298 



ANNUAL, EEPORT SMITHSONIAN INSTITUTION, 1931 



The second disk, which is 30 inches in diameter, is placed below 

 the sending disk and exposes through its holes the neon lamp, which 

 the observer sees through a magnifying lens in a position slightly 

 below that of the scanning beam. This neon lamp is, of course, 

 actuated by the signals coming in from the distant end of the 

 system, where there is a similar arrangement of two disks, photo- 

 electric cells, and neon lamp. 



The two parties to the conversation take their places in sound- 

 proof and light-proof booths, where, sitting in front of the photo- 

 electric cells, they look at the image of the person at the other end 



TRANSMITTING 

 DISK 



MICROPHONE 



(Sr PHOTO 

 ^-^ELECTRIC 



FiGUKE 1. — Two-way television is essentially the same in principle as the television 

 demonstrated three years ago. A beam of light from an arc light is thrown by a 

 scanning disk on the speaker's face, and reflected light is picked up by photoelectric 

 cells and transmitted electrically to the distant end. The incoming image is seen 

 by means of the lower scanning disk and a neon tube. A concealed microphone and 

 loud speaker act as speech terminal elements to complete the television-telephone 

 system 



at the same time that the scanning beam plaj^s over their faces. A 

 problem of illumination is immediately encountered in that the 

 scanning beam is of necessity intensely bright and tends to dazzle 

 the eyes to the extent that the somewhat faint neon lamp image is 

 hard to see. This difficulty is met by using light for scanning to 

 which the photo-electric cells are extremely sensitive, but to which 

 the human eye is relatively insensitive, that is, blue light. By inter- 

 posing a filter in the path of the scanning beam, the spot of light in 

 the lens which projects it is seen as a blue disk of light not bright 

 enough to interfere with clear vision of the neon lamp which 

 provides the image of the person located at the distant end. 



