The next step is to fill up all the cells of [Fig. 2] with portions of the device, proportionate to their position in [Fig. 1]. For instance, in [Fig. 1] the nose is in the second vertical division from the left, and in the third and fourth horizontal divisions from the top, and that portion of the face must accordingly be placed in a corresponding part of [Fig. 2].
By these means we procure the anamorphosis seen in [Fig. 2], which when viewed from a particular position, will lose all its distortion, and assume an appearance resembling that in [Fig. 1]. This position lies immediately over the point v, and at a height above it equal to the length of the line s v; and the means of determining it are as follows.
Place the drawing horizontally before a window; take a slip of card, and rest its lower edge on the line s v, the card being accurately vertical; pierce a small hole in the card vertically over the point v, and at a height from it equal to the length of the line s v, then with the eye placed immediately behind the card, look through the orifice at the anamorphosis, and it will be found that as soon as it has become accustomed to the novelty of the experiment, the anamorphosis will lose its distortion, and appear almost exactly like the symmetrical figure.
It would be very difficult, and would require geometrical reasoning of a lengthened kind, to show why this particular form of construction should lead to such results.
THE COSMORAMA.
The principle upon which the cosmorama is formed is so simple, that any person may easily fit up one in a small summer house, &c. Nothing more is necessary than to fix in a hole a double convex lens of about three feet focus, A, and at rather less than this distance a picture, B, is to be hung. To absorb all the rays of light but those necessary for seeing the picture, a squared frame of wood blackened on the inside is placed between the lens and the picture. The picture may be hung in a large box having a light coming in upon it from above, or in a small closet illuminated in the same manner. Should it be wished to show it by candle-light, a lamp, c, may be placed on the top of the wooden frame, and if the light of this be converged by a lens to a moderate radius, it will be more effective.