AND SOME ANALOGOUS RAYS. 
485 
The tube was opened and resealed to the pump in a vertical position, so that the 
diaphragm remained constantly in its seat. The electrode K was used as cathode, 
and the support S as anode, daring the exhaustion, which was continued until the 
spark-gap in the alternative circuit could be increased to about 25 millims. There 
were now clear shadows of aluminium, platinum, and glass, all equally dark. But on 
allowing the discharge to pass for 8 or 10 seconds, during which time the exhaustion 
diminished, the shadow of the aluminium arm faded out, leaving those of the arms of 
platinum and glass as dark as before. At this stage of things a barium platino- 
cyanide screen, placed directly over the bulbous end of the tube, showed a faint 
luminosity, but no shadow of the cross could be distinguished. Exhaustion was 
carried gradually higher, until the spark-gap was 4 inches; hut at no stage could the 
shadow of the platinum upon the mica screen he made darker than that of the 
aluminium without first letting the dischai’ge pass for a few seconds. The glass walls 
of the tube showed yellow-green luminescence only on the lowest region between the 
cathode and the diaphragm, save a slight trace on the upper end of the tube, which 
also showed yellow-green. Yet, on examining the side of the tube with a barium 
platino-cyanide screen, it was found that Kontgen rays were being emitted both 
above and below the diaphragm, the luminescence being almost as bright above as 
below. The place w^here the diaphragm lay across the tube was marked by a strong 
clean black line between two regions of almost equal luminosity. Hence it would 
seem that in this case the rays which produce the yellow-green luminescence are 
stopped by the aluminium diaphragm, whilst other rays pass through, which are 
competent both to cast an internal deflectible shadow upon a scheelite screen, and to 
produce R5ntgen rays, which cause luminescence outside the tube on a platino- 
cyanide screen. 
To test the sifting action of aluminium foil more thoroughly, the tube [No. M 14], 
(fig. 18), was designed. The bulb is provided with two concave cathodes, one, K^, at 
the side, to focus upon a small oblique anticathode of platinum at the centre, the 
other, Kj, at the summit, being larger and shallower, intended to focus past the anti¬ 
cathode to a point in the narrow aperture at the bottom of the bulb. The object of 
this design was to allow either ordinary (ortho-)cathodic rays to be thrown directly 
down from K 2 , or by the use of the lateral cathode and the oblique target, to 
throw down Bontgen rays, accompanied by para-cathodic rays. Below the aj)erture 
at the bottom of the bulb a horizontal glass tube, Hj^, was fixed, and below this, with 
a short vertical neck intervening, a second horizontal tube. Ho, at right angles to the 
first. Each of these tubes was fitted at one end with a ground glass stopper, their 
other ends being closed. A platinum wire was sealed, in through the stopper of H^ 
Nitrate of silver was used as a cement to make the stoppers vacuum-tight. In the 
upper of the two horizontal tubes was laid a short slip of sheet-lead, having eight 
holes punched in it. These holes were covered with pieces of aluminium foil in 
various thicknesses, to act as screens or filters to sift the rays. In the experiments 
