I regret to be obliged to differ from MR. SHADBOLT, but there is a point in his communication which appears to me to arise from a misconception of the stereoscopic problem. He says (p. 557.), "for _distant_ views there is in nature scarcely any _stereoscopic_ effect." Now, surely visual distance is merely visual stereosity; for, to see an object solid is merely to see its parts in relief, some of them appearing to project or recede from the others. It is the difficulty of producing this effect in landscapes, by the ordinary camera process, that renders views taken by such means so deficient in air, or, as the artists term it, aerial perspective, most distant objects seeming almost as near as those in the foreground. This indeed is the main defect of all photographs: they are true representations of nature to one eye--cyclopean pictures, as it were--appearing perfectly stereoscopic with one eye closed, but seeming absolutely flattened when viewed by the two eyes. I remember being shown a huge photograph of the city of Berlin, taken from an eminence; and a more violent caricature of nature I never set eyes upon. It was almost Chinese in its perspective: the house-tops appeared to have been mangled. It was a wonderful work of art, photographically considered; but artistically it was positively hideous. But the same defect exists in _all_ monophotographic representations, though in a less degree, and consequently less apparent than in views to which a sense of distance is essential. In portraits, the features appear slightly flattened; and until photographers are able to overcome this, the chief of all obstacles to perfection, it is idle to talk of the art giving a correct rendering of nature. This is what is wanted, more than colour, diactinic lenses, multiplication of impressions, or anything else. And when it is remembered that the law of an ordinary convex lens is, the farther the object from the lens the nearer the focus, and, _vice versa_, the nearer the object the farther the focus, it becomes evident that by such an instrument distant objects must be made to appear near, and near objects distant, and nature consequently mangled. The stereoscope gives us the only demonstrably correct representation of nature; and when that instrument is rendered more simple, and the peep-show character of the apparatus disconnected from it, the art of photography will transcend the productions of the painter--but not till then. I am anxious to obtain all the information I can from such of your photographic readers as are practically acquainted with the stereoscopic portion of the art relative to the angles under which they find it best to take their pictures for given distances. Mr. Fenton, the secretary of the Photographic Society, takes his stereoscopic pictures, when the objects are 50 feet and upwards from the camera, at 1 in 25. This is, as MR. SHADBOLT states, Professor Wheatstone's rule for distances. {17}