272 BELL SYSTEM TECHNICAL JOURNAL 



entire frame of the picture and the lenses /; P represents the glow lamp 

 electrode. A great advantage of this optical arrangement is that the 

 cathode of the glow lamp can be made quite small, and can be removed, 

 as shown, to a considerable distance from the glass wall of the contain- 

 ing tube. In consequence of these changes in lamp design, a very high 

 current density can be obtained for a relatively low expenditure of 

 energy, with at the same time a long lamp life. 



The condenser lens disc is observed exactly as the simple disc, by 

 the eye placed at E. According to Nipkow, when lenses are used on 

 the disc, the holes should be covered with diffusing material. This is 

 not necessary in the present case, because in the two-way booth, the 

 observer has very little latitude of motion, and it is only necessary that 

 his eyes lie in the overlapping cones of rays from the extreme holes in 

 the field. By making the lenses / of large diameter compared with 

 their focal length, the solid angle through which an image is visible 

 is entirely adequate. 



The general characteristics of the lamps used are shown in Figs. 4 

 and 5. The cathode is a heavy slug of copper, into which a hollow 

 cylindrical aluminium electrode is screwed, shielded from the copper by 

 mica and glass. Because of the large mass of the copper, the water- 

 cooling is no longer necessary. With lamps of this type, the amplifier 

 circuit used before makes it possible to obtain images of much greater 

 brilliancy, whereby the contrast between the image and the scanning 

 light is still further increased beyond what w^as before found satisfac- 

 tory. This margin of brightness is so large that it has been found 

 possible to use lamps filled with helium in place of neon, giving a much 

 whiter image, more pleasing to some people. 



