CH. LV.J COLOUR VISION. 779 



Bering's theory appears to meet the difficulty best, for if the 

 red element of Helmholtz were absent, the patient ought not to 

 be able to perceive white sensations, of which red is a constituent 

 part : whereas, according to Hering's theory, the white-black 

 visual substance remains intact. 



The two theories that have just been described may be called 

 the classical theories of colour vision, at any rate they are those 

 which have been for the longest time before the scientific world. 

 As facts have accumulated it has been for some years recognised 

 that many such facts could not be reconciled with either theory ; 

 and modifications of one or the other theory have been from 

 time to time introduced. 



The observations recently made by C. J. Burch are of con- 

 siderable importance ; the following is a brief account of his 

 methods and results. 



He finds that by exposing the eye to bright sunlight in the focus 

 of a burning glass behind transpai*ent coloured screens it is pos- 

 sible to produce temporary colour blindness. After red light, the 

 observer is for some minutes red-blind, scarlet geraniums look 

 black, yellow flowers green, and purple flowers violet. After 

 violet light, violet looks black, purple flowers crimson, and green 

 foliage richer than usual. After light of other colours, corre- 

 sponding effects are produced. If one eye is made purple-blind, 

 and the other green-blind, all objects are seen in their natural 

 colours, but in exaggerated perspective, due to the difficulty the 

 brain experiences in combining the images from the two eyes. 



By using a brightly-illuminated spectrum, and directing the 

 eye to certain of its colours, the eye in time becomes fatigued 

 and blind for that colour, so that it is no longer seen in the 

 spectrum. Thus, after green blindness is induced the red ap- 

 pears to meet the blue, and no green is seen. If, however, the 

 eye is exposed to yellow light, it does not similarly become blind 

 for yellow only, but for red and green too. This supports the 

 Young-Helmholtz theory that the sensation yellow is one com- 

 pounded of the red and green sensations. By an exhaustive 

 examination of the different parts of the spectrum in this way it 

 thus becomes possible to differentiate between the primary colour 

 sensations and those which are compound. By a study of this 

 kind, Burch concludes that the phenomena of colour vision are in 

 accordance with the Young-Helmholtz theory, with the important 

 addition that there is a fourth primary colour sensation, namely, 

 blue. He could not discover that colour sensations are related 

 to each other in the sense indicated by Bering. Each may be 

 exhausted without either weakening or strengthening the others. 



