440 ON THE THEORY OF COMPOUND COLOURS. 



G the colour is mixed, varying from "yellow" to "blue," and becoming neutral 

 or "white" at a point near F. In this part of the spectrum, the total inten- 

 sity, as given by the dotted line, is decidedly less than on either side of it, and 

 near the line F, the retina close to the "yellow spot" is less sensible to light 

 than the parts further from the axis of the eye. This peculiarity of the light 

 near F is even more marked in the colour-blind than in the ordinary eye. 

 Beyond F the " blue " element comes to a maximum between F and G, and 

 then diminishes towards H ; the spectrum from this maximum to the end being 

 pure " blue." 



In fig. 10, Plate VII. p. 444, these results are represented in a different 

 manner. The point D, corresponding to the sensation wanting in the colour-blind, 

 is taken as the origin of coordinates, the "yellow" element of colour is represented 

 by distances measured horizontally to the right from D, and the " blue " element 

 by distances measured vertically from the horizontal line through D. The 

 numerals indicate the different colours of the spectrum according to the scale 

 shewn in fig. 9, and the coordinates of each point indicate the composition of 

 the corresponding colour. The triangle of colours is reduced, in the case of 

 dichromic vision, to a straight line "B" "Y," and the proportions of "blue" 

 and "yellow" in each colour are indicated by the ratios in which this line is 

 cut by the line from D passing through the position of that colour. 



The results given above were all obtained with the light of white paper, 

 placed in clear sunshine. I have obtained similar results, when the sun was 

 hidden, by using the light of uniformly illuminated clouds, but I do not consider 

 these observations sufficiently free from disturbing circumstances to be employed 

 in calculation. It is easy, however, by means of such observations, to verify the 

 most remarkable phenomena of colour-blindness, as for instance, that the colours 

 from red to green appear to differ only in brightness, and that the brightness 

 may be made identical by changing the width of the slit ; that the colour 

 near F is a neutral tint, and that the eye in viewing it sees a dark spot in 

 the direction of the axis of vision ; that the colours beyond are all blue of 

 different intensities, and that any "blue" may be combined with any "yellow" 

 hi such proportions as to form " white." These results I have verified by the 

 observations of another colour-blind gentleman, who did not obtain sunlight for 

 his observations ; and as I have now the means of carrying the requisite 

 apparatus easily, I hope to meet with other colour-blind observers, and to obtain 

 their observations under more favourable circumstances. 



