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 a sculptor, as was the case, for instance, with the famous Joseph Kleinhaus, who died in Tyrol, 1853, having been a sculptor from his fifth year.[80] For, no matter from what cause it may have derived its data, perception is invariably an operation of the Understanding. [80] The Frankfort "Konversationsblatt," July 22, 1853, gives the following account of this sculptor:--"The blind sculptor, Joseph Kleinhaus, died at Nauders, in Tyrol, on the 10th inst. Having lost his eyesight through small-pox when he was five years old, he began to amuse himself with carving and modelling, as a pastime. Prugg gave him some instructions, and supplied him with models, and at the age of twelve he carved a Christ in life-size. During a short stay in Nissl's workshop at Fügen, his progress was so rapid, that, thanks to his good capacities and talents, his fame as the blind sculptor soon spread far and wide. His works are numerous and of various kinds. His Christs alone, of which there are about four hundred, bear special witness to his proficiency, particularly if his blindness is taken into consideration. He sculptured many other objects besides, and, but two months ago, he modelled a bust of the Emperor Franz Joseph of Austria which has been sent to Vienna." [Illustration: _Fig. 2._]