This remarkable property of binocular vision being thus clearly established by preceding writers, and admitted by himself, as the cause of the vision of solidity or distance, Mr. Wheatstone, as Mr. Elliot had done before him, thought of an instrument for uniting the two dissimilar pictures optically, so as to produce the same result that is obtained by the convergence of the optical axes. Mr. Elliot thought of doing this by the eyes alone; but Mr. Wheatstone adopted a much better method of doing it by reflexion. He was thus led to construct an apparatus, to be afterwards described, consisting of two plane mirrors, placed at an angle of 90°, to which he gave the name of _stereoscope_, anticipating Mr. Elliot both in the construction and publication of his invention, but not in the general conception of a stereoscope. After describing his apparatus, Mr. Wheatstone proceeds to consider (in a section entitled, “Binocular vision of objects of different magnitudes”) “what effects will result from presenting similar images, differing only in magnitude, to analogous parts of the retina.” “For this purpose,” he says, “two squares or circles, _differing obviously_ but not extravagantly in size, may be drawn on two separate pieces of paper, and placed in the stereoscope, so that the reflected image of each shall be equally distant from the eye by which it is regarded. _It will then be seen that notwithstanding this difference they coalesce and occasion a single resultant perception._” The fact of coalescence being supposed to be perfect, the author next seeks to determine the difference between the length of two lines which the eye can force into coalescence, or “the limits within which the single appearance subsists.” He, therefore, unites two images of equal magnitude, by making one of them visually less from distance, and he states that, “by this experiment, _the single appearance of two images of different size is demonstrated_.” Not satisfied with these erroneous assertions, he proceeds to give a sort of rule or law for ascertaining the relative size of the two unequal pictures which the eyes can force into coincidence. The inequality, he concludes, must not exceed the difference “between the projections of the same object when seen in the most oblique position of the eyes (_i.e._, both turned to the extreme right or the extreme left) ordinarily employed.” Now, this rule, taken in the sense in which it is meant, is simply _a truism_. It merely states that the difference of the pictures which the eyes _can_ make to coalesce is equal to the difference of the pictures which the eyes do make to coalesce in their most oblique position; but though _a truism_ it is not _a truth_, first, because no real coincidence ever can take place, and, secondly, because no apparent coincidence is effected when the difference of the picture is greater than what is above stated.