io 9 o VISION. 



since there is no doubt that the most obvious characteristic is confusion 

 between red and green, the condition may, without prejudice, be spoken 

 of as red-green blindness. It is now generally agreed that to individuals 

 with this defect the spectrum is divided into two parts, separated by an 

 interval which can be matched with white, known as the neutral band. 

 In most cases the neutral band is very narrow, and its existence can 

 only be demonstrated with a spectrum in which each part is free from 

 mixture with other rays, or it may be demonstrated more roughly by 

 finding where the colour of one end passes into that of the other. This 

 neutral band is situated in the green or blue-green, somewhere between 

 the lines b and F, the wave-lengths 505 a and 492 a. In other cases the 

 neutral band is broader, and more easily demonstrated. On either side 

 of the neutral band the spectrum appears to be of one colour, differing 

 in its different parts in brightness and saturation only. By suitable 

 adjustment of intensities any part of one half can be matched with any 

 other part of that half, and a match may also be made between two 

 parts of one half by adding white light to the one that appears more 

 saturated. The colour of each half appears to increase in saturation 

 from the middle outwards. Further, in all cases of this defect, it is 

 possible to find a red and a green, each of which may be matched with 

 white ; this may be done with spectral, and less easily with pigment 

 colours ; the red which is so matched is one in which a certain amount 

 of blue has to be added to the extreme red of the spectrum. In any 

 given case the red and green so found are complementary, and mixed 

 together make white to the normal eye. 



Another feature common to all cases of red-green blindness is, that 

 a colour equation which is good for normal vision is good for dichromatic 

 vision. 



Cases of red-green blindness seem to show the changes in colour 

 matches with alteration of intensity more distinctly than in normal 

 vision. The first observation of the kind was made by Preyer, 1 who 

 found that the match between the neutral band and white was no 

 longer good on change of intensity, the neutral band shifting its 

 position towards violet when the intensity was increased. This was 

 confirmed by Konig and Brodhun, 2 and denied by Hering, who pointed 

 out, however, the great difficulty of determining the position of the 

 neutral band, owing to its varying with the character of the source of 

 light and the colour adaptation of the eye ; while, if the eye is adapted 

 to the dark in order to get rid of the latter factor, there is great 

 diminution of the sensitiveness for slight differences of colour upon 

 which the determination of the neutral point depends. 



Brodhun 3 investigated the influence of intensity on a number of 

 equations in which a mixture of light of 615 a and 460 X was matched 

 with intermediate parts of the spectrum. He found that a match with 

 light of 480 a was not affected by change of intensity, but, as the wave- 

 length of the homogeneous light increased, marked changes occurred; 

 thus, with an intensity of one unit, light of 560 7. was matched with a 

 mixture of 49 parts of 615 a, and 428 parts of 460 a; while, with 

 an intensity of 32 units, the proportions were 76 and 114 re- 

 spectively. 



1 Arch./, d. ges. Physiol., Bonn, 1881, Bd. xxv. S. 41. 



2 Sifzungxb. d. Tc. Akad. d. Wisscnsch., Berlin, 1887, S. 311. 



3 Ztschr. f. Psychol, u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1893, Bd. v. S. 323. 



