Among the ingenious and beautiful deceptions of the 17th century, we must enumerate that of the re-formation of distorted pictures by reflexion 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; but I have shown in Fig. 11 a simple and practical method of executing them. Let MN be an accurate cylinder made of tin-plate or of thick pasteboard. Out of the farther side of it cut a small aperture _abcd_; and out of the nearer side cut a larger one ABCD, the size of the picture to be distorted. Having perforated the outline of the picture with small holes, place it on the opening ABCD, 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 ABCD 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 A´B´C´D´, which, when sketched in outline with a pencil, and shaded or coloured, will be ready for use. If we now substitute a polished cylindrical mirror of the same size in place of MN, then the distorted picture, when laid horizontally at A´B´C´D´, will be restored to its original state when seen by reflexion at ABCD in the polished mirror. It would be an improvement on this method to place at ABCD a thin and flexible plate of transparent mica, having drawn upon it with a sharp point, or painted upon it, the figure required. The projected image of this figure at A´B´C´D´ may then be accurately copied. The effect of a cylindrical mirror is shown in Fig. 12, which is copied from an old one which we have seen in use. [Illustration: _Fig. 12._] The method above described is equally applicable to concave cylindrical mirrors, and to those of a conical form; and it may also be applied to mirrors of variable curvature, which produce different kinds of distortions from different parts of their surfaces. [Illustration: _Fig. 13._]