38 PHOTO-TELEGRAPHY 



the selenium in the sending apparatus is not con- 

 stant, but decreases as the illumination increases. 

 The angle at the apex of the triangle has some- 



FIG. 17. Photograph of galvanometer as used 

 by Prof. Korn. 



times to be altered to suit a particular cell, but is 

 usually about 50 to 60. The shutter, and hence 

 the shadow on E, is constantly moving more or less 

 towards the base of the triangle according to the 

 strength of the current and therefore to the density 



