800 THE EYE AND VISION [CH. LVI. 



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 Bering's theory, the white-black visual sub- 

 stance remains intact. It likewise explains to some extent the 

 phenomena of total colour blindness, which is an almost inconceiv- 

 able condition, if the Young-Helmholtz theory is accepted. 



These two theories have been for a long time before the scientific 

 world. As facts have accumulated, it has been for some years 

 recognised that many facts could not be reconciled with either ; 

 and modifications of one or the other have been from time to time 

 introduced. 



The observations made by C. J. Burch are of considerable 

 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 transparent coloured screens, it is possible 

 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, corresponding effects are pro- 

 duced. If one eye is made purple-blind, and the other green-blind, 

 all objects are seen in their natural colours, but in exaggerated per- 

 spective, 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 appears 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 compounded 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 Hering. Each may 

 be exhausted without either weakening or strengthening the others. 

 These observations were confirmed by examining in a similar way 

 the colour sensations of seventy other people, but there are individual 

 differences in the extent to which the colour sensations overlap. 



