In common with all dioptric instruments the eye has certain optical defects which tend to interfere with the distinctness of the image.
Chromatic aberration is due to the breaking up of white light into several colors, owing to the different colored rays of which ordinary light is composed, possessing different degrees of re-frangibility. We see this in the spectrum and in the colored rings of the marginal part of a biconvex lens made of a single kind of glass. This form of aberration can be corrected by making lenses of two kinds of glass, one of which counteracts the dispersion caused by the other. Optical instruments may thus be made achromatic. This defect is minimized by the iris, which cuts off the marginal rays in which it is most apt to occur. Pos-sibly the different density of the various parts of the dioptric media may have a correcting effect on the chromatism of the eye. Further correction takes place in the nerve centres which receive the sensation, for just as we mentally reinvert the image, we are unconscious of the color. At any rate, the chromatic aberration is so slight that it needs certain artifices to make it observable.
Spherical aberration depends upon the fact that luminous rays, on passing through a convex lens, strike the various parts of its surface at different angles, and hence are differently refracted. The rays striking the margin of the lens are more bent than those passing through the centre, and hence the former come sooner to a focus. Thus, a luminous point gives rise to a diffused figure, which is circular in perfectly centred dioptric systems, but is stellate in our eyes where the centering of the lenses is not absolutely accurate. Spherical aberration causes us no inconvenience, as the iris only allows the more central rays to pass, in which its influence is not noticed.
Another optical defect in our eyes is astigmatism, depending upon some irregularity of the curvature of the cornea, which may be bent more horizontally than vertically, or vice versa. In either of these cases the light in the vertical and horizontal planes will be differently refracted, so that lines drawn in the two directions will require different adjustments to see them distinctly. This may be recognized if we gaze with one eye at a centre from which many sharply-defined lines radiate; some of the lines cannot be seen distinctly, unless we move the eye or change its accommodation. When the greater curvature extends evenly over the whole diameter of the cornea it gives rise to what is called regular astigmatism, and when the unevenness is localized to one part of the cornea surface it is called irregular astigmatism.
The astigmatism which may be called physiological is not noticed by the individual, but pathological astigmatism often occurs and requires cylindrical glasses to correct it.
Entoptic images are those which depend on the presence of some opacity or difference in density in the transparent media of the eye itself. They look like variously-shaped specks moving over the field of vision. They are only remarkable when we look at an evenly-colored object or through a pin hole in a black card. In using the microscope they often annoy the unpracticed observer.