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.
In reply to the very sensible inquiry of Simplicitas, there is an essential difference between the calotype of Talbot and the waxed-paper process, the picture in the first being almost entirely superficial, whilst in the latter it is much more in the body of the paper; this causes the modification of the treatment. A tolerably-strong solution of (A9O NO5) nitrate of silver is required to decompose the (KI) iodide of potassium, with which the paper is saturated, in any reasonable time, but if this were allowed to dry on the surface, stains would be the inevitable result; therefore it is floated in distilled water, to remove this from the surface; and it seems to me that the keeping of the paper depends on the greater or less extent to which this surface-coating is removed. There can be no doubt that the paper would be far more sensitive, if used immediately, without the washing, simply blotting it off; but then the great advantage of the process would be lost, viz. its capability of being kept.
William Pumphrey.
Camera for Out-door Operations.—I should be glad to see a clear description of a camera so constructed as to supersede the necessity for a dark room. Such a description has been promised by Dr. Diamond (Vol. vi., p. 277.); and if he could be induced to furnish it at an early period, I at least, amongst the readers of "N. & Q.," should feel much additionally indebted to him.
E. S.