together at right angles to each other, the two pictures being placed at each side, at a distance corresponding to their size. The reflecting instrument, although not
Fig. 69.—The Principle of the Refracting Stereoscope.
so portable, is in some sort superior to the other, inasmuch as pictures of any size can be seen by it, whilst in the smaller instrument the size of the photograph is limited by the distance at which the eyes are placed.
It should be mentioned, that no optical instrument of any kind is absolutely necessary to obtain a stereoscopic effect from two suitable drawings or photographs, as it is quite possible by a little management of the eyes to cause the two images to combine with each other. Referring again to fig. 67, it will be perceived that the two figures of the dice are about an inch and a half from each other. Holding the book at about ten inches from the eye, they are viewed by squinting strongly until the right eye looks at the left die, and the left eye at the right. This may be also done by converging the eyes on a point beyond the centre of the figure, which may be easily done by looking at a point midway between the two. In both cases the images at first appear doubled, and we see four dice, but a little practice will soon enable you to cause the two inside images to coalesce,
