THE SENSES 829 



Sensibility of Different Parts of the Retina. The perception of 

 colours, like the perception of white light, is not equally distinct 

 over the whole retina. We have repeatedly had occasion to refer to 

 the fovea centralis as the region of most distinct vision ; but it 

 would be a mistake to suppose that it is therefore necessarily more 

 sensitive than the rest of the retina. As a matter of fact, when the 

 minimum intensity of white light which will cause an impression at all 

 is determined for each portion of the retina, it is found that the fovea 

 centralis requires a somewhat stronger stimulus than the zone im- 

 mediately surrounding it. Objects only a little brighter than the 

 general ground on which they lie, e.g., very faint stars, are best seen 

 when the eye is directed a little to one side. This has been attri- 

 buted to the absence of visual purple from the fovea, the visual 

 purple being supposed to act as a mechanism which ' adapts ' the 

 retina for the perception of light of varying intensity. But, with 

 this exception, the sensibility of the retina diminishes steadily from 

 centre to periphery, both for white and for coloured light. Konig 

 has, indeed, upheld the paradoxical view that the fovea is absolutely 

 blind for blue rays, supporting this assertion by two main experi- 

 ments : (a) that when a number of feebly illuminated blue points are 

 looked at, those that fall on the fovea disappear ; (b] that when the 

 moon is examined through a blue glass, her image is blotted out as 

 soon as it falls on the fovea. But, as Gad has pointed out, the 

 moon's image is of such dimensions that it would lie well within the 

 fovea, and there ought, therefore, to be no difficulty in getting it to 

 disappear if Konig's theory were true. Yet Konig himself admits 

 that his second experiment is difficult, and succeeds only under 

 special conditions. Hering, too, seems to have shattered Konig's 

 first argument by showing that the disappearance of the weakly 

 illuminated blue points is only an illustration of the phenomenon 

 known as Maxwell's spot, a dark-blue or almost black blot, seen in 

 the visual field when the eye, after being kept closed for a short time, 

 is directed to a surface illuminated by a weak blue light. It is due 

 to the absorption of blue light by the pigment of the yellow spot, and 

 stands out as a rose-coloured disc when a source of white light is 

 looked at through a solution of chrome alum, since all the light 

 which the greenish solution permits to pass is absorbed by the macula 

 lutea, except the red rays. Hering, indeed, asserts that the fovea is 

 the most sensitive part of the retina for colours, in opposition to 

 Charpentier, who finds it slightly less sensitive for blue than the zone 

 immediately external to it. When the eye is fixed and the visual 

 field (that is, the whole space from which light can reach the retina 

 in the given position, or, what comes to the same thing, the pro- 

 jection of the visual field on the retina by straight lines passing 

 through the nodal point) explored by means of a perimeter (Figs. 318, 

 319), it is found that, under ordinary conditions, a white object is 

 seen'over a wider field than any coloured object, a blue object over 

 a wider field than a red, and a red over a wider field than a green 

 object. The exact shape, as well as size, of the visual field also 

 differs somewhat for different colours. And although it has been 



