Joty— On a Method of Photography in Natural Colours. 131 
If, from the knowledge afforded by Keenig’s curves of the compound nature 
of the green sensation, Maxwell’s curves be examined with reference to their 
suitability to serve the purposes of the photographic method, it will be found that, 
assuming Maxwell’s E green to excite the proportionate amounts of red and violet 
sensation revealed by Kcenig’s curves, a correct synthesis of the F green by 
Maxwell’s curves is impossible. Although such a comparison is not. strictly 
allowable owing to the red and violet curves of Maxwell being based on different 
wave-lengths to those used by Koenig, the fact of some inaccuracy is certain. 
This fact will appear if the spectrum is photographed according to Maxwell’s 
curves. ‘The blue-green will then be found to be reproduced too yellow in 
tone. 
In order to apply the colour sensation curves of Keenig to the photographic 
method, we have to find by trial examinations of his curves the green most 
suitable for backing the ‘green’ positive; for we see that the several green 
wave-lengths excite very different amounts of red and violet sensation. We find 
as suitable a wave-length a little to the left side of the E line, about 550 py. If we 
take this colour to back the green positive, we must, in order to find the correct 
red and violet curves which are to control the densities of the red and violet 
images, replot the red and violet curves with allowance for the proportionate 
amounts of red and violet which will be carried to all those points where in the 
image of the spectrum the green curve operates. The red and violet curves must 
be lowered by amounts obtained by ascertaining from the height of the green 
curve at any point the amount of red and violet sensations excited by the amount 
of our selected green present at that point. 
The necessity for the alteration of the curves will, perhaps, be more clearly 
understood if it be considered what consequences would ensue if we take three 
negatives of the normal spectrum through light filters, selecting according to 
the unaltered Keenig’s Colour Vision curves. 
In the process of synthesis we would, in this case, have upon the screen, at the 
point D (suppose) of the projected image a quantity of fundamental red repre- 
sented by the height DR, and of green represented by the height DG. Now, 
whatever wave-length we make choice of to excite the fundamental green 
sensation in the optical synthesis, we inevitably excite both the red and violet 
sensations by the chosen wave-length. If we adopt the EK green, we see by the 
curve that this wave-length excites the large amount of red sensation shown by 
the vertical height of the red sensation curve at the point E, and the small amount of 
violet sensation measured to the violet sensation curve at this point. In projecting 
then at the point D of the image a quantity of the E green, according to the height 
DG, we excite amounts of red and violet sensation which can be ascertained by 
