144 
MR. W. CROOKES ON THE ILLUMINATION OF LINES OF 
the green light is still better, and there is no violet light on the positive pole. 
At '9 M the green phosphorescence is about at its maximum. When the exhaustion 
reaches 15 M the spark has a difficulty in passing, and the green light only appears in 
flashes occasionally, and then only in patches over the tube. At '06 M the vacuum 
is almost non-conducting, and a spark can only be forced through by increasing the 
strength of the coil and well insulating the tube and wires leading to it. Beyond 
that exhaustion nothing can be observed. 
513. It is difficult to experiment at these high vacua. After the induction spark 
has been forced through a few times, gas is liberated from the metallic poles or from 
the surface of the glass, conduction through the residual gas begins, and the appear¬ 
ance of light commences. Further exhaustion again impairs the conductivity. The 
greatest care must be taken to keep the conducting wires away from the tube except 
just where they touch the terminals ; they should be well insulated and no metal 
should approach the tube. With all these precautions, however, it is difficult to 
prevent a spark passing through the side of the tube, when the vacuum is at once 
spoiled. 
FOCUS OF MOLECULAR ENERGY. 
514. To investigate still further the phenomena attending the concentration of 
force to a focus, another apparatus was made, as shown in fig. 12. a is a hemi- 
Fig. 12, 
cylinder, 22 millims. diameter, accurately made of polished aluminium ; it is supported 
in position by the glass tube b, through which passes a fine copper wire connecting 
shadow is thrown upon the side, since it excludes such rays of the cathode as impinge upon it from reach¬ 
ing the side. If the solid body after some time is removed, the shadow disappears, but an image of the 
body remains, distinguished from the surrounding luminous surface by its greater brightness, and exactly 
reproducing the shape of the former shadow.” 
