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7. After my experiments on the transparency of increasing thickness of different media, I proceeded to investigate whether the X rays could be deflected by a prism. Investigations with water and carbon bisulphide in mica prisms of 30 degrees showed no deviation either on the photographic or the fluorescent plate. For comparison, light rays were allowed to fall on the prism as the apparatus was set up for the experiment. They were deviated 10 mm. and 20 mm. respectively in the case of the two prisms.
With prisms of ebonite and aluminium, I have obtained images on the photographic plate, which point to a possible deviation. It is, however, uncertain, and at most would point to a refractive index 1.05. No deviation can be observed by means of the fluorescent screen. Investigations with the heavier metals have not as yet led to any result, because of their small transparency and the consequent enfeebling of the transmitted rays.
On account of the importance of the question, it is desirable to try in other ways whether the X rays are susceptible of refraction. Finely powdered bodies allow in thick layers but little of the incident light to pass through, in consequence of refraction and reflection. In the case of the X rays, however, such layers of powder are for equal masses of substance equally transparent with the coherent solid itself. Hence we cannot conclude any regular reflection or refraction of the X rays. The research was conducted by the aid of finely powdered rock salt, fine electrolytic silver powder, and zinc dust already many times employed in chemical work. In all these cases the result, whether by the fluorescent screen or the photographic method, indicated no difference in transparency between the powder and the coherent solid.
It is, hence, obvious that lenses cannot be looked upon as capable of concentrating the X rays; in effect, both an ebonite and a glass lens of large size prove to be without action. The shadow photograph of a round rod is darker in the middle than at the edge; the image of a cylinder filled with a body more transparent than its walls, exhibits the middle brighter than the edge.
8. The preceding experiments, and others which I pass over, point to the rays being incapable of regular reflection. It is, however, well to detail an observation which at first sight seemed to lead to an opposite conclusion.
I exposed a plate protected by a black paper sheet, to the X rays, so that the glass side lay next to the vacuum tube. The sensitive film was partly covered with star-shaped pieces of platinum, lead, zine, and aluminium. On the developed negative the star-shaped impression showed dark under platinum, lead, and, more markedly, under zinc; the aluminium gave no image. It seems, therefore, that these three metals can reflect the X rays. As, however, another explanation is possible, I repeated the experiment with this only difference, that a film of thin aluminium foil was interposed between the sensitive film and the metal stars. Such an aluminium plate is opaque to 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.
If one considers this observation in connection with others, namely, on transparency of powders, and on 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.
Since I have obtained no evidence of refraction at the surface of different media, it seems probable that the X rays move with the same velocity in all bodies, and in a medium which penetrates everything, and in which the molecules of bodies are embedded. The molecules obstruct the X rays the more effectively as the density of the body concerned is greater.
9. It seemed possible that the geometrical arrangement of the molecules might affect the action of a body upon the X rays, so that, for example, Iceland spar might exhibit different phenomena according to the relation of the surface of the plate to the axis of the crystal. Experiments with quartz and Iceland spar on this point lead to a negative result.
10. It is known that Lenard, in his investigations on cathode rays, has shown that they belong to the ether, and can pass through all bodies. Concerning the X rays the same may be said.
In his latest work, Lenard has investigated the absorption coefficients of various bodies for the cathode rays, including air at atmospheric pressure, which gives 4.10, 3.40, 3.10 for 1 cm., according to the degree of exhaustion of the gas in discharge tube. To judge from the nature of the discharge, I have worked at about the same pressure, but occasionally at greater or smaller pressures. I find, using a Weber's photometer, that the intensity of the fluorescent light varies nearly as the inverse square of the distance between screen and discharge tube. This result is obtained from three very consistent sets of observations at distances of 100 and 200 mm. Hence air absorbs the X rays much less than the cathode rays. This result is in complete agreement with the previously described resuit, that the fluorescence of the screen can be still observed at two metres from the vacuum tube. In general, other bodies behave like air; they are more transparent for the X rays than for the cathode rays.
