As the wet process required that the plates be prepared at the time of exposure, that process naturally found its most extensive employment. for indoor work. and continual efforts were made to devise portable plates. Finally plates were introduced which were sensitized, washed, dipped in some organic preservative, such as a solution of tannin. and then dried.
In 1ST] Maddox introduced the earliest. form of the `gelatin-emulsion process.' which has since been considerably improved, and is now used very largely. In this process the sensitive silver salt is held in suspension in either collodion or gelatin. The collodion emulsion may be made by dissolving in one portion of collodion a solution of zinc bromide. in another a solution of silver nitrate, and then thoroughly mixing the two por tions. The gelatin emulsion may be prepared by bringing into solution the gelatin with potassium bromide. while a solution of silver nitrate to which sonic ammonia is added is gradually poured into the liquefied solution of gelatin. These emulsions are floated on plates and dried. Modern plates of this nature are found to be from ten to a hundred times more sensitive than the older wet plates, and have thus made possi ble the photography of moving objects and of those which are only dimly lighted. They also make possible the flash-light pictures by means of which instantaneous views are taken of a dark interior during the burning of a small quantity of magnesium. A large variety of developing agents are employed with the modern dry plate. Pyrogallie acid in connection with an alkali is perhaps the most common. Sodium Itypostilphite almost entirely used as the 'fixing agent.
The camera consists of a box, either of wood or a framework of wood with a bellows-expand ing body of leather, so that when closed the cam era may occupy but little space and be conven iently carried. In the front of the camera is the lens. and at the hack a piece of ground glass on which the image of the object to he photographed may he focused, together with an arrangement for lengthening or shortening the body of the camera. The camera should he placed so as to insure perfect rigidity when fixed. and for in door work shoold be strong and steady. while for outdoor photography the camera stand should be made light so as to be easily portable. or the stand or tripod may he dispensed with altogether.
The lens is of very ancient origin. There is a lens in the British Museum which was found in the ruins of Nineveh. and during the Middle Ages the manufacture and properties of simple lenses were well understood in Europe. In general use 'low, four varieties of lenses are ensomon: (I) A single lens consisting usually of a combination of two converging meniscus lenses separated 1w a diverging meniscus, arranged as a combination in the the of the lens at the end nearest the cam era, the convex side of the combination being to ward the plate: (2) a rapid rectilinear lens eon sIstmg of a converging meniscus lens cemented to a diverging meniscus. the latter occupying the outside position in the tube: (3) a wide-angle rectilinear lens of somewhat similar construction, having two combinat;rms formed of meniscus lenses. but of much deeper curvature than those employed in other combinations, and placed closer together in the lens tube: and (4) the portrait lens. consisting of a back and front combination, arranged at some little distance apart, the front combination containing a double convex and a plano-convex lens cemented together, the double convex lens taking the position next the object, while the back combination consists of a double convex and a diverging meniscus lee. which are
frequently mounted so as to leave a small space between the two.
In photographing an object the camera is set up, and, after the image is properly focused on the ground glass plate, the holder containing the sensitized plates, which are carried in double backed dark frames arranged to carry two plates, each back to back. is inserted in the camera and the cover of the slide is withdrawn, expos ing the sensitized plate so that it receives the picture. The shutter is then closed and the cover of the dark slide placed over the plate, which is later taken to the dark room for development.
For the bringing out or 'developing,' as it is called, of the latent image that has been pro duced on the sensitive plate. it is treated with various solutions: thus. in the daguerreotype the vapor of mercury is the agent used, and in the wet collodion process, as has already been men tioned, the development is effected by ferrous sul phate o• pyrogallic acid. In the case of the dry plates. a developer, prepared by adding one part of a saturated solution of ferrous sulphate to four parts of a saturated solution of potassium oxalate, to which has been added a very small quantity of a solution of potassium bromide, is sometimes employed. A ferrous citro-oxalate is frequently used instead. As the plates are ex ceedingly sensitive. they must be developed in a dull ruby light, and the operation is accomplished by laying the plate in the developing tray. face upward, after which the liquid is run gently from one end of the plate to the other' Excellent re sults have also been obtained by the use of hydro quinone, eikonogen. metol, etc. These substances, when mixed with potassium bromide and sodium sulphite to act as reducing agents. produce very clear negatives. But pyrogallic acid is still most largely used as a developing agent.
For the reproduction of printed copies from the negative. a paper sensitized by dipping in a solu tion of silver nitrate is employed, and is then ex posed to the under the negative. The paper. which should be of some light and mod erately strong quality, is first coated on the sur face with a smooth layer of albumen to which either ammonium or sodium chloride has been added, and then floated in a solution of silver ni trate of strength, according to the condi tion of the negative. and of which 50 grains of the salt for every ounce of water is an average proportion. The exposiu•e to light also varies to the brightness of the light and the character of the negative. The print thus ob tained is toned by immersion in a neutral solu tion of gold containing certain metallic salts, and finally the image is fixed by immersing the print in a solution of sodium hyposulphite which re moves the unaffected silver salt. The final opera tion consists in thoroughly washing the print, after which it is ready for mounting.
The gradual disappearance or fading out of prints made with silver salts led to numerour at tempts to Lind satisfactory substitutes, and va rious metallic salts have been employed for the purpose of producing permanent prints. some of which are described in the following paragraph.: