The absence of double vision is in fact the only evidence that can be adduced in favour of the exclusion theory; this negative fact, however, proves nothing, and is, moreover, capable of other explanations, as soon as one abandons the theory of congenital retinal identity. The examination of those who squint demonstrates the untenability of this theory. People who squint seldom complain of diplopia, but double images can be rendered apparent in a comparatively large proportion of cases, usually with the greatest ease, by covering the best eye with a red glass and squinting with a vertically deviating prism. Many squinters now admit the presence of double images, but their position by no means corresponds to the identity theory, their lateral displacement is far too slight, or patients find themselves unable to localise the position of the image. It sometimes happens that alternating vision with both eyes is mistaken for diplopia, the images are then invariably specified as homonymous; however, with attention it is easy to distinguish this alternating vision from the simultaneous perception of two images of one and the same object. There can be no doubt that in most cases the position of the double images does not correspond to the principle of identity, and just as little doubt that one to whom double images are easily made apparent cannot possess the confirmed habit of always suppressing the image of the squinting eye. A certain number of cases remain in which it is impossible to produce diplopia; that these, however, do not constantly suppress the image of the squinting eye may be proved in the very simple way I have indicated. An object of fixation is placed in a darkened room, on one side of and behind the squinting eye is placed a small flame, the reflection of which, by means of a plane mirror before the squinting eye is thrown upon its retina. The reflection of the flame is seen on the cornea of the squinting eye, by slight rotation of the glass it can be brought into the area of the pupil, and at the same instant the patient sees the light, the reflection of which can easily be made to coincide with the image of the fixation object seen by the other eye. The experiment has then an entirely objective basis, it always succeeds, a fact on which I lay special stress, even in eyes whose vision is very defective; therefore here also the habit of suppression of the retinal images of the squinting eye is not present.