The exposure required will vary not only with the intensity of the light and the sensibility of the surface used, but also with the degree of reduction or enlargement of the image. In printing upon collodion the resulting picture is positive by transmitted light; it should be backed up with white varnish, and then becomes positive by reflected light. The tone of the blacks is improved by treating the plate first with bichloride of mercury, and then with ammonia. Mr. Wenham, who has written a most practical paper on the mode of obtaining positives of the life size, operates in the following way:--he places the camera, with the slide containing the negative in a dark room, and reflects the sunlight in through a hole in the shutter, so as to pass first through the negative and then through the lens; the image is received upon iodized paper, and developed by gallic acid. _On Printing Collodion Transparencies for the Stereoscope._--This may be done by using the camera to form an image of the negative in the mode described at the last page; but more simply by the following process:--Coat the glass, upon which the print is to be formed, with collodio-iodide of silver in the usual way, then lay it upon a piece of black cloth, collodion side uppermost, and place two strips of paper of about the thickness of cardboard and one-fourth of an inch broad, along the two opposite edges, to prevent the negative being soiled by contact with the film. Both glasses must be perfectly flat, and even then it may happen that the negative is unavoidably wetted; if so, wash it immediately with water, and if it be properly varnished no harm will result. A little ingenuity will suggest a simple framework of wood, on which the negative and sensitive plate are retained, separated only by the thickness of a sheet of paper; and the use of this will be better than holding the combination in the hand. The printing is conducted by the light of gas or of a camphine or moderator lamp, diffused daylight would be too powerful. The employment of a concave reflector, which maybe purchased for a few shillings, ensures parallelism of rays, and is a great improvement. The lamp is placed in the focus of the mirror, which may at once be ascertained by moving it backwards and forwards until an _evenly illuminated circle_ is thrown upon a white screen held in front. This in fact is one of the disadvantages of printing by a naked flame--that the light falls most powerfully upon the central part; and less so upon the edges, of the negative. * * * * * (From Humphrey's Journal, No. 17, Vol. 8.) On the Use of Alcohol for Sensitizing Paper. [Sidenote: TO SENSITIZE PAPER.]