When they reach that part of the establishment, the first thing that is done with them is to throw them face down upon the surface of a salt bath. Their purple changes at once to a dull red. They are then washed in clean water for a few minutes, and after that laid, face up, in a solution of chloride of gold with a salt of soda. Here they must lie for some minutes at least; for the change, which we can watch by the scanty daylight admitted, goes on slowly. Gradually they turn to a darker shade; the reddish tint becomes lilac, purple, brown, of somewhat different tints in different cases. When the process seems to have gone far enough, the picture is thrown into a bath containing hyposulphite of soda, which dissolves the superfluous, unstable compounds, and rapidly clears up the lighter portions of the picture. On being removed from this, it is thoroughly washed, dried, and mounted, by pasting it with starch or dextrine to a card of the proper size. The reader who has followed the details of the process may like to know what are the common difficulties the beginner meets with. The first is in coating the glass with collodion. It takes some practice to learn to do this neatly and uniformly. The second is in timing the immersion in the nitrate-of-silver bath. This is easily overcome; the glass may be examined by the feeble lamp-light at the end of two or three minutes, and if the surface looks streaky, replunged in the bath for a minute or two more, or until the surface looks smooth. The third is in getting an exact focus in the camera, which wants good eyes, or strong glasses for poor ones. The fourth is in timing the exposure. This is the most delicate of all the processes. Experience alone can teach the time required with different objects in different lights. Here are four card-portraits from a negative taken from one of Barry's crayon-pictures, illustrating an experiment which will prove very useful to the beginner. The negative of No. 1 was exposed only two seconds. The young lady's face is very dusky on a very dusky ground. The lights have hardly come out at all. No. 2 was exposed five seconds. Undertimed, but much cleared up. No. 3 was exposed fifteen seconds, about the proper time. It is the best of the series, but the negative ought to have been intensified. It looks as if Miss E.V. had washed her face since the five-seconds picture was taken. No. 4 was exposed sixty seconds, that is to say, three or four times too long. It has a curious resemblance to No. 1, but is less dusky. The contrasts of light and shade which gave life to No. 3 have disappeared, and the face looks as if a second application of soap would improve it. A few trials of this kind will teach the eye to recognize the appearances of under- and over-exposure, so that, if the first negative proves to have been too long or too short a time in the camera, the proper period of exposure for the next may be pretty easily determined.