The structure and distribution of the two principal types of fovea are briefly described. Using the convexiclivate central fovea of the eagle as a type the effect of refraction at the boundary of retina and vitreous humour on the foveal image is computed. It is shown that, when aberration and diffraction are taken into account, no improvement in acuity can result from refraction at the fovea. On the contrary, the results strongly suggest that the convexiclivate fovea is a device for achieving improved fixation and improved sensitivity to movement of objects in the visual field at some sacrifice of acuity.
The application of these results to other variants of fovea is discussed, and a scheme indicating possible relations between function and geometrical form is proposed.