Joty—On a Method of Photography in Natural Colours. 133 
the red side of D by about one-sixth the interval D to C; for the green and 
violet-taking screens the correct tints are found in the same manner by scaling 
from the figure. Good results are thus obtained, but I do not assert that these 
details of procedure are final. 
Although referred to in what has preceded, the necessity of the separate 
screens, according to Maxwell’s theory, cannot be too clearly understood. The 
necessity will be evident if the effects of photographing, and viewing the spectrum 
through the same screens be considered. Evidently, if these are the fundamental 
colours, the final result will show colour at three separated regions of the spectrum 
only. The red, green, and violet wave-lengths would alone affect the plate, and the 
purer we choose the fundamental colours the more restricted would the action be, 
and the less the action of the intermediate wave-length. On the other hand, if the 
negatives are taken through the ‘taking’ colours, and the positives subsequently 
viewed through the same, we can, of course, have no pure red or violet upon 
the screen, for, in fact, these colours are entirely absent from our individual 
images. In short, the three primary colours by which correct syntheses of all 
colours of the spectrum can be alone made are indispensable to the final projec- 
tion, but are quite unsuited to serve as the taking colours, as they will transmit 
to the plate only a limited selection of the various wave-lengths of nature, 
and that not at all according to the degree which these excite ‘ Young’s 
three sets of nerves separately.” The use of the same screens for taking 
and viewing having intermediate tints can, of course, only mitigate the evils 
referred to. 
It is further necessary to observe that, as no photographic plate has as yet been 
prepared which is uniformly colour sensitive, allowance must be made for this 
in the choice of pigments to act as light filters, and only by careful photo- 
spectroscopic work can these be selected. 
Had we a plate possessing a uniform distribution of colour sensitiveness the 
curves could be directly applied to the choice of colours, and these be selected 
simply by eye observation of their effects upon the solar spectrum. 
Any method of photography in natural colours must possess the character- 
istics not only of accuracy of colour rendering, but also of convenience of 
application and permanency of colour, if it is to possess value as a scientific 
method. For use under the various circumstances of travel the naturalist requires 
a method no more cumbersome than the present dry plate. In the method of 
composite colour photography, as described, the ordinary camera will not serve. 
The cumbersome necessity of obtaining three images remains, and subsequently 
no concrete image in natural colours is actually obtained. One can only be 
realised by triple projection upon a screen, or by using some optical contrivance 
