Joty—On a Method of Photography in Natural Colours. 137 
of the experiences of other senses underlies our visual inferences. The fact that 
Maxwell’s photographic method, based on the three-colour theory of vision, can 
thus interpret nature, appears strong confirmation of Young’s theory. 
It is to be remembered that these results are attained by no new photographic 
operations. It is necessary to use good orthochromatic plates sensitised into the 
red, and also to have affixed in the lens an orthochromatic screen cutting off the 
ultra-violet light in the usual manner. ‘The exposure is longer than the ordinary 
exposure, for we can of course only use visible light, and of this a part is stopped 
by the taking screen. The ordinary backs may be used. The displacement of 
the sensitive film from accurate register with the ground-glass camera screen, 
owing to the presence of the taking screen in front of it, may be corrected (if 
thought necessary) by simply reversing the surface of the ground-glass camera 
screen, turning the muffled side outward. ‘This secures that the image will be 
accurately focussed in the plane of the sensitive surface. Negatives and positives 
may be used as ordinary negatives or positives till it is desired to recall the 
original colours. Thus, for those who wander with the camera, the possession of 
but the one seeing screen to test results is sufficient, and of course the one taking 
screen suffices to take an indefinite number of plates. 
These considerations lead us naturally to observe that the registration of 
colour being really carried in the silver image, which with very little care in 
manipulation may be made permanent, secures that the colours are permanent. 
A faded screen may at any time be made good by a fresh screen; the colours in 
all cases being spectroscopically chosen, we are assured of the reproduction of the 
original colour. In this aspect the necessity of the detached colour screen is no 
disadvantage, but rather a necessary safeguard against the inevitable fading 
attending most pigment colours. 
The question of course naturally suggests itself if results on paper, that is 
results seen by reflected light, are not also possible on the principles described. 
My early experiments in this direction were not encouraging. The difficulties of 
correctly superposing the viewing colours upon a paper positive are considerable. 
For book illustration, the correct and sufficiently rapid register of printing sur- 
faces, having the required minuteness, presents grave difficulties, and in all such 
printing processes the truth of colour vanishes with the uncertainty attending 
the amount of colour transferred to the paper. Again, if any fixed combination 
of the colours and the sensitive salt is sought for, the difficulties of preserving | 
these colours during development or toning, &c., present themselves. Finally, 
perhaps the gravest difficulty resides in the enfeeblement of the reflected light, 
already enfeebled by the inevitable loss due to absorption in the reflecting 
material. 
