Chap, hi.] SIGHT. 913 



§ 570. 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 periph- 

 eral parts of the retina, the colour sensations change and the 

 peripheral parts may be spoken of as colour-blind. Thus the 

 sensation of red is lost towards the periphery, which may be 

 spoken of as red-blind, while in the same region other sensa- 

 tions, at all events that of blue, are still felt. At the extreme 

 periphery even blue is wanting, that is, all the primary sensa- 

 tions are wanting, and }^et we receive by it uncoloured sensa- 

 tions, sensations of black and white. But these phenomena of 

 peripheral colour vision also need a fuller discussion than we 

 can afford to give them here. 



§ 571. Influence of the pigment of the yellow spot. In the 

 macula lutea, or yellow spot, the yellow pigment which is dif- 

 fused 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 follow- 

 ing experiment. A solution of chrome alum, which only trans- 

 mits 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 consequently 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 owing to the thinness or absence of all retinal layers ex- 

 cept the cones and cone fibres, still in ordinary vision we make 

 large use of the whole yellow spot, and our sensations of the 

 colour of external objects must be to a certain extent influenced 

 by the pigment of the spot. The light which reaches the rods 

 and cones of this region from objects which we call white, is in 

 reality more or less tinged with yellow; in other words what we 

 call white is more or less yellow. Indeed variations in the 

 amount of pigment present in the yellow spot have been offered 

 in explanation of some of the differences in colour vision dis- 

 cussed above. 



§ 572. In speaking of the relation between a visual sensa- 

 tion and the intensity of the stimulus (§ 550) we were confining 

 our remarks to white light ; when we inquire into the behaviour 

 of our colour sensations under variations in the intensity of the 

 stimulus, we come upon results which are in many ways com- 

 plicated. We must be content with pointing out one or two 

 only of these. 



Each of our colour sensations, when the light giving rise to 



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