IMAGE TRANSMISSION SYSTEM 



455 



shown at L, Fig. 4. This lens is used to magnify the incoming image 

 to such a size that the image structure is just on the verge of visibiUty, 

 under which condition the face of the distant person appears as though 

 he were approximately eight feet away. In addition to acting as 

 a magnifier, this lens serves to position the incoming image to fit the 

 height of the user. For, by raising or lowering it by means of a 

 knob, the operator, using the information as to the observer's height 

 obtained from the position of the scanning beam, locates the lens so 

 that the virtual image appears in the proper position. 



Photoelectric Cells and Associated Circuits 



The photoelectric cells used in this apparatus are similar in shape 

 to those used in the first demonstration, but somewhat larger. Each 



100 



80 



I- 



z 

 u 

 Cj 60 



40 



20 



0.3 



0.4 



0.5 0.6 



WAVE LENGTH IN p. 



0.7 



o.a 



Fig. 5 — .4. Relative optical transmission of the blue filter through which the 



scanning beam passes. B. Relative sensitivity of the photoelectric cells to various 



parts of the spectrum. C. Relative sensitivity of the eye to A-arious parts of the 

 spectrum. 



cell is twenty inches long and four inches in diameter, giving it an 

 area of approximately eighty square inches for collecting light. The 

 anode is made in the form of a hollow glass rod wound with wire. 

 This construction prevents the electrical oscillations that would 

 otherwise result from mechanical vibrations of the anode. The sen- 

 sitive cathode consists of a coating, covering the rear wall of the tube, 

 of potassium sensitized with sulphur.'^ This kind of cell is consider- 

 ably more sensitive than the older type of potassium-hydride cell 



3 A. R. Olpin, Phys. Rev., 33, 1081 (1929). 

 30 



