454 



BELL SYSTEM TECHNICAL JOURNAL 



have to do with the positioning of the outgoing and incoming images. 

 A practical problem which is encountered when customers of various 

 heights use the apparatus is that the scanning beam, if fixed in its posi- 

 tion, would strike too high or too low upon many faces. In order 

 to direct the beam up or down as is required, a variable angle prism, 

 consisting of two prisms arranged to rotate in opposite directions, 



Fig. 4 — Optical means for controlling heights of scanning and viewing beams. 



is interposed in the path of the scanning beam. This prism, which 

 lies directly in front of the projection lens used with the upper disc, 

 is shown in Fig. 4 at P. Its rotation is controlled by a knob with a 

 numbered dial. The exact position is determined by the operator 

 by reference to a monitoring image which will be described below. 

 Another optical element which serves two purposes, is a large con- 

 vex lens lying between the receiving disc and the observing frame, 



