658 THE EYE IN EVOLUTION 



A further device for increasing the resolving power is the develop- 

 ment of an excavated fovea within the central area. The classical 

 view of the rationale of this pit -like configuration is that the out- 

 spreading of the cellular layers of the retina and the consequent 

 thinning of this tissue in the central pit reduce the absorption and 

 scattering of the light as it traverses the retinal layers to reach the 

 receptor cells. It is questionable, however, if the retinal tissue is much 

 less transparent than the vitreous and it seems probable that in weU- 

 developed foveae at any rate, a refractive magnification of the image 

 is a more important optical effect (Walls, 1937). It was shown by 

 Valentin (1879) that the refractive index of the retina is considerably 

 higher than that of the vitreous ; this being the case, incident light 

 will be diverged as it strikes the curved sides of the pit (Fig. 791). 



VISUAL CELLS 



VITREOUS 



Fig. 791. — The Magnifying Effect of the Fovea. 



Owing to the fact that the index of refraction of the retina is higher than that 

 of the vitreous, incident light striking the chvus of the foveal depression is 

 refracted laterally so that the image is magnified. 



This theory, advanced by Walls (1937), demands that the most 

 efficient fovea will have a deej) pit with highly convex sides, and this 

 is indeed the case ; in Birds, for example, the linear magnification 

 thus obtained is of the order of 13% and the areal magnification, 30%. 

 As Walls puts it, when the area centralis has done everything possible 

 to increase the number of receptor imits over which an image will fall, 

 a further increase in efficiency is gained optically by the magnification 

 of the image. A shallow or broad fovea thus probably acts by 

 eliminating the dispersion of light as it traverses the retina, a deep 

 well-formed fovea with a steeply curving clivus acts also as an effective 

 magnifying device (Figs. 792-5). 



A further and equally interesting function for the fovea has been suggested 

 by Puniphrey (1948). From the optical point of view he reasoned that a deep 

 convex -clivate fovea vi^ould produce a distorted image peripherally and a clear 

 image only at the centre of the depression ; the shape of fovea could thus be 

 interpreted as a mechanism to maintain accurate fixation of the eye and might 

 be used to appreciate in exaggerated form the angular movements of objects 

 which are being fixated. This function, of course, would be attained at the 



