The apparatus consists of a metallic case at (Fig. 51), only slightly larger than the plate for which it is intended, which can be closed light-tight by means of the lid b. It is furnished with two tubes, one g, entering at the bottom of the bath, which can be connected by a piece of rubber tubing e, to the funnel d; the other near the top, just above the level of the plate p. The former serves to introduce the developer into the bath, and the latter / allows the air to escape, and also acts as an overflow.
The funnel d, and rubber tube e, are snpported by means of hooks A, which fit jnto an eye, The tubes are so bent that no light can enter the bath. An end view of the bath is shown in Fig. 52, in which a movable hook i, fitting into the socket j, serves to fix the bath to any convenient support. The film side of the plate p, is kept from contact with the side of the bath by means of metallic strips. When the bath is intended for the development of films or paper prints, it is furnished with a carrier (Fig. 53), into which the film or paper is inserted before being put into the bath, and when required for the development of several plates at one time it is constructed with grooves, as in an ordinary plate box.
The bath is used as follows: - The (lark side containing the exposed plate and the bath are placed in a changing bag, into which it is only necessary to insert the hands. The plate is then transferred from the slide to the bath, care being taken to keep the film side toward the back of the bath. After being closed, the bath is withdrawn from the bag into daylight, and fixed vertically to any convenient support by means of the hook. A vessel is placed under the overflow tube, and the inlet tube is attached to the funnel, which, after being filled with the developing solution, is raised and lowered several times to alternately fill and empty the bath, and so ensure the removal of air-bells from the surface of the plate. After standing sufficient time to complete the development, the solution is run off from the bath. The plate is washed by running water through the bath (either by means of the funnel or by attaching the rubber tube direct to a water-tap), after which it is taken out in day-light and "fixed" in a covered tray. If it be desired to "fix" entirely in the dark, the operation may be conducted in the same bath, or in a similar one to which the plate has been transferred in the changing bag.
The final washing of the plate may be done either in the bath or in the usual way.
The best developer for use with the dark bath is ferrous oxalate, on account of its property of not producing "fog" even where a plate has been left in it for an hour. It should be made just before use from the following solutions, prepared according to Thomas's formula: - No. 1. 300 grin, potassium oxalate, 2.2 grm. potassium bromide, and water to make 1000 c.c. No. 2. 400 grm. ferrous sulphate, 3 c.c. sulphuric acid, and water to make 1000 c.c.
Take 4 parts of No. 1, and 1 part of No. 2, adding 3 parts water.
When the developer is used at once with Thomas!s "extra rapid" plates, 12 to 15 minutes in the bath at about 60° F. will suffice to give a satisfactory negative, if the exposure has been correct. The time required to obtain a suitable density is found by placing the exposed plate in the bath, and adding the amount of developer required to fill the bath, in four equal portions, at intervals of 5 minutes. Four densities are thus obtained by developing for 5, 10, 15, and 20 minutes respectively, from which observations the time that gives the desired density is selected. (A. Watt.)