11. A further distinction, and a noteworthy one, results from the action of a magnet. I have not succeeded in observing any deviation of the X rays even in very strong magnetic fields.
The deviation of cathode rays by the magnet is one of their pecu liar characteristics; it has been observed by Hertz and Lenard, that several kinds of cathode rays exist, which differ by their power of exciting phosphorescence, their susceptibility of absorption, and their deviation by the magnet; but a notable deviation has been observed in all cases which have yet been investigated, and I think such deviation affords a characteristic not to be set aside lightly.
12. As the result of many researches, it appears that the place of most brilliant phosphorescence of the walls of the discharge tube is the chief seat whence the X rays originate and spread in all directions; that is, the X rays proceed from the front where the cathode rays strike the glass. If one deviates the cathode rays within the tube by means of a magnet, it is seen that the X rays proceed from a new point, i. e., again from the end of the cathode rays.
Also for this reason the X rays, which are not deflected by a magnet, cannot be regarded as cathode rays which have passed through the glass, for the passage cannot, according to Lenard, be the cause of the different deflection of the rays. Hence I conclude that the X rays are not identical with the cathode rays, but are produced from the cathode rays at the glass surface of the tube.
I have obtained
13. The rays are generated not only in glass. them in an apparatus closed by an aluminium plate two mm. thick. I purpose later to investigate the behavior of other substances.
14. The justification of the term " rays," applied to the phenomena, lies partly in the regular shadow pictures produced by the interposition of a more or less permeable body between the source and a photographic plate or fluorescent screen.
I have observed and photographed many such shadow pictures. Thus, I have an outline of part of a door covered with lead paint; the image was produced by placing the discharge tube on one side of the door, and the sensitive plate on the other. I have also a shadow of the bones of the hand, of a wire wound upon a bobbin, of a set of weights in a box, of a compass, card, and needle completely inclosed in a metal case, of a piece of metal where the X rays show the want of homogeneity, and of other things.
For the rectilinear propagation of the rays, I have a pin-hole photograph of the discharge apparatus covered with black paper. is faint but unmistakable.
15. I have sought for interference effects of the X rays, but, possibly in consequence of their small intensity, without result.
16. Researches to investigate whether electrostatic forces act on the X rays are begun but not yet concluded.
17. If one asks, What, then, are these X rays? Since they are not cathode rays, one might suppose, from their power of exciting fluorescence and chemical action, them to be due to ultra-violet light. In opposition to this view a weighty set of considerations presents itself. If X rays be indeed ultra-violet light, then that light must possess the following properties:
(a) It is not refracted in passing from air into water, carbon bisulphide, aluminium, rock salt, glass, or zinc.
(b) It is incapable of regular reflection at the surface of the above bodies. (c) It cannot be polarized by any ordinary polarizing media.
(d) The absorption by various bodies must depend chiefly on their density. That is to say, these ultra-violet rays must behave quite differently from the visible, infra-red, and the hitherto known ultra-violet
These things appear so unlikely that I have sought for another hypothesis.
A kind of relationship between the new rays and light rays appears to exist; at least the formation of shadows, fluorescence, and the production of chemical action point in this direction. Now it has been known for a long time, that besides the transverse vibrations which account for the phenomena of light, it is possible that longitudinal vibrations should exist in the ether, and, according to the view of some physicists, must exist. It is granted that their existence has not yet been made clear, and their properties are not experimentally demonstrated. Should not the new rays be ascribed to longitudinal waves in the ether?
I must confess that I have in the course of this research made myself more and more familiar with this thought, and venture to put the opinion forward, while I am quite conscious that the hypothesis advanced still requires a more solid foundation.
The problem indicated above by Professor Röntgen, as to the real nature of the X rays and their relation to the phenomena of light, electricity, sound, and other familiar subjects of hypothesis, can be solved only after much careful investigation. To this end the mere indefinite repetition of taking prints by means of the rays makes no contribu
tion. The inquiry is still in its initial stages: not yet have been determined even the precise conditions under which the new rays manifest themselves: we even have yet to see laid a sure experimental basis for speculation.
