36 THE WONDERFUL CENTURY. CHAP. v. 



The method is as follows: A sensitive film, of some of 

 the usual salts of silver in albumin or gelatin, is used> 

 but with much less silver than usual, so as to leave the 

 film quite transparent. It must also be perfectly homo- 

 geneous, since any granular structure would interfere 

 with the result. This film on glass must be placed in a 

 frame so constructed that at the back of it there is a 

 shallow cell that can be filled with mercury which is in 

 contact with the film. It is then exposed in the usual 

 way, but much longer than for an ordinary photograph, 

 so that the light-waves have time to produce the required 

 effect. The light of each particular tint, being reflected 

 by the mercury, meets the incoming light and produces 

 a set of standing waves that is, of waves surging up 

 and down, each in a fixed plane. The result is that the 

 metallic particles in the film become assorted and strati- 

 fied by this continued wave-action, the distance apart of 

 the strata being determined by the wave-length of the 

 particular colored light for the violet rays about eight 

 millionths of an inch ; so that in a film of ordinary thick- 

 ness there would be about five hundred of these strata 

 of thinly scattered metallic particles. The quantity of 

 silver used being very small, when the film is developed 

 and fixed in the usual way the result is not a light-and- 

 shade negative, but a nearly transparent film which 

 nevertheless reflects a sufficient amount of light to pro- 

 duce a naturally colored picture. 



The principle is the same for the light-waves as that 

 of the telephone for sound-waves. The voice sets up 

 vibrations in the transmitting diaphragm, which, by 

 means of an electric current, are so exactly reproduced 

 in the receiving diaphragm as to give out the same sue- 



