Again, your correspondent [Phi]. (Vol. vii., p. 16.) says, that for portraits he finds 1 in 10 a good rule. Let the sitter hold, straight from the front, _i.e._ in the centre, a box 2-1/2 inches in width. The result would be, that in the stereographs the box would have both its sides represented, and the front, instead of being horizontal, consisting of two inclined lines, _i.e._ unless the cameras were {110} placed on _one line_, when it would be horizontal. In such treatment the departure from both is as great as in the first example, and the outrage greater, inasmuch as, under these circumstances (I mean a boy with a box), to any person of common sense, the caricature would be at a glance obvious. This rule, then, although it produces stereosity enough, being false, should also be rejected. I believe that 2-1/2 inches will be found to be right under any circumstance; but should sufficient reasons be offered for a better rule, I trust I am open to conviction, and shall hail with great pleasure a demonstration of its correctness. Should it, however, turn out that I have given a right definition, and a correct solution of this most interesting problem, I shall rejoice to know that I have rendered an essential service to a great number of anxious students in photography. T. L. MERRITT. Maidstone. _Yellow Bottles for Photographic Chemicals._--The proposal of your correspondent CERIDWEN to employ yellow glass bottles for preventing the decomposition of photographic solutions has been anticipated. It was suggested by me, in some lectures on Photography in November 1847, and in January of the present year, that yellow bottles might be so used, as well as for preventing the decomposition, by light, of the vegetable substances used in pharmacy, such as digitalis, ipecacuanha, cinchona, &c. For solutions of silver, however, the most effectual remedy against precipitation is the use of very pure water, procured by slow redistillation in glass vessels at a temperature much below the boiling point. HUGH OWEN. * * * * * Replies to Minor Queries.