage of the X-rays, as shown as in the last experiment, we are led to the conclusion already stated that regular reflection does not take place, but that bodies with reference to the passage of the X-rays, as shown also in the last experiment, we are led to the conclusion already stated that regular reflection does not take place, but that bodies behave toward X-rays as turbid media do towards light.

Since moreover, I could detect no evidence of refraction of these rays in passing from one medium into another it would seem that X-rays move with the same velocity in all substances; and further, that this speed is the same in the medium which is present everywhere in space and in which the particles of matter are embedded. These particles hinder the propagation of the X-rays, the effect being greater, in general, the more dense the substance concerned.

9. Accordingly it might be possible that the arrangement of particles in the substance exercised an influence on its transparency; that, for instance, a piece of calcite might be transparent in different degrees for the same thickness, according as it is traversed in the direction of the axis, or at right angles to it. Experiments, however, on calcite and quartz gave a negative result.

10. It is well known that Lenard came to the conclusion, from the result of his beautiful experiments on the transmission of the cathode rays of Hittorf through a thin sheet of aluminum, that these rays are phenomena of the ether, and that they diffuse themselves through all bodies. We can say the same of our rays.

In his most recent research, Lenard has determined the absorptive power of different substances for the cathode rays, and among others, has measured it for air from atmospheric pressure to 4.10, 3.40, 3.10 referred to 1 cm, according to the rarefaction of the gas contained in the discharge-apparatus. Judging from the discharge pressure as estimated from the sparking distances I have had to do in my experiments for the most part with rarefactions of the same order of magnitude, and only rarely with lesser or greater ones. I have succeeded in comparing by means of the L. Weber photometer--I do not possess a better one--the intensities, taken in atmospheric air, of the fluorescence of my screen at two distances from the discharge-apparatus--- about 100 and 200 mm, and I have found from three experiments, which agree very well with each other, that the intensities vary inversely as the squares of the distances of the screen from the discharge-apparatus. Accordingly, air absorbs a far smaller fraction of the X-rays than of the cathode rays. This result is in entire agreement with the observation mentioned above, that it is still possible to detect the fluorescent light at a distance of 21 meters from the discharge-apparatus.

Other substances behave in general like air; they are more transparent to X-rays than to cathode rays.

Can x-rays be deflected by a magnet? Are they like cathode rays in that respect?

11. A further difference, and a most important one, between the behavior of cathode rays and of X-rays lies in the fact that I have not succeeded in spite of many attempts, in obtaining a deflection of X-rays by a magnet, even in very intense fields.

The possibility of deflection by a magnet has, up to the present time, served as a characteristic property of cathode rays; although it was observed by Hertz and Lenard that there are different sorts of cathode rays, "which are distinguished from each other by their production of phosphorescence, by the amount of their absorption, and by the extent of their deflection by a magnet." A considerable deflection, however, was noted in all cases investigated by them; so that I do not think that this characteristic will be given up except for stringent reasons.

The source of x-rays within the tube is identified.

12. According to experiments especially designed to test the question, it is certain that the spot on the wall of the discharge-tube which fluoresces the strongest is to be considered as the main center from which the X-rays radiate in all directions. The X-rays proceed from the spot where, according to the data obtained by different investigators, the cathode rays strike the glass wall. If the cathode rays within the discharge-apparatus are deflected by means of a magnet, it is observed that the X-rays proceed from another spot--namely, from that which is the new terminus of the cathode rays.

For this reason, therefore, the X-rays, which it is impossible to deflect, cannot be cathode rays simply transmitted or reflected without change by the glass wall. The greater density of the gas outside the discharge-tube certainly cannot account for the great difference in the deflection, according to Lenard.

I therefore reach the conclusion that the X-rays are produced by the cathode rays at the glass wall of the discharge apparatus.

The production of x-rays is not limited to glass.

13. This production does not take place in glass alone, but as I have been able to observe in an apparatus closed by a plate of aluminum 2 mm thick, in this metal also. Other substances are to be examined later.

Photographic images can be created by casting shadows with the x-rays.

14. The justification for calling by the name "rays" the agent which proceeds from the wall of the discharge-apparatus I derive in part from the entirely regular formation of shadows, which are seen when more or less transparent bodies are brought between the apparatus and the fluorescent screen (or the photographic plate).

I have observed, and in part photographed, many shadow-pictures of this kind, the production of which has a particular charm. I possess, for instance, photographs of the shadow of the profile of a door which separates the rooms in which, on one side, the discharge apparatus was placed, on the other the photographic plate; the shadow of the bones of the hand; the shadow of a covered wire wrapped on a wooden spool; of a set of weights enclosed in a box; of a galvanometer in which the magnetic needle is entirely enclosed by metal; of a piece of metal whose lack of homogeneity becomes noticeable by means of the X-rays, etc.

15. I have tried in many ways to detect interference phenomena of the X-rays in a pin-hole photograph which I was able to make of the discharge-apparatus while it was enveloped in black paper; the picture is weak but unmistakably correct.

Do electrostatic forces have an effect on x-rays?

16. Experiments have been begun, but are not yet finished, to ascertain whether electrostatic forces affect the X-rays in any way.

17. In considering the question what are the X-rays--which, as we have seen, cannot be cathode rays--we may perhaps at first be led to think of them as ultraviolet light, owing to their active fluorescence and their chemical actions. But in so doing we find ourselves opposed by the most weighty considerations. If the X-rays are ultraviolet light, this light must have the following properties:

(a) On passing from air into water, carbon disulphide, aluminum, rock-salt, glass, zinc, etc., it suffers no noticeable refraction.

(b) By none of the bodies named can it be regularly reflected to any appreciable extent.

(c) It cannot be polarized by any of the ordinary methods.

(d) Its absorption is influenced by no other property of substances so much as by their density.

That is to say, we must assume that those ultraviolet rays behave entirely differently from the ultra-red, visible and ultraviolet rays which have been known up to this time.

I have been unable to come to this conclusion, and so have sought for another explanation.

There seems to exist some kind of relationship between the new rays and the light rays; at least this is indicated by the formation of shadows, the fluorescence and the chemical action produced by them both. Now, we have known for a long time that there can be in the ether longitudinal vibrations besides the transverse light vibrations; and according to the views of different physicists, these vibrations must exist. Their existence, it is true, has not been approved up to the present, and consequently their properties have not been investigated by experiment.

Ought not, therefore, the new rays to be ascribed to longitudinal vibrations in the ether?

I must confess that in the course of the investigation I have become more and more confident of the correctness of this idea, and so, therefore I permit myself to announce the conjecture, although I am perfectly aware that the explanation given needs further confirmation.


In the Nov. 1, 1895 edition of The X-ray Century we examined the history of gas discharge tubes.

In the Nov. 8, 1895 edition of The X-ray Century we were there when Prof. Roentgen discovered a new kind of ray.

In the Dec. 1, 1895 edition of The X-ray Century we looked at the investigation which led Dr. Roentgen to write this paper.

The next edition of The X-ray Century will be published on February 1.

Webmaster: medjep@emory.edu