MR. WILKINSON, on the other hand, asserts that 3 feet in 300 yards is sufficient separation for the cameras: this is only 1 in 300,--a vast difference truly. "For views across the Thames," says the editor of the _Photographic Journal_, "the cameras should be placed 12 feet apart, and with this separation the effect is declared to be astonishing." MR. WILKINSON, however, asserts that from 4 to 6 feet in a mile will do _well enough_! Farther, Mr. Latimer Clark (the inventor of an ingenious stereoscopic camera) states that with regard to the distance between the two positions of the cameras, he knows no good reason why the natural distance of the eyes, viz. 2-1/2 inches, should be much exceeded. "A little extra relief is obtained," he adds, "without visible distortion, by increasing the separation to about 4 or 5 inches; but if this distance be greatly exceeded, especially for near objects (I give the gentleman's own words), they become apparently diminished in size, and have the appearance of models and dolls rather than natural objects." The reason for making the separation between the cameras greater than that between the two eyes, is exceedingly simple. The stereograph is to be looked at much _nearer_ than the object itself, and consequently is to be seen under a much larger angle than it is viewed by the two eyes in nature. Hence the two pictures should be taken at the angle under which they are to be observed in the stereoscope. Suppose the object to be 50 feet distant, then of course it is seen by the two eyes under an angle of 2-1/2 inches in 50 feet, or 1 in 240. But it is intended that the stereograph should be seen by the two eyes when but a few inches removed from them, or generally under an angle of 2-1/2 in 12 inches, or nearly 1 in 5. Hence it is self-evident that the stereoscopic angle should be considerably larger than that formed by the optic axes of the two eyes when directed to the object itself. But there is great diversity of opinion as to the extent of the angles requisite for producing the precise stereoscopic or distantial effect of nature. For myself I prefer Professor Wheatstone's rule, 1 in 25 for objects beyond 50 feet distant. For portraits I find the best angle 1 in 10 when the sitter is 10 feet off, and for busts about 1 in 5 when placed about 5 or 6 feet from the cameras. But I should be happy to receive information from any of your readers concerning this important branch of the photographic art. For months past I have been engaged in a series of experiments in connexion with the subject, and wish for larger experience than it is possible for any single operator to acquire for himself. Mr. Fenton, I may observe, does not keep the cameras parallel in taking landscapes, but inclines them so that the same object may occupy as nearly as possible the centre of the ground glass plate.