A Manual Of Photography

250 PRACTICE 0E PHOTOGRAPHY. Fig. 63 represents a small dark chamber fixed outside the apartment in which the operations are carried on, hut opening into it by means of sliding glass doors. On either side of the chamber are placed pieces of yellow glass, through which, 63. on opening the shutters by which they are covered, a sufficient quantity of light is admitted to serve any useful purpose, with- out in any way interfering with the sensitive surfaces ot the plates. Within this chamber are placed two mercury boxes, each containing a small quantity of mercury. One of these is shewn in section. Each box is placed over a water bath, supplied by means of a pipe with water from a cistern above, and a small sand bath is placed between the mercury box and the water bath. By means of the gas-burner beneath the box, the water is heated, and the mercury volatilized slowly and deposited on the plates, which are fixed on the grooves shown on the sides of the box in section. The windows being closed, any mercurial vapours which may escape from the box may pass ou