ON COLOUR PHOTOMETRY. 
437 
It will be noticed that whereas in our curve there is a marked convexity about 
scale number 48'5, with K that part of the curve is much less convex. In his paper 
(Phil. Trans., vol. 150 (1860) p. 57), Prof. Clerk Maxwell notices the same thing 
with regard to one of his observers at another part of the spectrum. It will, however, 
be seen that in comparing K’s curve with ours, the total areas are nearly the same. It 
would appear that if the trichromatism of the normal eye be an established fact, the 
slight deficiency in K’s curve would be accounted for by a slight want of sensitiveness 
in the curves which give the sensation of green. 
§ XYI.— Colour-Blindness. 
We are not aware that hitherto any direct and accurate determination has been 
attempted of the relative luminosities to a colour-blind person of different parts of 
the spectrum. We have been favoured by several gentlemen who have submitted to 
an examination of their perceptiveness of different colours. The luminosity curves of 
the electric arc light as perceived by four of these gentlemen, who are distinctly 
“ colour-blind,” were determined in the way above described. While at certain parts 
their curves fall below ours, their maxima are considerably higher than ours. The 
reason of this evidently is that their eyes not being equally sensitive to the whole 
of the spectrum, the light of the comparison-candle is to them depreciated by the 
proportion of the rays which do not affect their eyes, and their measurements are 
therefore proportionally exaggerated, this exaggeration being, of course, most evident 
in the parts of the spectrum to which their eyes are most sensitive. We have 
plotted each of their curves with the ordinates reduced in such a proportion in each 
case that no part of the curve lies outside the normal curve. Assuming for the 
moment that their eyes are not more sensitive than ours for any part of the spectrum, 
the difference between the curve of any one of them and the normal will show the 
amount and the position in the spectrum of the deficiency in his colour perception. 
Observer G. —The first to whom we call attention is G, who kindly put his eyes 
at our disposal on two occasions for measuring the luminosity of the spectrum. 
Reducing the ordinates of his original curve by ^ we obtained a curve which touched 
our curve, taken under the same circumstances, in the green, but dropped below it 
considerably in the red, and very slightly in the blue-green. Fig. 5, Plate 24, gives 
the two curves, the maximum of the normal curve being taken as 100. G’s prin¬ 
cipal defect is evidently in the red; in fact, when he matched the luminosity in the 
part of the spectrum near C with the candlelight, to our eyes, the shadow illumined 
by the spectrum was a brilliant red, whilst that illumined by the candle was nearly 
black. There may be a very slight falling off in the blue-green, which on the 
trichromatic theory can be accounted for by a slight want of sensitiveness in the 
“ blue-perceiving ” nerves. Apparently, also, his perception of violet is not normal. 
