In connection with his factorial system of development (q.v.), A. Watkins produced in 1894 a factorial calculator and a dark room clock. With the later extension of "time and temperature" and tank developing methods, a number of dark room clocks and tanks of various patterns have been introduced, some of the former being provided with alarms which can be set to ring on the expiration of a given number of minutes, while others merely mark the progress of time on a dial with large hands.
For printing, the simplest method is to place the negative in a "printing frame" used to hold the sensitive paper in close contact with it during exposure to light. Pressure is usually by means of springs acting on a hinged back. Even for amateur use, however, the replacement of other printing materials by developing papers has involved the general employment of simple printing boxes containing an electric light for which the current is controlled by a switch which is actuated when a platen is brought down on the negative and sensitive paper placed in contact with it.
Professional photographers develop their plates very largely in tanks, and the flat films, which to a considerable extent have replaced plates in professional work, are developed in holders which are placed in tanks and transferred from the developer to the fixing bath and then to the washing tank and finally to a dry ing cabinet. In this way a large number of negatives can be handled together and the day's work can be developed and made ready for printing very expeditiously.
Almost all of the printers used in professional work are of the printing box type, exposure being made to electric light operated when the platen is closed over the negative and printing paper. Professional printers are provided with attachments for masking the negatives and for the insertion of special masks of ground glass or celluloid on which local work can be done, so that parts of a negative can be printed to a lesser extent than other portions.
Prints are usually developed in large dishes and of ter fixing and washing may be dried either on racks covered with muslin or, if the output is very large, on one of the various belt driers.
Commercial photographs are often made with a glossy surface produced by squeegeeing the print on to a lacquered metal plate known as a "ferrotype plate" and are usually mounted in contact with backing cloth or paper in order to stiffen the print.
For mounting prints, a method of dry mounting by means of tissues which have been saturated with shellac is frequently adopted. Tissue and print are laid on the mount in the dry mount ing press, in which heat is applied, which causes the shellac to melt, with the result that the print is cemented to the mount.
The business of "photo-finishing," as it is termed in the United States or "developing and printing" (D. & P.), as it is generally
known in England, is now a very considerable portion of the photographic industry, many firms developing several million spools of amateurs' cartridge film every year. In the case of the large photo-finishers, the finishing plant collects film through the agency of a number of photographic dealers and returns the fin ished pictures to the dealers for distribution to the retail customer. Two important factors in the business are the short time in which it is necessary to get the work done, often 24 hours or less, and the great variation in the amount of business throughout the year, the output in England, for instance, being ten times as much at the maximum in August as at the minimum in January. This variation in the amount of business is causing great attention to be paid to the provision of automatic machines in order to diminish to a minimum the amount of skilled labor required. When the rolls of film come in, they are unwrapped in the dark room and numbered, the number being either punched into the film or light-printed on it. They are then held in clips which can be slipped over rods, so that the films with a weighted clip at the end are immersed in deep tanks of developer. After a fixed time of development, they are transferred to a rinse tank, fixed, washed, and again transferred to a drying cabinet.
Several forms of continuous machines have now been devised for developing the rolls of film. In these, the rods carrying the films held in clips are moved continuously by a conveyor system through a system of tanks and finally through a drying cupboard. The dried negatives go forward to a printing room, where they are printed by contact upon developing paper, the printers being . adapted for rapid operation, frequently by foot treadles, and equipped with numbering devices. Automatic power driven print ers have been designed and some printers automatically give the correct printing exposure in accordance with the density of the negative.
From the printers, the paper goes to the developers. Most prints are developed in flat trays, rinsed in water and fixed, and are then washed in a rotating cage immersed in flowing water or in a series of flat trays through which water flows, this being the form used particularly in England, where it is known as a "cascade washer." The washed prints are dried by being placed between two fabric belts which travel over a heated drum, these machines being known as "belt driers," which deliver the prints dry within a few minutes.
Glossy prints are dried upon ferrotype plates in cupboards or, with the most recent machines, upon polished metal drums to which they are carried while wet by traveling belts.
The finished prints may be trimmed at the edges in order to give them a smooth finish and are usually delivered unmounted.