EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 137 



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 shew how the functions which Young 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 

 undulations of the various rays of light and the sensations as felt by us, as 

 the threefold partition of the properties of light may be effected by physical 

 means. The remarkable correspondence between the results of experiments on 

 different individuals 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 subsid'ary evidence which we may possess. Such evidence is 

 furnished by those individuals who have the defect of vision which was 

 described by Dalton, and which is a variety of that which Dr G. Wilson has 

 lately investigated, under the name of Colour-Blindness. 



Testimony of the Colour-Blind with respect to Colour. 



Dr George Wilson 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 phenomena 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 experiments 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 appears faintly luminous in the red; 



VOL. i. 18 



