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as I used in my researches on the velocity of propagation of “X” rays, might perhaps in this case play the part of analyzer, inasmuch as the properties of a spark may be different in the direction of its length, which is also that of the electric force producing it, and in directions normal to its length. Starting from this, I arranged an apparatus as shown in the accompanying diagram, so as to obtain a small spark during the emission of “X” rays.

A focus tube is connected to an inductioncoil by wires BH, B'H', covered with guttapercha (Fig. 1). Two other wires, also covered with gutta-percha, Alc and A'Ic', terminate at A and A' in two loops, which surround BH and B'H' respectively; a bit of glass tubing, not shown in the figure, keeps each loop separate from the wire which it surrounds. The wires AI, A'I are then twisted together, and their sharply pointed ends, c and c', are fixed opposite each other, at a very small distance, adjustable at will, so as to form a small sparkgap. By virtue of this disposition, the electrostatic influence exercised by the wires BH and B'H' on the loops A and A' produces

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at each break of the current in the coil a small spark at the gap cc', at the same time as “X” rays are being emitted by the tube. Owing to the flexibility of wires, AIC, A'I'c', the straight line cc', along which the spark occurs, can be set in any direction we please. A sheet of aluminium foil, 40 cms. square, is interposed between the tube and the spark, so as to prevent any direct influence of the electrodes of the tube on cc'.

In order to define easily the relative positions of the tube and the spark cc', take three rectangular axes, of which one, Oz, is vertical.

Fix the focus tube so that its length, and, consequently, the pencil of cathode rays, coincides with OY, the anticathode being placed near the origin, and sending “X” rays in the positive direction of OX.

Place the gap cc' at a point on the positive side of Ox, so that its direction is parallel to OY. The spark being properly regulated one observes that the “X” rays act upon it in such a way as to increase its luminosity, for the interposition of a sheet of lead or glass manifestly diminishes the brightness.

Now, without altering the position of the gap, turn it so that it comes parallel to OZ, i.e. normal to the cathode rays. The influence of the “X” rays on the spark is then seen to disappear, and the interposition of a lead or glass plate causes no change in its brightness.

“X” rays have therefore a plane of action, which is the one passing through each “X” ray and the cathode ray which gives rise to it. If the direction given to the spark-gap is intermediate between the two above mentioned, the action is seen to diminish from the horizontal position to the vertical.

The following is another experiment, still more striking : if the spark is made to turn about OX, parallel to plane YOZ, the spark is seen to pass from a maximum brightness when horizontal to a minimum when vertical. These variations of brightness are similar to those observed when a pencil of polarized rays traverses a rotating Nicol's prism, the small spark playing the part of analyzer. The pencil of X” rays presents the same asymmetry as a pencil of polarized light. According to Newton's definition, it has sides differing from

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