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Rh The “calculator” can be used with any ordinary clock or watch, but the “dark-room clock” (fig.73) has been specially constructed for the factorial system. It is an improvement on the earlier forms of Watkins’ “Eikronometer,” and) has a 4 in. dial with 10 minute and 100 seconds divisions, very plain for dark rooms, centre seconds hand, stop action and outside indicator to mark the completed time. The seconds hand completes the revolution in 100 seconds, while the minute hand does so in 10 minutes, or sufficient for the longest ordinary development, though it runs on, if necessary, very much longer, both hands starting together always at O.

In 1908 Watkins brought out another system of “thermo-development” by time dependent on the use of a standard “time developer,” the duration of the development, at a given temperature, being modified according to the make and speed of the particular plate in use. The temperature variations are indicated by a movable scale, or “thermo-calculator,” on the bottle of developer, the variations for development speed of various plates being given approximately on the “Watkins’ Plate Speed List,” which thus shows the “speed of plate” and “speed of development” with the standard developer at 60°. This method is well adapted for plates, films and stand development in tanks or machines, no observation of the plate being required, and the times are most conveniently observed with the “dark-room clock.” Full details of these two distinct methods of development will be found in the 4th edition of the Watkins’ Manual of Exposure and Development.

C. W. Piper’s “photographer’s stop clock” (1906) is a more elaborate clock, intended or use not only in “time development” but for all photographic operations in which accurate control in regard to time is of importance. It is fitted with a gong and arranged to work by “time” or “bulb.” Once started, by pressure on a lever or on the bulb, it will continue to go until stopped, striking the gong at the completion of every minute, when the seconds hand reaches the zero point. A second pressure on the bulb stops the clock, so long as the pressure is continued, while pressure on a lever stops it permanently. It is thus useful for timing any intermittent operations, whilst the clock adds up the separate times and prevents the occurrence of errors difficult to avoid when timing with an ordinary watch. By an additional attachment a prolonged time exposure with the camera may be terminated, or an "instantaneous” or short “time” exposure given at any prearranged time. Messrs Houghton’s “Ensign” clock for time development has a dial with 60 divisions, a single hand, and is fitted with a gong. It can be set to ring an alarm bell at the expiration of any period from one minute to one hour, can be started or stopped immediately and is easily read in the dark-room. It requires no winding up, the action of setting providing the tension for the recording movements. It can be stopped and started at will and the bell arranged to give a short or prolonged ring. S. Stanley’s is another convenient form, with a 4 in. dial, divided into 60 seconds and 60 minutes, the thick hand recording the seconds and the thin hand the minutes.

Several forms of developing tanks and machines have been constructed for developing a number of exposed plates, together with ordinary or dilute developers, with the aid of the factorial system or independently of it. The Kodak “Automatic Developing Tank” (1905) is a useful arrangement by which bands of exposed roll films can be developed in daylight, without any need of a dark-room (fig. 74). The exposed film is wound from the spool

74.—Kodak Developing Tank. into a red celluloid apron contained in a box A, then laced in the tank B, where it is left in a dilute developer for about twenty minutes, and requires no attention. It gives very good results For the “Brownie” films a special daylight developing box is made. With the Kodak “Eastman Plate-developing Tank” (1908) the exposed plates are removed, in the dark-room, from the plate holders and placed, in pairs back to back, in a special framework holding six pairs, which is lowered into a metal tank containing the developer, and is fitted with a watertight lid so that it can be inverted during development A clock face, with pointer, by which the period of development may be noted is fitted outside the tank. Another apparatus of the kind is made for developing celluloid films expose in the “Premo Film Packs” (fig. 75).

Other forms are made, and in some the fixing and washing can also be effected. These tanks undoubtedly save much time and trouble in developing a large number of exposed plates or films, and have been found to work with efficiency and regularity. Eastman Kodak Co. brought out in 1907 a machine for developing paper prints on bromide or gaslight papers.

