_Spherical Aberration and Coma._—Suppose we focus on a screen, by means of a simple convex lens the image of a distant point of light. Suppose for simplicity that this image is located on the axis of the lens and that light of only one color is used, such as yellow. It will be found that the smallest image that can be obtained is not a point, but a small disc. This is due to the fact that the rays of light passing through the outer portions of the lens are bent more than those passing through the lens in the region near the center. This effect is shown in Fig. 12 by the usual mode of representing it graphically. Here the figures 1, 2, 3, 4, represent distances from the axis of the lens, and the letters A_{1}, A_{2}, A_{3}, A_{4}, the points of convergence of the rays from 1, 2, 3, 4, etc. These distances projected upward on to the produced lens points form a curve which shows at a glance the extent and direction of the error due to each part of the lens. This information is of value where the lens is fitted with an adjustable diafram. With some types of correction sharper definition may be obtained by reducing the aperture. With others, however, diaframing impairs definition, by destroying the balance between under and over correction which averages to make a good image. In aerial lenses it is not customary to use diaframs, as all the light possible is desired. Consequently the reduction of spherical aberration must be accomplished by proper choice of lens elements and their arrangement. Off the axis of the lens the image of a point source takes on an irregular shape, due to oblique spherical aberration or _coma_. _Chromatic Aberration._—Because of the inherent properties of the glass of which it is made, a simple collective lens does not behave in the same way with respect to light of different colors. If one attempts, with such a lens, to focus upon a screen the image of a distant white light, it will be found that the blue rays will not focus at the same point as the red rays, but will come together nearer the lens. Modern photographic lenses are compounded of two or more kinds of glass in such a way as to largely eliminate this defect, the presence of which is detrimental to good definition. Such lenses are called achromatic, and the property of a lens by virtue of which this defect is eliminated is called its _chromatic correction_. Chromatic correction is never perfect, but two colors of the spectrum can be brought to a focus in the same plane, and to a certain extent the departure of other colors from this plane can be controlled. Off the axis of the lens outstanding chromatic aberration results in a difference in the size of images of different colors, known as _lateral chromatism_.