This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
1. Photography is the art of producing images of objects by the agency of light. Such images may be made on many different materials by direct contact of the object to be imaged with a sensitive surface; or the image may be projected on the sensitive surface, by the aid of a lens. The underlying principle is the same in both cases, though the manipulations are different. The first mentioned method is that characteristically used in photographic printing processes; the second in the making of photographic negatives.
2. The effect of light on certain substances is to induce chemical changes in them which alter their properties. Many hundred such substances have been noted, and others are being added to the list continually. Those which are useful in photography may be roughly grouped under the general heads of first, gums and resins; second, the salts of various metals, chiefly those of the silver-platinum group.
3. A familiar example of the chemical action of light is the fading of colored paper in parts exposed to sunlight, while protected parts are unchanged from the original hue. Instead of fading, or becoming lighter, the reverse effect may take place, the exposed parts becoming darker. These results are due to some chemical employed in making or in tinting the paper. The "Indelible Ink" used in marking white cloth is colorless when fresh, and turns black by exposure to light. This is chiefly due to the fact that the "ink" really consists of a solution of silver nitrate, a salt very readily darkened by light.
4. If a freshly made solution of gum arabic in water is made and enough to form a thin film is poured into a flat-bottomed saucer, it may be allowed to dry in the dark. Place a coin in the center of the dried film, and set the dish where it will receive the overhead rays of the sun for an hour at mid-day. If then the coin be removed, and the film be held under a gentle flow of luke-warm water, the circular patch of gum that was underneath the coin, and thus protected from the light, will readily dissolve and be washed away. The remainder which received the full rays of the sun no longer dissolves readily, but requires warmer water or a longer washing to remove it. In this case the action of light has altered the property of solubility, although this alteration was not outwardly apparent, but "developed" only when the luke-warm water was applied.
5. If lampblack had been mixed with the gum arabic solution, and the washing arrested when the soluble part was gone, the image would be white (the color of the saucer), with a black border formed by the remaining gum with the lampblack mixed. If chalk had been used instead of lampblack and a black saucer instead of a white one, under the same conditions a black image surrounded by white would result. These experimental results are of importance as giving the key to many modern methods of photographic printing and other processes.
STREET PERFORMANCE Study No. 1 - See Page 369 By Grace E. Mounts.
6. If a piece of sensitized Ferro-Prussiate paper ("blue-print" paper) is exposed to daylight for a short time, and then developed by immersion in water, the sensitized side will turn blue all over. If another piece is similarly exposed under a drawing made with black ink on tracing-cloth (which is almost transparent), the inked side next the sensitized paper surface, and immersed in water, only those parts of the surface that were not under the black lines turn blue; the parts that were protected by the black ink are unchanged and wash away, leaving the image of the lines showing white on a blue ground. In this experiment the tracing cloth, with inked lines, was used as a "negative" for producing the print or "positive."
7. "Negatives" made by photography involve exactly similar principles. As a matter of convenience, photographic negatives are usually made from an image of the object which is greatly reduced in size, by the intervention of a lens in a miniature "camera obscura." Some light-sensitive material is spread upon a support, preferably transparent, for greater facility in subsequent "printing" operations, and the light is allowed to act under suitable conditions. The results of this action are "developed" to a certain point, the development arrested and the unacted on material removed or rendered inactive; this latter operation constituting the "fixing" of the negative. From the negative thus produced, positive images or prints may then be reproduced in any required quantity, by the same or other processes.
8. Negative making or printing involves the principles of photography; whatever special applications may be made of these principles.