It has been implied, in the comments on visual acuity, that the limiting factor is one of an anatomical arrangement of photoreceptors and of their neural organization. A very important feature, however, must be the accuracy of the formation of an image of external objects by the optical system of the eye. It may be calculated, for example, that the image of a grating produces lines 0.5 micron wide on the retina, but this is on the basis of ideal geometrical optics. In fact, the optics of the eye are not perfect, and diffraction of light by its passage through ...(100 of 31643 words)