As I have experienced the excellence of these preparations, I hope they may be useful to your photographic students. WELD TAYLOR. Bayswater. _After-dilution of Solutions._--There are in general use two methods of preparing sensitive paper. In one, as in Mr. Talbot's, the iodide of silver is formed in a state of purity, before being rendered sensitive: and as, for this end, a small quantity only of nitrate of silver is necessary, a very dilute solution will answer the purpose as well, or even better, than a strong one; but by the other method, the paper being prepared with iodide of potassium only, or with some other analogous salt, the iodide of silver has to be formed by the same solution that renders it sensitive. Now as for every 166.3 parts of iodide of potassium 170.1 parts of nitrate of silver are required for this purpose, it is evident that a dilute solution could not be employed unless a very large bulk were taken, and the paper kept in a considerable time. The after-washing is to remove from the surface of the paper the great excess of silver, which is of but little service, and prevents the paper from keeping. WILLIAM CROOKES. Hammersmith. _Stereoscopic Pictures from one Camera._--Your correspondent RAMUS will easily obtain stereoscopic pictures by either of the following plans:--After the first picture is taken, move the subject, as on a pivot, either to the right or left, through an angle of about 15°; then take the second impression: this will do very well for an inanimate object, as a statue; but, if a portrait is required, the camera, after taking the first picture, must be moved either to the right or left, a distance of not more than one-fifth of the distance it stands from the sitter; that is, if the camera is twenty feet from the face of the sitter, the distance between its first and second position should not exceed four feet, otherwise the picture will appear distorted, and the stereosity unnaturally great. Of course it is absolutely necessary in this plan that the sitter do not move his position between the taking of the two impressions, and also that the distance between him and the camera be the same in both operations. {49}