This is a very curious optical effect, producing a distorted and grotesque figure from a regular one. The term is derived from two Greek words, signifying a distortion of figure, and by its means many optical puzzles may be produced geometrically.
Take any subject, such as the portrait of a head; divide it vertically and horizontally with parallel lines, of which the outer sides shall form the boundary, A, B, C, D, and the whole shall be equi-distant. Then, on a separate piece of paper, or cardboard, prepare a drawing similar to Fig. 2 by the following means:
1. Draw a horizontal line, a b, equal to A B, and divide it into as many equal parts as the latter is divided.
2. Let fall a perpendicular line, e v, from the middle of a b, and then draw s v parallel to a b.
3. Both e v and s v may be any length at pleasure, but the longer the first is, and the shorter the other, so will the anamorphoses be more and more deformed. The proportions in our figures are sufficiently different.
4. After having drawn from the point v right lines, v 1, v 2, v 3, v 4, to the divisions of a b, draw the line s b, and through each point where s b intersects the divergent lines, draw other horizontal lines parallel to a b. We now have a trapezium, a b c d, divided into as many cells as the square in Fig. 1.
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 follow:
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 the eye has become accustomed to the novelty of the experiment the anamorphosis will lose its distortion, and appear almost exactly like the symmetrical figure.