844 THE EYE AND VISION [CH. LYIII. 



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. 



C. J. Burch found 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 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 

 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. 



Testing for Colour-blindness. The test formerly adopted by the 

 Board of Trade consists in matching skeins of wool from a heap of 

 skeins of different colours (Holmgren's worsteds). It has, however, 

 been shown that the test is not trustworthy, and it has been supple- 

 mented by one in which the subject is required to name the colours of 

 lights in a lantern. The Edridge-Green lantern is one of the best to 

 employ ; in it the intensity and colour of the light and the order in 

 which the colours are shown can be easily varied. If the colour- 

 blind person is made to examine and report on the colours of a 

 spectrum, or in portions of the spectrum exposed to view, the results 

 obtained are more accurate, but this is not so simple as the lantern 

 test. 



After-Images. These are the after-effects of retinal excitation, 

 and are divided into positive and negative. Positive after-images 

 resemble the original image in distribution of brightness and 

 colour. In negative after-images bright parts appear dark, dark 

 parts bright, and coloured parts in the complementary colours. 



If a bright white object is looked at, and the eyelids are then 

 closed, a positive after-image is seen which fades gradually, but as it 

 fades it passes through blue, violet or red, to orange ; according to 



