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Rh 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'Ic, 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.