Fig. 173. The silver salt on the paper remains white where it was shaded by the key.
As a matter of fact, the film or plate is exposed such a short time that there is not time for the change to be completed. So the photographer develops the negative; he washes it in some chemicals that finish the process which the light started.
If he exposed the whole plate to the light now, however, all the unchanged parts of the silver salt would also be changed by the light, and there would be no picture left. So before he lets any light shine on it, except red light which has no effect on the silver salt, he dissolves off all the white unchanged part of the silver salt, in another kind of chemical called the fixing bath. This is called "fixing" the negative.
The only trouble with the picture now is that wherever there should be a patch of white, there is a patch of dark silver particles; and wherever there should be a dark place, there is just the clear glass or celluloid, with all the silver salt dissolved off. This kind of picture is called a negative; everything is just the opposite shade from what it should be. A white man dressed in a black suit looks like a negro dressed in a white suit.
How a photographic print is made. The negative not only has the lights and shadows reversed, but it is on celluloid or glass, and except for moving pictures and stereopticons, we usually want the picture on paper. So a print is made of the negative. The next experiment will show you how this is done.
Experiment 101. In a dark room or closet, take a sheet of blueprint paper from the package, afterwards closing the package carefully so that no light can get to the papers inside. Hold the piece of blueprint paper under your waist or coat, to keep it dark when you go into the light. Now lay it, greenish side downward, on a negative. Hold the two together, or place them in a printing frame, and turn them over so that the light will shine through the negative upon the greenish side of the blueprint paper. Be sure that the paper is held firmly against the negative and not moved around. Let the sun shine through the negative upon the paper for 1 or 2 minutes according to the brightness of the sun, or let the gray light of the sky, if it is cloudy, shine on it for 5 or 10 minutes. Now quickly put the blueprint paper (not the negative) into a basin of water, face down. Wash for a couple of minutes. Turn it over and examine it. If it has been exposed to the light too long, it will be dark; if it has been exposed too short a time, it will be too light; in either case, if the print is not clear, repeat with a fresh piece of blueprint paper, altering the time of exposure to the sunlight to improve the print.
Figs. 174 and 175. Where the negative is dark, the print is light.
You can make pretty outline pictures of leaves and pressed flowers, or of lace, by laying these on the blueprint paper in place of the negative and in other respects doing as directed above.
In making blueprints you are changing an iron salt instead of a silver salt, by the action of light. Regular photographic prints are usually made on paper treated with a silver salt rather than with iron salt, and sometimes a gold or platinum salt is used. But these other salts have to be washed off with chemicals since they do not come off in water, as the unchanged part of the iron salt comes off when you fix the blueprint paper in the water bath.
Since the light cannot get through the black part of a negative, the coating on the paper behind that part is not affected and it stays light colored; and since the light can get through the clear parts of the negative, the coating on the paper back of those parts is affected and becomes dark. Therefore, the print is "right side out,"—there is a light place on the print for every white place on the object photographed, and there is a dark place on the print for every black place on the object.