In this passage the author makes a distinction between _ordinary binocular vision_, and binocular vision through the stereoscope, whereas in reality there is none. The theory of both is exactly the same. The muscles of the two eyes unite the two dissimilar pictures, and exhibit the solid, in ordinary vision; whereas in stereoscopic vision the images are united by reflexion or refraction, the eyes in both cases obtaining the vision of different distances by rapid and successive convergences of the optical axes. Mr. Wheatstone notices _the degree of indistinctness_ in the parts of the picture to which the eyes are not immediately directed; but he does not notice the “_confusion and incongruity_” which Aguilonius says ought to exist, in consequence of some parts of the resulting relievo being seen of one size by the left eye alone,—other parts of a different size by the right eye alone, and other parts by both eyes. This confusion, however, Aguilonius, as we have seen, found not to exist, and he ascribes it to the influence of a _common sense_ overruling the operation of physical laws. Erroneous as this explanation is, it is still better than that of Mr. Wheatstone, which we shall now proceed to explain. In order to disprove the theory referred to in the preceding extract, Mr. Wheatstone describes two experiments, which he says _are equally decisive against it_, the first of them only being subject to rigorous examination. With this view he draws “two lines about two inches long, and inclined towards each other, on a sheet of paper, and having caused them to coincide by converging the optic axes to a point nearer than the paper, he looks intently on the upper end of the resultant line without allowing the eyes to wander from it for a moment. The _entire line will appear single_, and in its proper relief, &c.... The eyes,” he continues, “sometimes become fatigued, which causes the line to become double at those parts to which the optic axes are not fixed, _but in such case all appearance of relief vanishes_. The same experiment may be tried with small complex figures, but the pictures should not extend too far beyond the centre of the retinæ.” Now these experiments, if rightly made and interpreted, are not _decisive against_ the theory. It is not true that the entire line appears single when the axes are converged upon the upper end of the resultant line, and it is not true that the disappearance of the relief when it does disappear arises from the eye being fatigued. In the combination of more complex figures, such as two similar rectilineal figures contained by lines of unequal length, neither the inequalities nor the entire figure will appear single when the axes are converged upon any one point of it.