On a New Species of Light (March 23, 1903).
The radiations emitted by a focus tube are filtered through a sheet of aluminium foil or a screen of black paper, in order to eliminate the luminous rays which might accompany them. While studying these radiations by means of their action on a small spark, I discovered that they are plane-polarized as soon as emitted. I further proved that when these radiations traverse a plate of quartz in a direction at right angles to its axis, or a lump of sugar, their plane of action undergoes a rotation just like the plane of polarization of a pencil of light (see pp. 5 and 6).
I then asked myself if a rotation could also be obtained by passing the radiations of the focus tube through a pile of Reusch mica sheets. I observed, in fact, a rotation of from 25° to 30° in the same direction as that of polarized light. This action of a pile of micas made me at once infer that a single sheet of mica must act, and that this action must be depolarization, or, rather, the production of elliptic polarization; this is indeed what occurs. The interposition of a sheet of mica, set so that its axis makes an angle of 45° with the plane of action of the radiations emitted by the tube, destroys their rectilinear polarization, for their action on a small spark remains sensibly the same, whatever be the direction of the spark-gap. If a second sheet of mica is interposed, identical with the first, so that the axes of the two sheets are perpendicular to each other, rectilinear polarization is re-established. This result can also be obtained by the use of a Babinet's compensator. Consequently, we are dealing with elliptic polarization.
Now, if the sheet of mica changes rectilinear into elliptic polarization, such a sheet must be doubly refractive for the radiations thus transformed. But if double refraction exists, a fortiori simple refraction must exist; and I was thus led to examine whether, in spite of the fruitless attempts to discover the refraction of "X" rays, I could not obtain a deviation by a prism. I then arranged the following experiment: a focus tube sends through an aluminium screen a pencil of rays, limited by two vertical slits cut in two parallel sheets of lead, 3 mms. thick. The small spark is placed on one side of the pencil at such a distance that it cannot be reached, even by the penumbra; this is ascertained by proving that the interposition of a sheet of lead causes no diminution of its brightness. Now let us interpose in the pencil an equilateral quartz prism, with refractive edge on the side away from the spark. If the prism is properly set, the spark becomes much more brilliant; when the prism is removed, the spark reverts to its former faintness. This phenomenon is certainly due to refraction, for if the setting of the prism is altered, or if the prism is replaced by a plate of quartz, no effect is observed. The experiment may also be carried out in a different manner: the pencil is first made to impinge directly on the spark, then it is deviated by means of the prism, and the brightness of the spark wanes. If, now, the spark is moved laterally towards the base of the prism, it recovers its previous brightness, proving that the rays in question have been deviated in the same sense as rays of light.
Refraction being thus proved, I at once sought to concentrate the rays by means of a quartz lens. The experiment is unattended with difficulty. An image of the anticathode is obtained, extremely well-defined as to size and distance by a heightened glow of the small spark.
The existence of refraction rendered that of regular reflection extremely probable; as a matter of fact, regular reflection does take place. By means of a quartz lens, or a lens formed by a very thin horn envelope filled with turpentine, I produce a conjugate focus of the anticathode; then I intercept the emerging pencil by a sheet of polished glass, placed obliquely; I then obtain a focus exactly symmetrical, in respect to the plane of reflection, with the one which existed before the glass was interposed. With a plate of ground glass there is no regular reflection, but diffusion is observed.
If one half of a lamina of mica is roughened, the polished half lets pass the radiations, and the other half stops them (note 3).
This allows of the repetition of the refraction experiments under much more precise conditions, by the use of Newton's arrangement for obtaining a pure spectrum.
From all that precedes, the fact results that the rays which I have thus studied are not Röntgen rays, since these undergo neither refraction nor reflection. In fact, the little spark reveals a new species of radiations emitted by the focus tube, which traverse aluminium, black paper, wood, etc. These are plane-polarized from the moment of their emission, are susceptible of rotatory and elliptic polarization, are refracted, reflected, diffused, but produce neither fluorescence nor photographic action.
I had expected to find that amongst these rays some existed whose refractive index for quartz is about 2; but probably quite a spectrum of such rays exists, for in the refraction experiments with a prism, the deviated pencil appears to cover a broad angle. The study of this dispersion remains to be pursued, as well as that of the wave-lengths of the rays.
By progressively diminishing the intensity of the current actuating the induction-coil, one still gets these new rays, even when the tube no longer produces any fluorescence, and is itself absolutely invisible in the dark. They are fainter, however, in this case. They can also be produced continuously by means of an electric machine giving a spark a few millimetres in length.
At first I had attributed to Röntgen rays the polarization which in reality belongs to the new rays, a confusion which it was impossible to avoid before having observed the refraction, and it was only after making this observation that I could with certainty conclude that I was not dealing with Röntgen rays, but with a new species of light.
It is interesting to collate these remarks with the view expressed by M. Henri Becquerel, that in certain of his experiments "manifestations identical with those giving refraction and total reflection of light may have been produced by luminous rays which had traversed aluminium" (see Comptes Rendus, t. xxxii., March 25, 1901, p. 739).