49? 
gram on the back side of the door is shown in Fig. 3 (A). The mica is 
held in place by means of four pices of brass, 1.7 mm. thick and 1.5 cm. 
wide. These pieces are screwed onto the door in such a way that they not 
YT, = 2 BY Fa 
AY 
only hold the mica in place over the window but they also form a close 
fitting rabbeted joint of the door to the box. 
The back side of the electroscope is constructed like the front side, 
with a mica window of the same dimensions for illumination. This side 
may be either hinged to the box or it may be held in place by means of 
two little hooks. 
