Fig. 267.
a b, d h. (No. 2) represent two parallel rays incident on the convex surface b h, the one (a b) perpendicularly, the other (d h) obliquely. c is the centre of convexity. h e is the reflected ray of the oblique incident one, d h; whilst c h i is the perpendicular.
Convex mirrors are not employed in any optical deception on a large scale, although some ingenious delusions are producible from cylindrical and conical mirrors, and are thus described by Sir David Brewster:
"Among the ingenious and beautiful deceptions of the seventeenth century, we must enumerate that of the re-formation of distorted pictures by reflection from cylindrical and conical mirrors. In these representations, the original image from which a perfect picture is produced, is often so completely distorted, that the eye cannot trace in it the resemblance to any regular figure, and the greatest degree of wonder is of course excited, whether the original image is concealed or exposed to view. These distorted pictures may be drawn by strict geometrical rules, and I have shown a simple method of executing them. Let m be an accurate cylinder made of tin-plate or of thick pasteboard. Out of the further side of it cut a small aperture, a b c d, and out of the nearer side cut a larger one, a b c d (white letters), the size of the picture to be distorted; having perforated the outline of the picture with small holes, place it in the opening a b c d (white letters), so that its surface may be cylindrical; let a candle or a bright luminous object—the smaller the better—be placed at s, as far behind the picture a b c d (white letters) as the eye is afterwards to be placed before it, and the light passing through the small holes will represent on a horizontal plane a distorted image of the picture at a b c d, which, when sketched in outline with a pencil, shaded, and coloured, will be ready for use. If we now substitute a polished cylindrical mirror of the same size in place of m, then the distorted picture, when laid horizontally at a b c d, will be restored to its original state when seen by reflection at a b c d (white letters) in the polished mirror." The effect of a cylindrical mirror on a distorted picture is shown at No. 2, being copied from an old one seen by Sir D. Brewster.
Fig. 268.
By looking at a reflection of the face in a dish-cover or the common surface of a bright silver spoon or of a silver mug, the latter truly becomes ugly as the image is seen reflected from its surface, and assumes the most absurd form as the mouth is opened or shut, and the face advanced or removed from the silver vessel. (Fig. 269.)
Fig. 269.