In most stereoscopic pictures, the distances are too wide. For a portrait, two inches and half to three inches, at nine or twelve feet distant, is enough; and for landscapes much less is required than is generally given, for no very great accuracy is necessary. Three feet, at three hundred yards, is quite enough; and four to six feet, at a mile, will do very well. Let experiment determine: for every photographer must learn his profession or amusement; there is no royal road to be depended on. But a small aperture, a quarter of an inch diameter, may be considered a good practical size for a lens of three and a quarter inches, depending on light and time: the smaller the aperture, the longer the time; and no rules can be given by any one who does not know the size and quality of the lenses employed. Every one can make a few trials for himself, and find it out; which will be more satisfactory than any instructions derived from books or correspondence. I obtain all the information I can from every source, then try, and judge for myself. At worst, you only spoil a few sheets of paper, and gain experience. I perfectly agree with DR. DIAMOND, that it is much better not to wash the collodion pictures after developing; but pour on about one drachm of sat. sol. hypo. at once, and then, when clear, plenty of water; and let water rest on the surface for an hour or more, before setting on edge to dry. HENRY WILKINSON. _Collodion Negative._--Can you inform me how a collodion negative may be made? that is, how you can ensure the negative being always of a _dense enough character to print from_. This is rarely the case. F. M. _Developing Collodion Process._--I use to develope my collodion pictures M. Martin's plan, _i. e._ a solution of common copperas made a little acid with sulphuric acid. This answers very well and gives to the pictures, after they have been exposed an hour or two to the atmosphere, a silver-like appearance: but this copperas solution seems to destroy the _glass_ for using _a second time_, inasmuch as a haziness is cast upon the glass, and its former enamel seems lost, not to be regained even by using acids. The hyposulphite also seems to be affected by this manner of developing the {605} pictures after a short time, which is not the case with pyrogallic acid. The hypo., when thus affected with the copperas, appears also to throw a mist over the picture, which new hypo. does _not_. I should esteem it a favour if any of your numerous readers could inform me the cause of this. A. A. P. _An iodizing Difficulty._--May I request the favour, from some one of your numerous photographic correspondents, of a solution to the following apparent enigma, through the medium of "N. & Q."?