1244 COLOUR-BLINDNESS. [BOOK in. 



or, what amounts to the same thing put in another way, that 

 in such a case there had been a return to a primitive condition, 

 in which light produced one kind of visual sensation only, not 

 yet differentiated into colour sensations. But such a supposition 

 in laying one difficulty would raise many others. 



767. What we have said concerning colour vision refers 

 to the central parts of the retina only. If a coloured object be 

 moved so that its image travels from the central to the peripheral 

 parts of the retina, the colour sensations change and the peri- 

 pheral parts may be spoken of as colour-blind. In studying 

 the changes of colour which are thus undergone, some results 

 are obtained which seem to favour Hering's theory rather than 

 the Young-Helmholtz theory. Thus the sensation of red is lost 

 towards the periphery, which may be spoken of as red-blind, 

 while in the same region other sensations, at all events that of 

 blue, are still felt. If we suppose this peripheral red-blindness 

 to be due to the loss of the primary sensation of red, the image 

 of a white object ought to give in this region a sensation com- 

 pounded of the two remaining primary sensations, that is blue- 

 green ; but the sensation actually felt is white. Moreover at 

 the extreme periphery even blue is wanting, that is, all the 

 primary sensations are wanting, and yet we receive by it un- 

 coloured sensations, sensations of black and white. But the 

 phenomena of peripheral colour vision need a fuller discussion 

 than we can afford to give them here. We may however add 

 that in certain diseases of the eyes the central parts of the retina 

 may become more or less colour-blind, while the peripheral parts 

 suffer comparatively little ; but in these cases, though red and 

 green disappear first, there seems to be rather an unequal 

 diminution of all the primary sensations than a loss confined to 

 any particular one, and the failure to recognize colour is accom- 

 panied by failure to recognize form. 



768. Influence of tfie pigment of the yellow spot. In the 

 macula lutea, or yellow spot, the yellow pigment which ( 744) 

 is diffused through the retinal structures in this region absorbs 

 some of the greenish-blue rays of the light which falls upon it. 

 We may use this feature of the yellow spot for the purpose of 

 making the spot, so to speak, visible to ourselves, by the following 

 experiment. A solution of chrome alum, which only transmits 

 red and greenish-blue rays, is held up between the eye and a 

 white cloud. The greenish-blue rays are absorbed by the yellow 

 spot, and here the light gives rise to a sensation of red ; whereas 

 in the rest of the field of vision, the sensation is that ordinarily 

 produced by the purplish solution. The yellow spot is con- 

 sequently marked out as a rosy patch. This very soon however 

 dies away. 



Though, when we wish our vision to be most acute, we use the 

 fovea centralis in which the pigment is extremely scanty or absent 



