The plate, prepared by one of the methods directed, has been placed in the camera, and the image impressed upon it—attention being given to the points especially alluded to in the chapter " General Remarks on the use of the Camera Obscura".

The image is developed on the daguerreotype plate, as has been already described, by the use of mercurial vapour. In the original process (Fig. 10), one form of a mercurial vapour-box is given; and Fig. 63, d, represents another.

It matters little in what maimer the plate is placed in the mercurial bath; the mercury should be volatilized very slowly, and the image allowed to have its full development before it is removed from the box. Care should be taken that the operator avoids as much as possible contact with the mercurial vapour, since continued exposure to its influence might lead to serious inconvenience.

M. Claudet has adopted a most admirable arrangement in this respect, as is shown in the woodcut on the following page.

Fig. GO represents a small dark chamber fixed outside the apartment in which the operations are carried on, but opening into it by means of sliding glass doors. On either side of the chamber are placed pieces of yellow glass, through which, on opening the shutters by which they are covered, a sufficient quantity of light is admitted to serve any useful purpose, without in any way interfering with the sensitive surfaces of the plates. Within this chamber are placed two mercury boxes, each containing a small quantity of that metal. One of these is shown 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, 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 out into the air through proper ventilators, and the operator is thus protected from the injurious effects of the mercury.