130 Joty—On a Method of Photography in Natural Colours. 
wave-lengths of the spectrum, according as they excite the red sensation, we 
require to produce a greater photographic effect by the D wave-lengths than by 
To effect this analysis of the light, a screen transmitting as 
the C wave-lengths. 
a predominant wave-length, a wave-length near D, must be used for obtaining the 
image which is to represent the appreciation of light peculiar to the ‘ red’ nerves. 
Such a screen has a yellow-orange colour, which is not the sensation excited in or 
In the optical synthesis this must afterwards be 
transmitted by the ‘red’ nerves. 
The same remarks apply to the other screens. 
represented by a C red colour. 
In the foregoing description I have spoken of the method as if based directly 
on actual colour sensation curves. Upon the revival of Maxwell’s method, writers 
Although supplying a convenient 
quoting from Maxwell fell into this mistake. 
terminology in conveying a general idea as to the nature of the procedure, it is 
necessary now to be more precise. 
Maxwell’s curves (fig. 1) are not true colour sensation curves,* but represent 
the subjective synthesis of the prismatic spectrum out of three chosen wave-lengths— 
The question as to how far one or all these 
a red, a green, and a blue-violet. 
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chosen wave-lengths may excite more than the one set of nerves remains over, 
and indeed can only be gone into by examination of abnormal colour vision. In 
Keenig’s curves of colour vision, colour sensations are plotted to the normal 
These are shown in the named curves of fig. 2. 
spectrum. 
* Abney : ‘‘ Colour Vision,’’ Tyndall Lectures, 1895, 
