White reflecting screens are also of great use in lighting the dark portions of the object to be photo graphed.
Studio accessories may be left entirely to the taste and skill of the photographer. The intelli gent selection and use of these are of the highest importance in the production of artistic photo graphs.
Camera Stands.—The studio-stand should be a solid piece of furniture, and yet easily moved about. It is so made that, by simple mechanism, the camera may be raised, depressed, or angled, at convenience. Portable tripod stands are aupplied for outdoor work. These are so light as to be used as an alpenstock, and, when expanded, ao rigid as to render the camera perfectly immobile.
The foregoing summary of the instruments and appliances used in photography is far from exhaustive. It is simply suggestive of the progress made in this direction, and intended also to render the processes of photography more intelligible to the reader.
PROCESSES.—Collodion Process.—" Negative " and " positive " are terms employed to denote respec tively the photograph taken in the camera and the print obtained from the plate so taken. The negative usually consists of a sheet of glass, which affords support for the chemicals required to produce a photograph. Collodion charged with haloid salts is one of the vehicles used to coat the plate, and is made by dissolving gun-cotton (pyroxyline) in ether and alcohol. Pyroxyline may be
prepared in the following manner :-10 dr. cotton, 52 oz. sulphuric acid, 18 oz. nitric acid, 8 oz. water ; the cotton should be of fine quality, and boiled for hour in 2 oz. caustic soda and 1 gal. water. It must then be freed from the alkali by washing in water, and dried. The acids and water should now be mingled in a porcelain jar, and left to fall to 651° (150° F.), when the cotton may be plunged in, and left for 10 minutes. When taken out, it should be washed thoroughly with water, and dried, care being taken not to heat and ignite the compound, as it is highly explosive.
To make the collodion, take 10 oz. alcohol, 5 oz. sulphuric ether, and 100 gr. cotton. The above solution may be fitted for use by adding 5 oz. alcohol, 60 gr. ammonium iodide, 30 gr. cadmium iodide, 20 gr. cadmium bromide. Shake well, and allow to settle for 12 hours.
The collodion may now be used for coating the glass plate. This operation is best conducted in daylight. The fluid is flowed over the glass, and drained off at one corner (Fig. 1092). In about 2 minutes, it will have set, and may be carried to the dark room, and there plunged into an upright