(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
|Fig. 2.||Fig. 3.|
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