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.

The printing from the negative is less difficult, because we can examine the picture as often as we choose; but it may be well to undertime and overtime some pictures, for the sake of a lesson like that taught by the series of pictures from the four negatives.

The only other point likely to prove difficult is the toning in the gold bath.  As the picture can be watched, however, a very little practice will enable us to recognize the shade which indicates that this part of the process is finished.

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We have copied a picture, but we can take a portrait from Nature just as easily, except for a little more trouble in adjusting the position and managing the light.  So easy is it to reproduce the faces that we love to look upon; so simple is that marvellous work by which we preserve the first smile of infancy and the last look of age:  the most precious gift Art ever bestowed upon love and friendship!