largely employed to produce reversed negatives direct, is by means of a mirror or prism placed either before or behind the lens. The prism is the more convenient, but if large sizes are used it becomes a costly piece of apparatus. The mirror, which should be a plane of glass silvered on its surface, is a less expensive affair. By either of these means the reversed negatives can be made direct without suffering the least in quality.

With celluloid or other flexible films, printing can, of course, be done from either side. Practical men, however, say that, except with the very thinnest films, there is an undoubted loss of sharpness in the grain when these films are reversed and with some mechanical processes.

Against this, however, it may be said that better contact can be obtained in printing than if the film were upon a piece of uneven glass, as is often the case, for by backing it with a piece of plate-glass perfect contact is ensured everywhere.

We come now to the method of stripping the film from the glass. If the negative is made by the collodion process the matter is a simple one. The glass is treated with French chalk previous to collodionizing. After the negative is made and dried it is laid on a leveling stand and a solution of gelatine poured on it. When dry, it is readily stripped by running a knife all round. With ordinary dry-plates the method usually recommended is to immerse them in dilute hydrofluoric acid. The difficulty often experienced here is in the lateral expansion of the film. This will largely depend upon the plate, or rather the quality of the gelatine used. There are, however, two methods of securing the films to some medium unaffected by moisture, and so prevent expansion or distortion. The first is that recommended by Mr. A. Pumphrey and the second by Mr. H. J. Burton, modified descriptions of which are given in a recent number of The British Journal of Photography. If the negative is varnished this is removed. A thin film of gelatine is moistened in a dilute solution of hydrofluoric acid, one part of acid to sixty of water. This gelatine film is secured on paper by a coating of india-rubber.

The action of the dilute acid is to soften the gelatine, making it very adhesive. It can, in this state, be readily attached to the negative by squeegeeing. The acid in the film passes through the negative, and releases it from the glass. It can then be lifted off and pinned to a flat surface to dry. The paper can afterward be stripped off, when dry, by moistening the back with a little benzole to dissolve the india-rubber. In this manner we get the stripped negative in exactly the same size as when on the glass, to which it can be restored at any time desired.

Burton, in his method, employs collodion in place of paper as the support. The negative is first coated with a thick collodion, and this is allowed ten minutes or so to set. It is then immersed in plain water until the film loses all appearance of greasiness. A few drops of hydrofluoric acid are added to the water, and the dish gently rocked. The film will soon detach itself, when the plate should be at once rinsed. Another plate previously coated with gelatine, and dried, is placed in the dish, and the released film, after reversing, is floated upon it, the two removed together, and allowed to dry.

So far we have only treated upon reverse negatives, either obtained at once or reversed afterward. It often happens, however, that we have an ordinary negative, which is required to be reversed. This negative may be a valuable one, and the risk involved in stripping it be too great.

Another simple method of obtaining a reversed negative is by means of the powder process. Although this process is an old one, it appears to be but little known, for what reason we have never been able to define. It is by no means difficult, and by its means a negative can be obtained direct from a negative without the intermediate positive transparency.

The principle of the process is this: An organic tacky substance is sensitized with potassium dichromate, and exposed under a reversed positive to the action of light. All those parts acted upon become hard, the stickiness disappearing according to the strength of the light action, while those parts protected by the darker parts of the positive retain their adhesiveness. If a colored powder be dusted over, it will be understood that it will adhere to the sticky parts only, forming a visible image, the same being a reproduction of the positive printed from. The process is very useful for the production of lantern slides and transparencies, or for the reproduction of negatives. Any of the following formulæ may be employed for the manufacture of the organic substance:—

Solution A.