which, making AC = 8 inches, as before, will give the following results:—
| Depth. | Ab = AC + d | Cc′ |
|---|---|---|
| Cb = d | Inches. | |
| 1 | 9 | 0.139 |
| 2 | 10 | 0.25 |
| 3 | 11 | 0.34 |
| 4 | 12 | 0.4166 |
| 5 | 13 | 0.48 |
| 6 | 14 | 0.535 |
| 7 | 15 | 0.58 |
| 8 | 16 | 0.625 |
| 9 | 17 | 0.663 |
| 10 | 18 | 0.696 |
| 11 | 19 | 0.723 |
| 12 | 20 | 0.75 |
The values of h and d when Cc, Cc′ are known, will be found from the formulæ
| h = | 2AB · Cc | , and |
| E | ||
| d = | 2AB · Cc′ | . |
| E |
As Cc is always equal to Cc′ in each pair of figures or dissimilar pictures, the depth of the hollow cone will always appear much greater than the height of the raised one. When Cc = Cc′ = 0.75, h:d = 3:12. When Cc = Cc′ = 0.4166, h:d = 2:4, and when Cc = Cc′ = 0.139, h:d = 0.8:1.0.
When the solids of which we wish to have binocular pictures are symmetrical, the one picture is the reflected image of the other, or its reverse, so that when we have drawn the solid as seen by one eye, we may obtain the other by copying its reflected image, or by simply taking a copy of it as seen through the paper.
When the geometrical solids are not symmetrical, their dissimilar pictures must be taken photographically from models, in the same manner as the dissimilar pictures of other solids.
Portraits of Living Persons or Animals.
Although it is possible for a clever artist to take two portraits, the one as seen by his right, and the other as seen by his left eye, yet, owing to the impossibility of fixing the sitter, it would be a very difficult task. A bust or statue would be more easily taken by fixing two apertures 2½ inches distant, as the two points of sight, but even in this case the result would be imperfect. The photographic camera is the only means by which living persons and statues can be represented by means of two plane pictures to be combined by the stereoscope; and but for the art of photography, this instrument would have had a very limited application.
It is generally supposed that photographic pictures, whether in Daguerreotype or Talbotype, are accurate representations of the human face and form, when the sitter sits steadily, and the artist knows the resources of his art. Quis solem esse falsum dicere audeat? says the photographer, in rapture with his art. Solem esse falsum dicere audeo, replies the man of science, in reference to the hideous representations of humanity which proceed from the studio of the photographer. The sun never errs in the part which he has to perform. The sitter may sometimes contribute his share to the hideousness of his portrait by involuntary nervous motion, but it is upon the artist or his art that the blame must be laid.