. 75.—Premo Film Pack Tank (1908).

Photographic Printer Apparatus

For ordinary printing purposes pressure frames, with or without glass fronts, are used or holding the negative and sensitive paper in close contact during exposure to light. They are fitted with hinged backs enabling the progress of the printing to be seen. The pressure is usually given with springs or with screws or wedges acting on the back. They are made in different kinds shown in the dealers’ catalogues. For copying large tracings and engineers drawings by the cyanotype and similar processes large glazed frames are used, mounted on a stand with axle, so that they may be easily turned over for refilling or fixed at a suitable angle to the light. The pressure is given by an elastic cushion or vacuum arrangement, by which air is pumped out from under an india-rubber sheet covering the back of the frame, thus securing a perfectly uniform pressure of about 14 ℔ to the square inch without strain on the front glass. Such frames are also useful for various photo-mechanical printing processes with large negatives or metal plates.

For rapid printing of post-card and other negatives up to 8 × 6 in. a handy and simple apparatus the “Rapide” has been brought out, consisting of a lantern fitted for oil, gas or electric light, with a sloping front, in which a special printing frame is fixed and arranged so that the prints can be rapidly exposed one after another (B. J. A. 1909), p. 691. In another form arrangements are made for exposing a large number of printing frames on a suitable stand, in one or two tiers round a central arc lamp, which may be provided, as in the “Westminster” revolving printing frame, with a shade to protect the eyes of the operator when examining the prints or changing the frames.

For printing tracings, &c., in long rolls, cylinder and rotatory machines of various types are used, so that the tracing and sensitive paper ma be drawn together at a regulated speed in close contact round a glass cylindrical surface within which electric arc or mercury vapour amps supply the source of light. Several machines of this kind are described in Eder’s Jahrbuch for 1908, also in the patent records and photographic journals.

.—Apparatus in general: Sir W. de W. Abney, Instruction in Photography (11th ed., 1905); R. C. Bayley, The Complete Photographer (1906); Dr J. M. Eder, Ausführliches Handbuch der Photographie (2nd ed., pt. i. (2), 1892); Jahrbücher für Photographie und Reproductions Technik (E. Jb.), (1887–1908). Valuable for reference on all forms of apparatus: Dr C. Fabre, Tratté encyclopédique de photographie (T. E. P.) (vol. i., 1889; Supplements A, 1892; B, 1897; C, 1902; D, 1906), also gives much information about photographic apparatus and optics; Chapman Jones, An Introduction to the Science and Practice of Photograph (4th ed., 1904); British Journal Photographic Almanacs to 1909 B. J. A.); Patent Office, Abridgements of Specifications, class 98, “Photography”; Photography Annuals (1891 to 1899); Photographic Journal (''Ph. Journ); Year Books of Photography'' to 1907.

Lenses and Optics: C. Beck and A. Andrews, Photographic Lenses (6th ed.); W. K. Burton, Optics for Photographers (1891); R. S. Cole, A Treatise on Photographic Optics (1899); T. R. Dallmeyer, Telephotography (1899); J. A. Hodges, Photographic Lenses (1895); Captain Houdaille Sur une méthode d’essai scientifique et pratique des objectifs photographiques (1894); G. L. Johnson, Photographic Optics and Colour Photography (1909); O. Lummer, Contributions to Photographic Optics, translated and augmented by Professor S. P. Thompson (1900); Dr A. Miethe, Optique photographique sans dévellopements mathématiques, translation by A. Noaillon and V. Hassreidter (1896); Lieut-Colonel P. Moëssard, L’Optique photographique (1898), L’objectif photographique (1899); C. W. Piper, A First Book of the Lens (1901); Dr M. von Rohr, Theorie und Geschichte des photographischen Objectivs (1899), a most valuable theoretical and historical summary of photographic optics and its literature; Hans Schmidt, Das Fern-Objectiv im Porirät- Architectur- und Landschaftsfache (1898); Dr H. Schroeder, Die Elemente 