13 



tion of the object must be obtained upon the ground glass, which 

 must then be withdrawn, and the frame containing the prepared 

 plate introduced in its place. The shutter may then be drawn up, 

 and the plate exposed to the light which passes through the lens. 

 The time of exposure must be decided by observation and experi- 

 ment ; as so much depends on the size and construction of the lens 

 or lenses, and the brightness or dulness of the season. With a good 

 achromatic lens, from five seconds to a minute and a half, will be 

 sufficient in almost every case. In another part will be found 

 some Directions for taking Portraits, Views, etc. which will assist 

 the beginner. The instant the assigned time has elapsed, the 

 shutter must be closed, and the_frame may then be withdrawn in 

 readiness for the next operation. 



EXPOSING THE PLATE TO THE VAPOUR OF MERCURY. 



Into the cup at the bottom of the Mercury-box, put four or 

 five ounces of mercury, which must be pure, dry, and free from 

 moisture. It may be occasionally filtered by enclosing it in Chamois 

 leather, and gradually and carefully twisting the leather till the 

 mercury is forced through its pores clean and bright. The vapour 

 of the mercury is raised by the application of a spirit-lamp to the 

 cup which holds the mercury. When a thermometer is attached 

 to the mercury-box, a temperature of about 90 degrees will 

 raise the vapour of the mercury: if the box have no thermometer, 

 the cup may be heated until the mercury is pleasantly warm to the 

 finger. If the mercury cup is removed from the box in order to 

 its being heated, it is well after that operation to wipe the outside, 

 on which a slight steam from the spirit may have settled. The 

 plate, without having been removed from the slide is then placed 

 over the mercury, where it must remain till the picture is perfectly 

 developed. Its progress may be obseved by the light of a candle 

 through the yellow glass in the front of the box. It generally takes 

 eight to fifteen minutes, or even longer, to perfect the operation ; 

 if, however, no outline is visible in about three minutes, either the 

 mercury has not been sufficiently heated, or the picture has been 

 removed too soon from the influence of light in the camera. If the 

 former be the case, the mercury may be again gently heated ; but 

 if made too hot, the plate will become covered with small white 

 spots. The details are usually much better developed when the 

 picture has been brought out slowly, and with a moderate degree of 

 heat. 



