However perfect be the stereoscope which we employ, the effect which it produces depends upon the accuracy with which the binocular pictures are prepared. The pictures required for the stereoscope may be arranged in four classes:—
1. The representations of geometrical solids as seen with two eyes.
2. Portraits, or groups of portraits, taken from living persons or animals.
3. Landscapes, buildings, and machines or instruments.
4. Solids of all kinds, the productions of nature or of art.
Geometrical Solids.
Representations of geometrical solids, were, as we have already seen, the only objects which for many years were employed in the reflecting stereoscope. The figures thus used are so well known that it is unnecessary to devote much space to their consideration. For ordinary purposes they may be drawn by the hand, and composed of squares, rectangles, and circles, representing quadrangular pyramids, truncated, or terminating in a point, cones, pyramids with polygonal bases, or more complex forms in which raised pyramids or cones rise out of quadrangular or conical hollows. All these figures may be drawn by the hand, and will produce solid forms sufficiently striking to illustrate the properties of the stereoscope, though not accurate representations of any actual solid seen by binocular vision.
If one of the binocular pictures is not equal to the other in its base or summit, and if the lines of the one are made crooked, it is curious to observe how the appearance of the resulting solid is still maintained and varied.
The following method of drawing upon a plane the dissimilar representations of solids, will give results in the stereoscope that are perfectly correct:—
