CONCLUDING REMARKS.
No attempt has been made in this chap ter to deal with every variety of camera and accessory ; these are so numerous and so diverse in action that the subject would require a volume to itself if treated exhaustively. The main principles of camera construction, and the leading classes into which the various cameras and appliances may be divided, have been suf ficiently explained. Other special de signs of cameras and accessories will be considered in their proper place. In many cases full working details have been given for making standard patterns, and these, no doubt, will prove of great practical use and interest.
How Day PLATES AND FILMS DIFFER.
The photographic plate, as manufac tured nowadays, consists of a sheet of glass coated with either gelatine or collo dion containing sensitive salts of silver. The processes of manufacture may be divided into two sections, the emulsion processes. and the bath processes. The bath processes are generally known as " Wet Plate," and will be described later. They are now scarcely ever used except for photo-mechanical work, for which they are preferred owing to the great contrast obtainable, and by itinerant photo graphers. Nearly all plates made nowa days are coated with emulsion, and are commonly known as dry plates. Films are
sheets of celluloid, mica, or even gelatine, coated with an exactly similar emulsion to that used in the glass dry plate. The difference, therefore, between plates and films lies in the support only ; the actual image-forming substance being the same. For either, therefore, the first requirement l3 to make the emulsion.
How THE EMULSION IS MADE, Emulsions may be prepared with either of the silver halogens—bromide, iodide, or chloride. The iodide is seldom used except as an addition to the bromide, and the chloride is merely employed for the making of slow emulsions, such as may be used for lantern slides or printing pur poses ; for example, the ordinary gelatino chloride paper or P.O.P., but as this is referred to in another chapter noth ing further need be said of it. The gelatino-bromide emulsion, then, may be taken, either with or without iodide, as a type the proceeding to be followed. For the better understanding of the sub-1 ject, and to avoid confusion of the opera tion, it should at once be stated that emulsion making may be divided into the following operations, which are usually conducted in the order given, although that order is not arbitrary.