290 Mr. 8. Skinner on the 
exposure, whilst the protected parts were exposed only for 
half that time. On development the slit appeared as a light 
line crossing a dark field. This result follows at once from 
the diagram (fig. 1), for if OA represents the 10 minutes’ ex- 
posure the density will be AM, and for a time exposure OB 
the density is BN, a value less than AM. 
Here it will be recognized that the action on the plate 
is like that of light, for an overexposure produces a fainter 
image. The action, however, is more under control as its 
rate is slower. 
Fig. 2. 
ra) 
DENSITY 
ia) N a) Oo oN CO © 

{O' 20° SO) 40 50) 60" 70 VS0, SO-1007tIeF 120 
MINUTES. 
From the arrangement of the radium * I am inclined to 
think that the a-rays were all absorbed before reaching the 
' plate, and that we have to consider the action as coming 
from the @- and y-rays alone, these two sets of rays having 
much more penetrating properties than those of the a group. 
The next experiments were made with the view of finding 
out whether a plate exposed to the light of an electric spark 
would undergo any reversal if it was afterwards exposed 
to radium rays. In fact, I tried to obtain a reversal effect 
similar to that got by Clayden, by exposing a plate first to 
electric sparks and then to faint gas-light, only I substituted 
radium rays for the after-exposure to faint light. Clayden 
showed that it was easy to reverse the images of bright 
sparks by exposing the plate subsequently to a gas-flame for 
afew seconds. In this way he explained the reversed images 
* The radium bromide was contained in an ebonite capsule covered 
with a thin mica plate. The rays passed through the mica. 
