SOME OPTICAL FEATURES IN TWO-WAY TELEVISION 267 



lions of the cells (which are virtual light sources) are likely to be seen 

 reflected in eyeglasses. Since, however, the head is normally directed 

 somewhat downward, cells placed in these upper corner spaces are not 

 serious offenders in this respect. 



Two other features of the two-way system which needed revision 

 when the caesium cells were adopted, were the variable angle prisms 

 used to direct the scanning beam upward or downward, depending on 



F~ig. 1 — Incandescent lamp used for scanning light. 



the user's height, and the general illumination of the television-tele- 

 phone booth. As to the variable angle prisms, the only change called 

 for was the substitution of achromatic prisms, corrected for deep red 

 and blue light, in order to prevent the scanning beam from breaking 

 effectively into two beams for large angles of deviation. The problem 

 of general illumination of the booth is principally the choice of a color 

 of light which shall affect neither the potassium nor the ca'sium cells. 

 For this purpose, a monochromatic yellow-green was chosen, secured 

 by covering all the lights with a combination of orange and signal green 

 glasses. The potassium cells are insensitive to this color of light, and 



