The whole apparatus is now to be placed in a sand-bath, and heat cautiously applied. It is convenient to place a glass thermometer in the sand-bath as near the iron pan as possible. The heat is to be continued about an hour, and to be kept as near 420° Fah. as possible; on no account is it to exceed 450°. The vapour of the acid condenses in the hat, and the crystals are prevented from falling back into the pan by the bibulous paper diaphragm. When it is supposed that the whole of the acid is sublimed, the strings are to be untied, and the hat and diaphragm cautiously taken off together; the crystals will be found in considerable quantity, and should be removed into a stoppered bottle; they should be very brilliant and perfectly white; if there is any yellow tinge, the heat has been too great. I believe that close attention to the above details will ensure success to any one who chooses to try the process, but at the same time I must remind your correspondents that scarcely any operation in chemistry is perfectly successful the first time of trial. J. G. H. Clapham. _Stereoscopic Pictures with One Camera_ (Vol. vi., p. 587.).--In reply to the inquiry of RAMUS, allow me to say the matter is not difficult. My plan is as follows:--Suppose a piece of still-life to be the subject. Set up the camera at such a distance as will give a picture of the size intended, suppose it sixteen feet from the principal and central object; by means of a measuring tape or a piece of string, measure the exact distance from the principal object to the front of the camera. Take and complete the first picture; if it prove successful, remove the camera about two feet either to the right or left of its first station (_i.e._ according to the judgment formed as to which will afford the most artistic view of the subject), taking care by help of the tape or string to preserve the same distance between the principal object and the camera, and that the adjustment of focus is not disturbed. In other words, the camera must be moved to another part of the arc of a circle, of which the principal object is the centre, and the measured distance the radius. If the arc through which the camera is moved to its second station be too large, the stereoscopic picture will be unnaturally and unpleasingly distorted. The second picture is now to be taken.