From these principles, which will afterwards be shewn to be erroneous, Mr. Wheatstone proceeds “to examine _why_ two dissimilar pictures projected on the two retinæ _give_ rise to the perception _of an object in relief_.” “I will not attempt,” he says, “at present to give the complete solution of this question, which is far from being so easy as at first glance it may appear to be, and is, indeed, _one of great complexity_. I shall, in this case, merely consider the most obvious explanations which might be offered, and shew their insufficiency to explain the whole of the phenomena. “_It may be supposed_ that we see only one point of a field of view distinctly at the same instant, the one, namely, to which the optic axes are directed, while all other points are seen so indistinctly that the mind does not recognise them to be either single or double, and that the figure is appreciated by successively directing the point of convergence of the optic axes successively to a sufficient number of its points to enable us to judge accurately of its form. “That there is _a degree of indistinctness_ in those parts of the field of view to which the eyes are not immediately directed, and which increases with the distance from that point, cannot be doubted; and it is also true that the objects there obscurely seen are _frequently doubled_. In _ordinary_ vision, it may be said, this indistinctness and duplicity are not attended to, because the eyes shifting continually from point to point, every part of the object is successively rendered distinct, and the perception of the object is not the consequence of a single glance, during which _a small part of it only is seen distinctly_, but is formed from a comparison of all the pictures successively seen, while the eyes were changing from one point of an object to another. “_All this is_ IN SOME DEGREE _true, but were it entirely so_ no appearance of relief should present itself when the eyes remain intently fixed on one point of a binocular image in the stereoscope. But in performing the experiment carefully, it will be found, provided the picture do not extend far beyond the centres of distinct vision, that the image is still seen single, and in relief, when in this condition.”[22] [22] _Phil. Trans._, 1838, pp. 391, 392.