middle line f. E E' are panels, to which the pictures are fixed in such a manner that their corresponding horizontal lines shall be on the same level; these panels are capable of sliding backward and forward in grooves on the upright boards, D D'. The observer must place his eyes as near as possible to the mirrors, the right eye before the right-hand mirror, and the left eye before the left-hand mirror; and he must move the sliding-panels E E' to or from him, until the two reflected images coincide at the intersection of the optic axes, and form an image of the same apparent magnitude as each of the component pictures.
In using this stereoscope, of which a perspective view is given in Fig. 4, the two conjugate pictures must be on separate cards, but may be much larger than those which are now so extensively used with more modern instruments. The arrangement is obviously such that no side-images can be perceived, since it is impossible for either eye to receive more than one image, and this is reflected from the oblique mirror directly in front. As an instrument it is unwieldy and inconvenient in comparison with those to which we are accustomed; but
with it the great secret of binocular vision was brought into open daylight. Wheatstone had the genius to find out how the door was to be unlocked, and it was left for others to devise the special forms that would be employed in making most acceptable to the world the treasure which he had found. His predecessors had more or less distinct conceptions of an hypothetical treasure, just as something was known about the nature of steam before the low-pressure engine was invented,