“ The only mirror which can sharply reproduce an object-point as an image-point has for its section an ellipse, which is so placed that the object and image are at its foci. This follows from a property of the curve, viz. the sum of the focal distances is constant, and that the focal vectores are equally inclined to the normal at the point. More important than the elliptical mirror, however, is the parabolic, which has the property of converting rays parallel to the axis into a pencil through its focus ”

“ A point-eye at would see a point object at , i.e. at a distance below the mirror equal to its height above. If the object be a solid, then the images of its corners are formed by taking points at the same distances below as the corners are above the mirror, and joining these points. The eye, however, sees the image perverted, i.e., in the same relation as the left hand to the right. Fig. 2 shows how an extended object is viewed in a mirror by a natural eye. ”

“ The permissible deviation from smoothness depends on the wave-length of the light employed: it appears that surfaces smooth to within 1/8th of a wave-length reflect regularly; hence long waves may be regularly reflected by a surface which diffuses short waves. ”