The accompanying cuts serve to illustrate the ordinary method of obtaining prints by means of the X rays. The object of which a radiotype is desired-say the lead in a pencil, or a purse containing coins, or a human hand-is inserted in the path of the rays, between the vacuum tube from which they emanate and a light-tight box containing an ordinary highly sensitized photographic plate. The requisite time of exposure varies under conditions not yet well understood. The X rays, as they pass through the objects interposed between the tube and the sensitive plate, are obstructed in varying degrees according to the thickness or density of the material parts of the objects. Thus the lead in the pencil, the coins in the purse, or the bones in the hand, absorb a greater proportion of the rays than the wood, the leather, or the flesh, and are accordingly outlined in shadow upon the plate.
The honor of having been the first to obtain a genuine X-ray print, is claimed for the University of Pennsylvania. It was obtained February 22, 1890, by accident; but the phenomenon was not followed up by research, being treated as inexplicable.
The prevailing opinion seems to be, that in their essence the X rays are a hitherto unknown form of manifestation of the same ultimate energy which, under other conditions, appears as light and electricity. They are not light in the ordinary sense, for they do not affect the eye. Nor are they the same as the well-known infra-red or ultra-violet rays of invisible light, whose rapidity of vibration falls respectively below or above the range to which the human eye is susceptible; for, while it seems possible that they may differ from these latter not in kind, but merely in the degree of rapidity of their vibrations, their behavior ranks them in a separate class: they are not readily susceptible of refraction, diffraction, con centration by a lens, or even reflection by ordinary reflecting surfaces. That they pass through opaque substances is perhaps less important, because we already have examples of selective transparency to light vibrations, and this may be only another, though unusually striking It seems probable, however, that these new rays are intercepted by bodies mainly in proportion to their density or their atomic weight, in which case the selection differs in kind from what we are familiar with. Two things they have in common with ordinary light-they can produce fluorescence, and they can cause chemical changes in the sensitive film on a photographic plate.
If the X rays were simply ultra-violet rays of shorter wave length than any yet recognized, we might expect them to follow the analogy of shortening waves by being more highly refrangible than the ultraviolet. They are not apparently refrangible at all under ordinary
APPARATUS USED BY PROF. A. W. WRIGHT OF YALE FOR OBTAINING X-RAY PRINTS.
On a clamp support is carried the Crookes tube. Professor Wright used one of approximately spherical shape of the type originally used by Professor Crookes to show the dependence upon the negative pole of radiant state phenomena. The excitation was furnished by an induction coil, the primary of which was excited by a five-cell storage battery, and the secondary was taken as giving 200,000 to 300,000 volts potential, corresponding roughly to a spark length or distance between electrodes of two to three inches in air. Wires from the secondary were connected to the terminals of the Crookes tube as shown, the negative wire to the upper electrode. On the table, a few inches below the tube, the sensitized plate contained in an ordinary plate holder was placed, and on its slide of ebonite were placed the objects to be "photographed."
conditions. However, Professor J. J. Thomson of Cambridge University has shown that this result might follow were their wave lengths of an order of magnitude comparable to the dimensions of a molecule; so that the non-refrangibility of X rays, even were it absolute, would offer no clue to their relation to ultra-violet rays.
However, Mr. Lascelles-Scott, a British scientist, states that the X rays are refrangible under certain conditions; and the researches of Professor J. S. McKay of the Packer Institute, Brooklyn, N. Y., point toward double refraction of the rays by Iceland spar in the same way as ordinary light is doubly refracted-all of which favors the view of the kindred nature of the X rays and light rays.
Another important announcement-made by Mr. Tesla-is that the X rays are to some extent capable of reflection. Tesla claims that by means solely of the rays reflected from a thick plate of glass placed at an angle of 45 degrees to the axis of a thick copper tube containing a sensitive plate, he has obtained X-ray shadow-prints after an exposure of 45 minutes. The terminal bulb was so placed