Having ascertained this, take a chess-board, and place the pieces on the row of squares which run {134} from corner to corner; focus the middle one, whether it be king, queen, or knight, and take a picture; you will soon see whether the one best in the visual focus is the best on the picture, or whether the piece one or more squares in advance or behind it is clearer than the one you had previously in focus. The chess-board must be set square with the camera, so that each piece is farther off by one square. To vary the experiment, you may if you please stick a piece of printed paper on each piece, which a little gum or common bees'-wax will effect for you. In taking portraits, if you are not an adept in obtaining a focus, cut a slip of newspaper about four inches long, and one and a half wide, and turn up one end so as it may be held between the lips, taking care that the rest be presented quite flat to the camera; with the help of a magnifying-glass set a correct focus to this, and afterwards draw in the tube carrying the lenses about one-sixteenth of a turn of the screw of the rackwork. This will give a medium focus to the head: observe, as the length of focus in different lenses varies, the distance the tube is moved must be learned by practice. W. M. F. _Is it dangerous to use the Ammonio-Nitrate of Silver?_--Some time ago I made a few ounces of a solution of ammonio-nitrate of silver for printing positives; this I have kept in a yellow coloured glass bottle with a ground stopper. I have, however, been much alarmed, and refrained from using it or taking out the stopper, lest danger should arise, in consequence of reading in Mr. Delamotte's _Practice of Photography_, p. 95. (vide "Ammonia Solution"): "If any of the ammonio-nitrate dries round the stopper of the bottle in which it is kept, the least friction will cause it to explode violently; it is therefore better to keep none prepared." As in pouring this solution out and back into the bottle, of course the solution will dry around the stopper, and, if this account is correct, may momentarily lead to danger and accident, I will feel obliged by being informed by some of your learned correspondents whether any such danger exists. HUGH HENDERSON. * * * * * Replies to Minor Queries. _Burke's Marriage_ (Vol. vii., p. 382.).--Burke married, in 1756, the daughter of Dr. Nugent of Bath. (See _Nat. Cycl., s.v._ "Burke.") P. J. F. GANTILLON, B.A.