optic axis of our eyes (Query, How do persons with only one eye judge?), so, in short or medium distances, it were better to let the camera radiate from its centre to the principal object to be delineated. The result of this must be error, as the following illustration will show. Let the sitter (for it is especially recommended in portraits) hold before him, horizontally, and in parallelism with the picture, a ruler two feet long; and let planes parallel to the ruler pass through the sitter's ears, eyes, nose, &c. The consequence would be that the ruler, and all the other planes parallel to it, would have two vanishing points, and all the features be erroneously rendered. This, to any one conversant with perspective, should suffice. But, as all are not acquainted with perspective, perhaps the following illustration may prove more convincing. Suppose an ass to stand facing the observer; a boy astride him, with a big drum placed before him. Now, under the treatment recommended by MR. G. SHADBOLT, both sides of the ass would be visible; both the boy's legs; and the drum would have two heads. This would be untrue, absurd, ridiculous, and quite as wonderful as Mr. Fenton's twelve-feet span view from across the Thames. Once more, and I shall have done with the present arguments of MR. G. SHADBOLT. He says that the two pictures should have exactly the same range of vision. This I deny: for, were it so, there would be no stereoscopic effect. Let the object be a column: it is evident that a tangent to the left side of the column from the right eye, could not extend so far to the left as a tangent to the left side of the column from the left eye, and _vice versa_. And it is only by this difference in the two pictures (or, in other words, the range of vision) that our conceptions of solidity are created. This is not exactly the test to suit the views of MR. SHADBOLT, as I am quite aware; but I chose it for its simplicity, and because it will bear demonstration; and my desire has been to elicit truth, and not to perpetuate error. In conclusion, I beg to refer MR. G. SHADBOLT to my definition and solution of the stereoscopic problem--which I then said I _believed_--but which I now unhesitatingly _assert_ to be correct. T. L. MARRIOTT. _Ammonio-nitrate of Silver._--The inability of your correspondent PHILO-PHO. to form the ammonio-nitrate of silver from a solution of nitrate of silver, which has been used to excite albumenized paper, is in all probability owing to the presence of a small quantity of nitrate of ammonia, which has been imparted to the solution by the paper. Salts of ammonia form, with those of silver, double salts, from which the oxide of silver is not precipitated by the alkalies. I cannot however explain how it was that the solution had lost none of its silver, for the paper could not in such case have been rendered sensitive. J. LEACHMAN. 20. Compton Terrace, Islington. * * * * *