270 BELL SYSTEM TECHNICAL JOURNAL 



to the elimination of the arc lamps, much of whose effective radiation 

 was from the arc stream which always wanders somewhat. The 

 chief gain, however, is in the tone quality of the image of the face. 

 The difference is very clearly shown if shutters are arranged so that 

 either the potassium or the ca'sium cells may be used alone, alternately, 

 and can then be quickly exposed together. With the potassium cells 

 alone, as already noted, flesh tints are in general too dark, and tanned 

 or ruddy complexions show unnatural contrast with the whites. High- 

 lights due to reflection on the skin are often observed to be out of scale, 

 with a resultant effect of mottling of the skin. With the ceesium cells 

 alone, on the other hand, the flesh tints are in general too light, and 

 faces are apt to appear very flat. These differences were anticipated, 

 but others not so obviously to be expected, have been observed. For 

 instance, with the caesium cells, the pupil and iris of the eye are brought 

 out with rather startling blackness, while with the potassium cells. 

 the detail around the eyes is apt to be lost. The most satisfactory re- 

 sults are obtained with both sets of cells acting, for, as was hoped, the 

 combination of the two ends of the spectrum, gives, in the case of the 

 face, an effect very like that which light from the middle of the visible 

 spectrum would give, that is, an '"orthochromatic" image, as it would 

 be described in photography, while the definition of important points, 

 such as the eyes, is distinctly improved. 



Passing now to the receiving end of the two-way television apparatus 

 we recall that in the apparatus as originally set up and described, a 

 simple disc with a spiral of holes was used, immediately behind which 

 was a neon lamp with a large flat water-cooled electrode. On con- 

 tinued operation, it was found that the heavy current demanded in 

 these lamps, in order to secure an image of sufficient brightness, caused 

 rapid sputtering on the closely adjacent glass wall, necessitating fre- 

 quent renewals of lamps. A very radical change in the disc and lamp 

 design has been made by which this undesirable situation has been 

 remedied. 



The change in the disc consists in substituting for the simple Nipkow 

 disc, with its spiral of holes, an alternative form, suggested also by 

 Nipkow, in which each disc hole has associated with it a condensing 

 lens, positioned so as to focus, in combination with a fixed collimating 

 lens, and image of the source on the disc hole. The optical arrange- 

 ment is shown in Fig. 4, and a photograph of the disc with lenses and 

 lamp in place in Fig. 5. Referring to Fig. 4, D represents in section 

 the simple disc with a spiral of holes, h ; / represents a small short focus 

 lens, fixed in position with respect to // at a distance equal to its focal 

 length; L represents a fixed lens of diameter large enough to cover the 



