1901-2.] Photography in Natural Colours. 383 



appear natural to the green colour-blind, and of the red and green to the 

 violet colour-blind. 



The superposition and registration of the three images [s/w7i'n], 

 give us as you will agree, a very good reproduction of the original 

 colours. It may be as well to point out, however, that it is a mere 

 optical illusion, for the colours though visually similar are not optically 

 the same. In the one case we have, collectively at any rate, the whole 

 range of the spectrum and in the other only three narrow bands in the 

 red, green, and blue. The principal drawbacks to this method of 

 synthesis are the necessity of using three lanterns and the difficulty of 

 registration. These can be partiall}^ overcome by means of Ives' 

 lantern Kromskop a diagram of which may be shown (Fig. 5). A beam 

 of light from the source, preferabh' the electric arc, is rendered parallel 

 b\- the condenser £; and divided into three approximateh' equal 

 portions by the unsilvered reflectors /i and z and the silvered mirrors/ 

 and k ; these three beams then pass through the reproduction filters 

 a, b, c, and the three transparencies and are focussed and registered 

 upon the screen by the objectives d, e, f, which are adjustable for this 

 purpose. The three transparencies are made on a single oblong plate 

 from negatives on a single plate, these negatives being taken in a 

 special camera which insures the same relative position of the three 

 images in every case. Registration for one slide then will be registra- 

 tion for all and this difficulty is lessened. Only a comparativel)' small 

 picture can be successfully shown, however, owing to the loss of light in 

 the colour screens, a diameter of three or four feet being the limit for a 

 brilliant image. 



Instead of superposition on the screen the three images may be 

 united by superposition on the retina. This can be effected by another 

 device of Ives, called the Kromskop or photo-chromoscope, a diagram of 

 which is given in Fig. 6. It depends upon the principle of transparent 

 or partially transparent reflectors. In Ives' instrument A. B. C. are 

 the positions of the three transparencies from the red record negative 

 at A, from the blue-violet record negative at B, and from the green 

 record negative at C. At A is the red reproduction screen and at B is 

 a violet screen ; D is a transparent mirror of blue-green glass and E a 

 transparent mirror of yellow-green glass. The red image from A falls 

 upon the mirror at D and is reflected to the eye, any entering the 

 mirror being absorbed since its colour, blue-green, is complementary to 

 red. The violet image at B is reflected from the yellow-green mirror at 

 E and transmitted through D, becoming blue-violet and thus reaches 



