BELIN'S TELESTEREOGRAPH 117 



A photograph in relief is obtained by printing 

 from an ordinary negative upon what is termed 

 carbon tissue, this being paper coated with gelatine, 

 which is rendered sensitive to light by the addition 

 of a bichromate. When sufficiently exposed, the 

 paper is " developed " in hot water, when the 

 gelatine washes away from the unexposed parts, but 

 remains insoluble where there has been much 

 exposure ; moreover, in the " half-tones/' the 

 gelatine washes away only to an extent depending 

 on the amount of exposure, I.e., on the density of 



Shadow Shadow faint 



Middle part 



tone 



FIG. 54. 



the negative. The relief picture, if we were to 

 cut a fine section transversely through the film, 

 would therefore appear as shown in Fig. 54. 



Now let us suppose such a relief picture wrapped 

 round the drum DI (Fig. 55) of Belin's transmitter. 

 This is a heavy metal drum turning between centres, 

 the whole drum, etc., moving laterally so that a 

 stylus fixed at S traces a spiral path over the photo- 

 graph. Now this stylus consists in reality of a 

 sapphire or a hardened steel point fitted to the end 

 of a rod attached to a long arm movable about F, 

 the fulcrum. The diagram is not flrawn to scale, 

 S being actually close to the fulcrum. When a 



