THE PHOTOGRAPHY OF COLOUR 361 



that he worked " in accordance with the Young-Helmholtz- 

 Maxwell theory of colour vision." "Although the three funda- 

 mental colour sensations are red, green, and blue-violet, the three 

 images of the triple photograph are not made through red, 

 green, and blue-violet glasses, nor by the action of red, green, 

 and blue-violet rays, but each by the joint action of all rays 

 that have power to excite the respective fundamental colour 

 sensations, . . . according to the measurements of Maxwell and 

 Abney." Thus the colour screens used for taking the object 

 each transmitted a large proportion of the spectrum. But for 

 viewing or projecting the photographs Mr. Ives claimed to use 

 colour screens that transmit only a narrow band of the spectrum, 

 just that part in each case that will best excite the one sensation, 

 and, as far as possible, that only. The disadvantage of narrow- 

 band colour screens is that they transmit comparatively little 

 light, and it has been stated that the screens used by Mr. Ives 

 did not well conform to his theory. I have already referred 

 to the uncertainty of these matters, but assuredly Mr. Ives was 

 guided by the principles he professed, and he was remarkably 

 successful. 



The necessity for taking three separate negatives is obviously 

 a circumstance to be avoided if possible, and the method of 

 avoiding it was pointed out by Du Hauron in 1869. It appears 

 that the only practicable method of making one plate suffice is 

 to divide the surface of the plate among the three colours, using 

 such small parts of the plate for each that when viewed in the 

 usual way these parts merge into each other, as the fine detail 

 of the impression of a copper or steel engraving does. The 

 drawback to all such methods is the great loss of light. If any 

 colour is represented by only one-third the area of the patch 

 that stands for it, the area which might otherwise be all colour 

 is two-thirds black. Thus two-thirds, more or less, of the light 

 that would form the image in Ives' apparatus is lost in a one- 

 plate process, and the picture is correspondingly duller unless 

 the light is proportionately increased. This can be accomplished 

 if the picture is in the form of a lantern slide, by using the same 

 light in the lantern but reducing the diameter of the picture on 

 the screen from 10 ft. to 6 ft. or less, or from 8 ft. to something 

 under 5 ft. 



The first successful methods of this kind were worked out 

 independently by Dr. John Joly, of Dublin, and Mr. J. W. 



