362 



SCIENTIFIC AMUSEMENTS. 



previously treated with a small quantity of iodide of potassium 

 (about 2 grains for every ounce of solid silver nitrate), so as to 

 form enough silver iodide to saturate the liquid 

 with that salt, which, though nearly insoluble in 

 pure water, is somewhat more soluble in solution 

 of silver nitrate ; if this precaution were not 

 taken, the silver iodide formed by double de- 

 composition would be more or less dissolved out 

 of the film, which would thus be rendered much 

 less sensitive. The plate should be dipped into 

 the nitrate solution with a steady motion, resting 

 in a glass or porcelain holder sold for the purpose, 

 and after a minute lifted out, and again dipped 

 in for another minute; finally, it is removed 

 and placed in position in a dark slide * fitting 

 into the camera (Expt. 370), supported on a 

 tripod stand and previously properly focussed, so that a sharp 



Fig. 199. Mode of withdrawing Dark Slide, 

 image of the landscape and sitter is produced on a ground glass 



* The dark slide is simply a light wooden frame fitting into the camera, 

 resembling a picture frame, excepting that the front part is of wood instead 

 of glass, and can be removed by drawing it outwards (fig. 198), so that the 

 sensitised plate is introduced into the camera inside the frame, with the 

 sliding wooden panel closed to exclude light whilst placing the whole in 

 position ; the slide being then withdrawn (somewhat after the fashion of a 

 dark lantern, fig. 199), the sensitised plate is ready to receive the impression 

 on admitting light through the lens by uncovering the cap of the instrument. 



