thickest at the center, the retardation is greatest there, gradually diminishing toward the edge. The effect is to change the form of the wave front from a plane to a spherical shell, 
FIG. 19which advances toward the focus at O, and produces at this point a maximum of light, which is the image of the point whence the waves started.
Fig. 20 illustrates the case where the convex waves diverging from a luminous point O are changed to concave waves converging to form the image at O'.
It can readily be shown that the luminous point and its image are in the same line with the center of the lens—sufficiently 
FIG. 20near for a first approximation. Accordingly, if we take separate points of an object, we can construct its image by drawing straight lines from these through the center of the lens, as shown in Fig. 21. The size of the image will be greater the greater the distance from the lens, so that
