Report of the Committee on Colour-Vision, 285 



for yellow in the above description of red, orange, yellow, and 

 yellow-green, the brightest red would be called dark green, 

 and they would fail to see at all in the extreme red, the 

 spectrum being- shortened. These latter would also recognize 

 a white or grey band, but it would be in a position rather 

 nearer the blue of the spectrum than in the first case (see 

 Xo. 3, Plate I). It is needless to say that to normal vision this 

 white or grey band is non-existent, and whenever a person 

 under examination sees such a band the evidence is conclusive 

 that he is colour-blind. These differing descriptions of the 

 spectrum show that this form of colour-blindness may be 

 divided into two classes, which for convenience sake may 

 be termed green- and red-blindness. Another point of difference 

 between them is the part of the spectrum that appears 

 brightest. To the normal eye it is the yellow, and to the 

 green-blind it is nearly at the same place, but to the red- 

 blind it is the green. This, perhaps, may give a clue to 

 the designation of the spectrum colours by these two classes. 

 To the green-blind, red and yellow are the same colour, but 

 the yellow being, the brighter he looks on red as degraded 

 or darkened yellow. On the other hand, to the red-blind 

 green is brighter than yellow or orange, and these appear as 

 degraded green. 



Experiment has shown that every colour in nature, as seen by 

 a normal eye, can be expressed as a mixture of three, so that 

 normal vision is tri-chromatic. In a similar sense the more 

 pronounced types of ordinary colour-blind vision are di-chromatic. 

 These colour relations must be regarded as purely subjective, for 

 enough is now known of the nature of light to exclude the 

 possibility of a three-fold physical constitution. In the theory of 

 Young, subsequently, and independently, brought forward and 

 developed b}^ Helmholtz, light is supposed to be capable of 

 exciting three distinct primary sensations, combined in varying 

 proportions, and dependent upon the quality of the light. As to 

 the character of the three sensations, Young identified them 

 with red, green, and violet ; and no widely-differing choice 

 is possible, unless upon the supposition that the primary 

 sensations, in their purity, are quite outside the range of 

 our experience. The yellow of the spectrum, for example, cannot 

 be primary, for it is capable of being matched by a suitable 

 mixture of red and green. According to this view each primary 

 sensation is excited in some degree by almost every ray of the 

 spectrum ; but the maxima occur at different places, and the 

 stimulation in each case diminishes in both directions, as the 

 position of maximum is receded from. 



