134 Joty—On a Method of Photography in Natural Colours. 
which, by the aid of reflectors, enables all three images to be simultaneously 
projected upon the retina. 
I now proceed to describe a mode of applying the foregoing principles 
which is free of the objection of cumbersomeness, and which enables us to realise 
a concrete image in transparent colours. <A plate is finally produced which may 
be held in the hand, regarded against the light, and which bears an image of the 
object in natural colours, or such as are so nearly accurate as to seem so to the 
eye. In this new method there is but the one image photographed. The 
ordinary camera, lens, and backs, &c., are used without modification. The first- 
class isochromatic plates in the market, which are sensitised down to the C red, 
will give very good results. 
In the new method the idea is to carry the application of physiological 
principles still further, and divide up the plate like a hypothetical subdivision of 
the retina, so that all over the plate there should be minute regions uniformly 
distributed wherein the sensitive silver salt is excited to become reduced to the 
‘photogenic’ material in the same degree in which the sensations of redness, 
greenness, violetness, would have been actually excited in the several nerves of 
the retina had the image been formed upon it. Development builds upon this 
photogenic material the denser silver deposit, and ultimately in the positive the 
amounts of the sensations are registered in the degrees of transparency of the 
successive regions. The lined screen which can bring about this will only show 
its individual colours when placed under the microscope. It is then seen to con- 
sist of closely -ruled adjacent lines in reddish-orange, yellowish-green, and 
blue tints. This screen, applied closely to the sensitive surface, analyses the 
image in the camera. The screens I have used hitherto are coarse, about 200 
lines to the inch, and even with this coarseness will show plainly, I regret to say, 
the imperfections of the only apparatus at my command in preparing these 
screens. I may observe, in passing, that the colours are ruled on in pigments 
made up as inks in gelatine and gum arabic or dextrine, and upon plates coated 
with a preliminary layer of gelatine. Such lines may be put on so close as 800 
or 1000 to the inch. With between 300 and 400 to the inch, however, the eye is 
no longer annoyed by the structure of the plates. The lines may also be ruled 
on celluloid or on translucent paper. 
The appearance of both negative and positive taken through such a screen is 
shown in Pl. VI. 
Recalling now that the lines upon the positive register in their degrees of 
transparency, the degrees in which the three-colour sensations would have been 
excited, it becomes apparent that to complete the physiological parallel we must 
convert these degrees of transparency to quantities of the red, blue, and violet 
