380 Mr. H. P. Droop en 



fore if red-blindness and green-blindness be distinct things, 

 due to the absence of different colour-senses, we should expect 

 that the colour-blind members of the same family would be 

 either all red-blind or all green-blind. But this is not the 

 case. Among the colour-blind cases examined by Professor 

 Stilling (Klinische Monatsbldtter fur Augenheilkunde, 1875, 

 Beilage 1 and 2) there were two pairs of brothers both unable 

 to distinguish between red and green; and in each case one 

 brother was able to perceive light at the red end of the spec- 

 trum, while the other was not. 



All these reasons lead me to believe that the difference 

 between so-called red-blind and so-called green-blind is not 

 due to their having lost different colour-senses, but rather 

 to the loss of one pair of colour-senses, those for red and 

 green being, in the case of the so-called red-blind, accom- 

 panied by some disturbance of the other pair of colour-senses 

 — a disturbance varying in character and degree in different 

 cases, and similar to what is found to exist in different cases 

 of blue- or violet-blindness. 



The shapes of the curves Prufessor Donders has published 

 (Trans. International Medical Congress, 1881, vol. i. p. 280) to 

 represent the respective intensities of the light perceived by a 

 red-blind and a green-blind eye have suggested to me a pos- 

 sible explanation of the difference between these two eyes. 

 The curves representing the less-refrangible sensation of each 

 eye correspond very nearly in shape and dimensions; but that 

 for the red-blind eye is shifted some way further from the red 

 end of the spectrum. On the other hand, the curves repre- 

 senting the more-refrangible sensation of the two eyes are 

 almost identical in position as well as in shape and size. This 

 effect would be produced if the organization producing the 

 less -refrangible (or yellow) colour-sensation in the green-blind 

 eye were so modified in the red-blind eye as to make shorter 

 waves produce the same effects which longer waves pro- 

 duced in a green-blind eye. The change supposed would be 

 equivalent to shifting the tone of a musical instrument an 

 octave higher. 



But I do not suppose that all cases of shortened spectrum 

 could be thus accounted for. 



In order to show more in detail what my views of colour- 

 sensation are, I have prepared a diagram representing roughly 

 how the two pairs of colour-senses are affected by the dif- 

 ferent rays of the spectrum. The upper half of the dia- 

 gram represents the effects produced by the different rays 

 on the yellow and blue colour-senses, and the lower part 

 those they produce on the red and green colour-senses. 



