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each other; in other words, it is polarized in the complete sense of the term.

The phenomenon is easy to observe when the spark is well regulated; this means that the spark must be very small and faint.

If the focus tube is made to turn about its axis, which is parallel to the cathode rays, the observed phenomena do not change, so long as “X” rays reach the gap. The plane of action is thus independent of the orientation of the anticathode, being always the plane passing through the “X” rays and the generating cathodic rays.

The spark being kept in this plane, and turned round from the position in which it is at right angles to the “X” rays to that in which it is parallel to them, we observe that the effect of the “X” rays on the brightness of the spark is a maximum in the first position, and diminishes to nothing in the second

Now, an “X”ray and its generating cathodic ray only determine a plane when their directions are different. Again, amongst the emitted “X” rays, some are in a direction very nearly the same as that of the cathode rays, being

those which graze the cathode. One should expect these to be very incompletely polarized ; and, indeed the small spark enabled me to confirm this.

I noted several important facts, which, however, I will merely allude to in the present paper. Quartz and lump-sugar rotate the plane of polarization of “X” rays in the same sense as that of light. I obtained rotations of 40°.

Secondary rays, styled “S” rays, are also polarized. Active substances rotate the plane of polarization of these rays in a sense contrary to that of light. I observed rotations of 18° (note 2).

It is extremely likely that magnetic rotation also exists for “X” rays as well as for “S” rays. One can also surmise that the properties of these rays, with reference to polarization, extend to tertiary rays, etc. I intend shortly to publish the results at which I have arrived concerning these different points.

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

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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

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