6 TlIORl'. Grating Films and Colour Photography. 



By the device shown cliagramatically on Fig. i, one 

 source of Hght alone fulfils all purposes. 



A set of three small mirrors /;, g, and r, is arranged 

 on each of the inner surfaces of two ellipscjids ; a slit, 

 through which the light passes from a source at A (a 

 Welsbach burner being found very suitable) being in one of 

 the f(;ci of each ellipsoid and the stereoscopic pictures in 

 the other. 



Total reflecting prisms, B, are placed over each slit 

 to deflect the rays proceeding from the source arranged 

 between the two slits, on to the mirrt)rs, the effect being 

 as though there were six sources of light at a uniform 

 distance from the pictures. These mirrors being adjust- 

 able, light from any desired angle can thus be directed on 

 the picture. A central ray from each is shown in dotted 

 lines, showing the overlapping of the spectra formed at the 

 eye end, a slit being necessary to confine the view to the 

 axial rays. It will thus be seen that the blue of the 

 spectrum formed by the picture illuminated by light from 

 the mirror b, coincides at the eye slit with that of the 

 green from the mirror g, and that of the red from the 

 mirror r, when they are set at the proj^er angle, and white 

 light is produced. Pictures taken through colour screens, 

 superposed and viewed in such an apparatus appear simply 

 as ordinary photographs and without colour. To produce 

 colour effects, the pictures must therefore act independently, 

 and transmit to the e}'e only those colours corresponding 

 to the screen through which the original photographs were 

 taken. To enable this to be done, the lines of the grating 

 composing the pictures are arranged at a slight inclina- 

 tion to each other — about lo" being found suitable— and 

 the mirrors moved laterally to the corresponding angles on 

 the cllii)S()id. Se[)arate spectra are now [)roduced at the 

 e)e slit and the pictures are seen, as exemplified in the 



