In the last number of the _Photographic Journal_ (21st ult.), some mention is made of this letter. They say it proves to be one already long in use, Mr. Kilburn having practised it for four years. I am very desirous of obtaining more information about it. I want to know the length of the box or camera required; and also the focus of the lens, and the best size. Probably Mr. Kilburn or Sir W. Herschel would one of them be so kind as to say. W. M. F. What kind of lens should be used for taking enlarged copies of glass negatives according to Mr. Stewart's plan? and will the same lens also diminish the picture? Will not the usual camera lens act? PLY. [The usual compound lens is all that is required.] Would you have the goodness to explain, in some detail, the two methods by which Mr. Stewart and Mr. Kilburn multiply photographs in a reduced or magnified size; the one by reflected light, the other by transmitted. Mr. Stewart's experiments are upon glass, Mr. Kilburn's on cameras and daguerreotypes. I have never seen any description of this latter process, or of the method of preparing the stereoscope objects: vide _Athenæum_, July 30, 1853. I observe with great pleasure that the cost of apparatus is becoming less, &c. AMATEUR. [However much we may agree in the views expressed in the latter part of AN AMATEUR'S letter, we have been obliged to omit it, as it violates our rule of not opening the columns of "N. & Q." to the recommendation of any particular manufacturer.] _Is it dangerous to use the Ammonio-Nitrate of Silver?_ (Vol. viii., p. 134.).--No: it is now generally used as the best of _marking inks, without preparation_; and we have never yet heard of an explosion from its use. Mr. Delamotte has evidently confounded this preparation with the chloride of silver precipitated with _strong ammonia_, which, when dried, forms the article known as _fulminating silver_; or by adding to the oxide of silver lime-water, and afterwards a strong solution of ammonia, a black powder is thrown down, which, when dried, is known as _Berthollet's fulminating silver_. There is also one other, formed by adding chloric acid to oxide of silver; after drying this, and then adding potassa to a solution of it, the precipitate, by again being dried, becomes an explosive compound. The photographer forms a weak solution for his purpose with one of the least soluble and _weakest_ of the ammoniacal preparations, and which, by drying _around the stopper of the bottle_, is very unlikely to become explosive, from its wanting the addition of another element as necessary to the formation of an explosive compound. For my own part, I must say, that I have found, from experience, all the compound solutions of silver keep much better, and the photogenic effect more satisfactory, by mixing only so much as I may require for immediate use, at this time of the year especially. J. H. * * * * * Replies to Minor Queries.