Colour-Sensation. 37^ 



must be of a yellowish green capable of producing yellow and 

 orange when combined with the red sense, and very different 

 from the bluish green which is complementary to red. There- 

 fore if one class of colour-blind persons have lost the red sense 

 and the other the green sense, there ought to be considerable 

 differences in all the colour equations obtained from the two 

 classes of eyes, and especially in the proportions of blue and 

 yellow which will neutralize each other. But I have not met 

 with any trace of such differences having attracted attention, 

 except in equations between red and green, and it is clear 

 that in the spectrum the neutral point where the blue or violet 

 colour-sense neutralizes the other colour-sense is very nearly 

 the same for red-blind and green-blind persons. Professor 

 Donders fixes it for his red-blind case at a wave-length of 

 494*85 millionths of a millimetre, and for his green-blind case 

 at 502*3 millionths, the difference 7*5 being not one fiftieth part 

 of the difference between the greatest and least wave-lengths 

 in the visible spectrum; while Professor Preyer, in another 

 case, found that doubling the amount of light altered the 

 neutral point from 512*8to 506*6, L ^.nearly as much(Pfluger's 

 ArcJtiv, vol. xxv.). 



(B) If blindness to the red end of the spectrum were due 

 to the absence of the red sense, it would be the same in extent 

 in different red- blind persons, whereas in fact it differs con- 

 siderably. (Donders, Grafs Archiv, vol. xxvii.) 



(C) The extent to which the violet end of the spectrum 

 is obscured to violet- or blue-blind eyes also varies very much. 

 Professor Stilling (Klinisclie Monatsblatter fur Augenheil- 

 kmide, 1875, Beilage 2) met with three cases in which the 

 green thallium-line between E and D formed the boundary of 

 the visible spectrum; while in another case (Centralblatt fur 

 praktische Augenheilkunde, 1878, p. 99) the same observer 

 found that nearly the whole of the spectral green was per- 

 ceived, and grey or, in a faint spectrum, red beyond it. In 

 the case of one-eyed violet- or blue-blindness described by 

 Professor Holmgren (' Proceedings of the Poyal Society/ 

 vol. xxxii. p. 305) "the spectrum is continued over the place 

 where we see green, greenish blue, cyan- blue, and indigo to 

 the commencement of the violet, where it absolutely ended 

 with a sharp limit about Fraunhofer's line G." 



(D) As is well known, inability to distinguish between red 

 and green has in many cases been found to exist among dif- 

 ferent members of the same family, and especially among 

 brothers; and therefore when such colour-blindness is found 

 to exist among relations, there is a very strong probability 

 that they have inherited the same affection of the eves. There- 



