148 
MR. W. CROOKES OK THE ILLUMINATION OF LIKES OF 
tube; if, however, e or f is one pole and c the other, the luminosity only occupies the 
upper part of the tube, and if e and f are the two poles, the light keeps close to 
the top. The whole appearance shows that both poles are at work in producing’ the 
phenomena. 
526. When, however, the vacuum is sufficiently high for the dark space round the 
negative pole to have swollen out to the dimensions of the tube, there is little dif¬ 
ference in the phenomena of green phosphorescence and projection of the shadow of c 
on b, whichever is the positive pole, provided e be made the negative. The appearances 
are almost the same, and the shadows projected from the negative pole e are just as 
sharp and intense whether I make f or h the positive pole. The positive pole, in fact, 
seems to have little or nothing to do with the phenomena. (519, 527, 549.) 
Fig. 13. 
V 
527. The best and sharpest shadows are cast by the flat disks e and h. The shadows 
thrown by the pointed poles f and g are faint and undecided in outline. An aluminium 
ring scarcely makes any shadow; a spherical pole, owing to the rays from it diverging 
more, gives faint and broad shadows; a square pole acts the same as a disk. Using 
the upper flat pole e as the negative, the shadow of the star c is thrown distinctly on 
the uranium plate b, where it is seen magnified about two diameters, but perfectly 
sharp in outline ; either f g, or h, and even the star itself may be made the positive 
pole without affecting the appearance of its shadow on b. (519, 526, 549.) 
528. The whole upper part of the tube which is in the line of direct projection 
from the negative pole, glows with an intense yellowish-green fluorescent light. The 
uranium plate is still more brilliant, and of a greenish colour. Where the shadow 
of the star falls on it, no phosphorescence whatever is visible. The mica plate a, 
where uncovered at the side of the uranium plate, gives no phosphorescence, and no 
