This section is from the book "Modern Photography In Theory And Practice", by By Henry G. Abbott. Also available from Amazon: Modern photography in theory and practice: A hand book for the amateur.
Whether you develop your plates yourself, or have the work done for you by the professional, it is necessary to know something about plates and the chemical action of light and the more you know in this respect the greater your chances of having good negatives. You are doubtless aware of the fact that the action of light is quite different on various materials. With one material it has a tendency to bleach, while with another it has the reverse effect and darkens or discolors. The housewife lays her linen on the grass in the sunlight in order to bleach it and yet the same sunlight changes the color of nearly all red paint to brown or black. The whole principle of photography is based upon the action of light upon certain well known sensitive mediums. These mediums are known as salts, the most sensitive of which is bromide of silver and hence it is used in making photographic plates and certain forms of printing paper. Ordinary printing papers, however, are made not from bromide but from chloride of silver. Chlorine is a gas and it has a great affinity for metals and salts of various kinds and when united with silver forms chloride of silver or when united with sodium it forms chloride of sodium or common table salt. If you have a knowledge of chemistry you are aware that there are many other sensitive salts and chemicals but the principal ones which the photographer has to deal with are chloride and bromide of silver.
If you expose either bromide or chloride of silver to the action of white light, it will turn black in a very short time. A plate is made by coating the piece of glass with a thin film of gelatin, in which the bromide of silver is carried. Bromide of silver is preferred for plates because of its extremely rapid action when exposed to light. The paper on which the picture is printed is like the plate, sensitive but not to such a degree, for it is coated with collodion, gelatin or albumen, in which chloride of silver is carried. Bromide paper differs from the ordinary printing papers in rapidity of action, because it is coated the same as a plate with bromide of silver.
Now, we know that our plate is covered with a solution carrying in it bromide of silver and that when exposed for even the fraction of a second, by the opening of the shutter in the camera, it is sufficient to produce an effect upon the sensitive film on the plate and an image is formed. If we take the plate from the plate holder and examine it, we will see no change in it whatever, as it requires a development in order to produce the image formed thereon. Now, the image on the plate is formed more or less distinctly, according to the length of the exposure and the quality of the light. If the day be a clear one and the light strong, it is said to be actinic, that is, the light possesses those rays of the spectrum which are most powerful in producing chemical changes. To the novice light is light and all sunlight is white but to the scientist light is made up of several colors, red, yellow, orange, green, bluee, indigo and violet. Now these various colors act very differently on the salts of silver and so it is that the light does not always act the same, either on the plate or printing pa-per and is one day rapid and another day slow. Blue, indigo and violet rays work the most rapidly, green rays slower and the red, yellow and orange rays are the slowest to affect the salts.
In relation to light there are two other points to be considered, i. e., the time of day and the season of the year. Other things being equal, the light is the strongest in the middle of the day and hence at that time a shorter exposure is required than at any other but much depends on the season of the year and very much depends on the country we are living in. The quality of the light in April, May, June, July, August and September is much better than that of the other six months of the year and that of a dry climate much quicker than that of a moist one. As a rule, the light in the country is much more actinic than that of large cities, probably owing to the fact that there is less smoke in the atmosphere. For the same reason the light is much more actinic after a heavy rainstorm than before it, as the atmosphere has been cleared of floating particles of dust and soot by the rain. It must be borne in mind that surroundings have a very material effect upon light and cause it to be more or less actinic. A view taken over a body of water, as a lake or the ocean, will not require more than half the length of exposure that would be required for a view taken at the same time of an ordinary open landscape, i. e., one without trees in the foreground. This is caused by the reflected light from the water, so that we see that reflection, as well as light, is a factor in photography. The same is true of snow and the amount of snow on the ground, trees and surroundings, materially affect the length of the exposure.
Now, we have learned two things, first that red, yellow and orange rays act less quickly than blue, indigo, violet and green ones and that reflection is a prime factor in lighting. Therefore, a little reasoning will tell us that a landscape, in which the prevailing colors are green, from grass and trees and blue and indigo from sky, will necessarily require a shorter exposure than would be necessary if we were taking a photograph of a number of red, yellow and orange-colored flowers. If we look directly at a red brick house, a great proportion of the rays coming towards us are red and if we look at a green tree a large proportion of the rays are green. In just the same way the colors of the various objects photographed are reflected back through the lens and on to our plate and our exposure should be varied according as the subject being photographed predominates in color. Now, if we undertake to photograph a bouquet in which there are red, yellow, orange, blue, indigo and violet-colored flowers and green leaves, what will be the result and how shall we time our exposure? Here we have all the spectrum colors and many shades of these colors. We have the colors to which the silver salt is most sensitive and we also have those which have the least effect on this salt. If we give the proper exposure for the red and orange colors, then we will have exposed our plate too long for the blues and indigoes and thus we will have what is known technically as an "over exposure." On the other hand, if we expose just long enough for the blues and indigoes, then the red and orange will be under exposed. There are several remedies but they are all based on one general principle, that of correcting the color value by reducing them all to a common, or uniform value. Special plates are made by which the various colors are reproduced in monochrome in their correct values. These plates are known under the names of the various manufacturers, as "Isochromatic," "Or-thochromatic," "Erythro," etc. The "ray filter," or "color screen," is another method of correcting color values. The ray filter is usually a cell composed of two pieces of glass set into a piece of glass tubing and in this cell is confined a liquid carrying picrate of ammonium, bichromate of potash, or some other chemical which tends to correct the color values. The ordinary color screen consists of an orange-yellow glass, confined in a suitable frame and through which the light passes before reaching the plate. As a rule, the Isochromatic and Orthochromatic plates are a trifle slower in their action than ordinary plates and where a color screen or ray filter is used with ordinary plates the time of exposure must be lengthened materially. The use of these plates and screens is described in Chapter XIV.