624 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
of the two forms of attendant phosphorescence described in the previous section, and 
also to obtain them at will. If a long slip of tinfoil be laid along a high vacuum tube 
and connected to the positive terminal, and a considerable positive air-spark be used, the 
thin positive luminosity will be seen to lie along the slip till we arrive at the end of the 
strip nearest to the negative terminal. Here it will naturally cease clinging to the side 
of the tube and slant off to a more central position (Plate 29, fig. 30), thus causing an 
angle in its direction. The attendant phosphorescence can be distinguished under 
favourable circumstances all along the line of the slip of tinfoil; and the spot where 
the angle is formed in the direction of the positive luminosity as above described will 
be found to be covered with bright phosphorescence, precisely as was the case when 
the positive luminosity was examined by the methods detailed in the previous section. 
The best arrangement for securing these effects is by the use of the positive unipolar 
discharge. So clearly are all the details of the phenomenon shown thereby that the 
attendant phosphorescence is readily distinguished in the shape of two bright green 
lines running along the two edges of the slip of tinfoil, these edges being doubtless 
the operative parts of the tinfoil considered as creating a quasi-positive-terminal in the 
interior of the tube. By this arrangement it is perfectly easy to obtain these effects 
in a sufficiently brilliant form to permit of the ready determination of the direction of 
arrival of the molecular streams which cause the phosphorescence. It will be found 
that the results in all cases are in conformity with the conclusions already arrived at. 
But it is not only in the above manner that positive special produces phosphor¬ 
escence. If we recall to mind the special effect in low tension tubes with a positive 
air-spark, we shall remember that the hollow cone of luminosity which marked the 
positive discharge in the tube directly caused by the positive impulses within the tin- 
foil, was separated from the truncated positive column by a dark space, across which 
occurred the action by which the advancing positive discharge became satisfied by the 
negative electricity left behind in the tube beneath the tinfoil. The discharge of this 
latter from the glass under the influence of the advancing positive electricity is a case 
of true negative discharge, and accordingly we find that in tubes of sufficiently high 
tension with sufficiently long air-sparks it is accompanied by the usual molecular 
streams, and that it produces the usual phosphorescence. This phosphorescence must 
be due to a negative discharge commencing almost synchronously with that of positive 
relief, for they both take place through the action of the advancing positive electricity, 
but it is of much less violence. If a piece of tinfoil be placed on the spot where the 
positive-special-phosphorescence falls, and then be connected to earth, it will be found 
that it clears the glass beneath it and for a short distance round its edge from phos¬ 
phorescence; while, on the other hand, it is able to throw phosphorescence over the 
glass immediately below the tinfoil that is connected to the positive terminal. This 
superior intensity of relief phosphorescence will be found to be of importance later on. 
Since positive special is accompanied by a negative discharge which commences 
almost synchronously with that which gives rise to relief phosphorescence and virtual 
