object also. Now, it should be well observed, that in this process it is not the outer side of one retina which corresponds to the outer side of the other, and the inner to the inner of each, but the right side of one retina which corresponds to the right side of the other, and so forth; so that this symmetrical correspondence must not be taken in a physiological, but in a geometrical sense. Numerous and very clear illustrations of this process, and of all the phenomena which are connected with it, are to be found in Robert Smith's "Optics," and partly also in Kästner's German translation (1755). I only give one (fig. 2), which, properly speaking, represents a special case, mentioned further on, but which may also serve to illustrate the whole, if we leave the point R out of question. According to this illustration, we invariably direct both eyes equally towards the object, in order that the symmetrically corresponding places on both retinas may catch the rays projected from the same points. Now, when we move our eyes upwards and downwards, to the sides, and in all directions, the point in the object which first impinged upon the central point of each retina, strikes a different place every time, but in all cases one which, in each eye, corresponds to the place bearing the same name in the other eye. In examining (perlustrare) an object, we let our eyes glide backwards and forwards over it, in order to bring each point of it successively into contact with the centre of the retina, which sees most distinctly: we feel it all over with our eyes. It is therefore obvious that seeing singly with two eyes is in fact the same process as feeling a body with ten fingers, each of which receives a different impression, each moreover in a different direction: the totality of these impressions being nevertheless recognised by the Understanding as proceeding from one object, whose shape and size it accordingly apprehends and constructs in Space. This is why it is possible for a blind man to become
