Popular Science Monthly/Volume 5/July 1874/Hints on the Stereoscope
By FRANKLIN C. HILL.
THE following diagrams will help to explain the principle on which this instrument acts. The stereoscope-glasses are halves of the same lens, placed with their outer edges toward each other (L L), Fig. 1. Rays of light (R R) from the objects (0 0), striking the oblique surfaces of the lens, are refracted outward, toward the focus, and thus reach the eyes (E E) in an oblique direction, appearing to come from (0'), a point half-way between the objects. Each eye being thus deceived, the objects seem to be one.
This may be easily demonstrated by drawing on the plain back of a stereograph two similar circles, onfe with an horizontal and one with a vertical diameter, opposite the centres of the pictures, and, half-way between them, a heavy vertical line (as in Fig. 2). Then, looking at the card through the instrument, a circle will appear with crossed diameters, and with a vertical line on each side (as in Fig. 3).
By closing and opening the eyes alternately, the diameters and vertical lines will appear and disappear, but the circle will remain constant. The right eye is thus deceived into seeing the circle in the middle, and the line away to the left; and the left is deluded on the other side. The same effect may be quickly obtained by sticking a common pin upright on the middle of the rack, with a carpet-tack on each side, when a tack will be visible standing between two pins.
In selecting a stereoscope, first look that the glasses be large and heavy, and of perfectly clear glass, and see that they are wide enough
for the eyes to come opposite the middle of them, producing no feeling of being shut in by dark objects at the side. Many instrument-makers seem unaware that heads differ in width.
Then, placing a picture in the rack, see that the diaphragm hides no part of it when the rack is drawn up to a short focus. Near-sighted people have rights as well as others. Should the pictures not appear as one, remove the card and stick up a pin on each side of the rack equidistant from the middle, moving them in or out until they appear as one. If the pins have to be within two or two and a half inches of each other to "solidify," reject the instrument; but, if they meet well when farther apart, the fault was in the pictures.
A friend complained to me that the stereoscope she had bought, on my recommendation, was worthless. I had taken it on faith, supposing that all "Holmes" instruments must be good; but a short examination showed that the glasses were rather flat, and were placed so near together that the rays came to the eyes through the central part of the lens, and of course without refraction, and hence the eyes were not deceived. By whittling out the frame, and moving each glass outward about three-eighths of an inch, a good instrument was made.
If a view does not "stand out well" when seen in a good instrument, examine each picture by itself, and try to find a well-defined object, as a post or tree, in the foreground, and note its relations to some other object in the distance. If the two objects have the same relative position in both, the pictures are duplicates, and worthless.
If the distant object is to the right of the near one, they should appear farther apart in the right-hand picture, and vice versa; if not so, the pictures are mounted on the wrong ends of the card, and are worthless, or worse.