334 



SCIENCE PROGRESS 



synchronously with the cylinder ff in fig. i. But in trans- 

 mission, a prism is placed at q, which turns the light downwards 

 upon the compensating cell Se 2 . Between the poles of the 

 magnet m m is fixed a galvanometer unit consisting of two very 

 fine silver wires, to which are attached, where they cross the 



shutter 



i 



■# 



EZZZ2ZZ2Z3 



Fig. 2. 



optic axis, a tiny magnesium foil shutter, about J^th inch square. 

 The shadow of this is made by optical arrangement just to cover 

 the cell Se 2 in transmission, or, in receiving, to cover a tri- 



front of the dark 



angular aperture in a diaphragm fixed in 



Fig. 3. 



box in which the receiving drum rotates. When current passes 

 through the silver wires, a lateral displacement is produced, 

 which enables the light to reach the cell Se 2 . With maximum 

 current, the whole cell is illuminated, with i/xth of the maximum 

 current, i/xth of the cell is illuminated, and so on. 



