284 MR J. CLERK MAXWELL ON COLOUR, 
the art of photography. Let it be required to ascertain the colours of a land- 
scape, by means of impressions taken on a preparation equally sensitive to rays 
of every colour. 
Let a plate of red glass be placed before the camera, and an impression taken. 
The positive of this will be transparent wherever the red light has been abundant 
in the landscape, and opaque where it has been wanting. Let it now be put in 
a magic lantern, along with the red glass, and a red picture will be thrown on 
the screen. 
Let this operation be repeated with a green and a violet glass, and, by means 
of three magic lanterns, let the three images be superimposed on the screen. The 
colour of any point on the screen will then depend on that of the corresponding 
point of the landscape ; and, by properly adjusting the intensities of the lights, 
&c., a complete copy of the landscape, as far as visible colour is concerned, will 
be thrown on the screen. The only apparent difference will be, that the copy 
will be more subdued, or less pure in tint, than the original. Here, however, we 
have the process performed twice—first on the screen, and then on the retina. 
This illustration will show how the functions which Youne attributes to the 
three systems of nerves may be imitated by optical apparatus. It is therefore 
unnecessary to search for any direct connection between the lengths of the undu- 
lations of the various rays of light and the sensations as felt by us, as the three- 
fold partition of the properties of light may be effected by physical means. The 
remarkable correspondence between the results of experiments on different indi- 
viduals would indicate some anatomical contrivance identical in all. As there is 
little hope of detecting it by dissection, we may be content at present with any 
subsidiary evidence which we may possess. Such evidence is furnished by those 
individuals who have the defect of vision which was described by Datron, and 
which is a variety of that which Dr G. Wirson has lately investigated, under the 
name of Colour-Blindness. 
Testimony of the Colour-blind with respect to Colour. 
Dr GrorGE Witson has described a great number of cases of colour-blindness, 
some of which involve a general indistinctness in the appreciation of colour, 
while in others, the errors of judgment are plainly more numerous in those 
colours which approach to red and green, than among those which approach to 
blue and yellow. In these more definite cases of colour-blindness, the phenome- 
na can be tolerably well accounted for by the hypothesis of an insensibility to 
red light; and this is, to a certain extent, confirmed by the fact, that red objects 
appear to these eyes decidedly more obscure than to ordinary eyes. But by ex- 
periments made with the pure spectrum, it appears that though the red appears 
much more obscure than other colours, it is not wholly invisible, and, what is more 
curious, resembles the green more than any other colour. The spectrum to them 
