This section is from the book "The Fundamentals of Photography", by C. E. K. Mees,. Also available from Amazon: The Fundamentals Of Photography.
Fox Talbot was not only the first to develop a faint or invisible image; he was also the first man to make a negative and use it for printing. What is meant by a negative is this: If we look at our film after we have exposed and developed it, we shall find that the sky, which was bright in the picture, is shown in our film as very black,while any shadows in the picture, which, of course, were dark, will be transparent in the film, so that the light let through the film is in the reverse order of the scene photographed, all the bright parts in the scene being dark in the film and the dark parts bright. For this reason the film is called a "negative," and when it is printed on paper the same reversal happens again and the clear parts in the negative become dark in the print while the dark parts of the negative protect the paper from the action of the light, so that the print which we may call a "positive," represents the scene as it appeared.
Fig. 3. Negative Image.
Fox Talbot, then, made two of the great steps in the advancement of photography when he found how to expose his paper for a time insufficient to darken it completely, and then to develop a negative which he could print on paper covered by silver chloride. Of course, the paper was not transparent as our film is, but he made it more transparent by treating it with oil or wax. In this he was followed many years afterwards in the Eastman roll holder, which was the forerunner of all the Kodaks. In this roll holder at first a paper film was used to make the negative and then the paper was made transparent for printing.
Fox Talbot's paper negatives were succeeded by the method known as the wet collodion process, which has survived to the present day. This is the process chiefly used by photo-engravers for making the negatives from which they make the engraved metal plates for printing pictures.
Collodion is made by dissolving nitrated cotton, such as is now used for the film base, in a mixture of ether and alcohol. The worker of the wet collodion process had to make his own plates at the time when he wanted to take a picture. He would clean a piece of glass and coat it with the collodion in which the chemicals were dissolved and then put the plate in a bath of nitrate of silver, which formed silver iodide in the collodion film and made it sensitive to light. Then the glass had to be exposed in the camera while wet, and immediately after exposure it was developed by pouring the developer over it. It was then fixed and dried.
Fig. 4. Positive or Print.
In order to carry out these operations a photographer who wanted to take landscapes had to carry with him a folding tent which he could set up in the open air. The tent was dark except for a yellow or red window by which to see to make the plates and develop them.
All this difficulty in working disappeared with the coming of the gelatine emulsion process, which is the one now used. The sensitive coating on films and papers now consists of a bromide or chloride of silver held in a thin sheet of gelatine, the gelatine being dissolved in hot water, the silver salt formed in the solution, and the warm solution of gelatine containing silver then coated on the film or paper.
The gelatine solution with the silver in it is called an "emulsion" because of the way in which the silver remains suspended in the gelatine. The first gelatine emulsions were made in 1871 by Dr. Maddox. An emulsion made in much the way that we use now was first sold in 1873 by Burgess. At first the early experimenters made and sold the emulsion itself, drying it for sale so that photographers had to take this dried emulsion, melt it up in hot water, and coat it on their plates. After a time, however, people realized that this was a great deal of trouble and that there was no reason why the manufacturer of the emulsion should not coat the glass plates with it, and sell the ready prepared plates. In those days all negatives were made on glass plates. These plates were coated with the emulsion by hand and then when the emulsion was spread over them were put on to cold level slabs for the jelly to set before drying. Glass plates are cumbersome and heavy, and for this reason George Eastman continually experimented to substitute a light, flexible support for the brittle and heavy glass. As already mentioned, he first used paper as a support for the negative, waxing it to make it transparent for printing. This was followed by a paper from which the film carrying the image was stripped, the film being transferred to a glass plate coated with gelatine so that this gelatine made a support for the film.
Fig. 5. Early Photographer with His Equipment.
While experimenting to find a more satisfactory material for coating the film than gelatine it was found that a solution of nitrated cotton would make a clear, transparent and flexible support, and after a period of further experimenting this material was adopted and a roll film was made, the emulsion being carried on the clear, transparent sheet of film support. The only remaining difficulty with this was its tendency to curl owing to the gelatine coating on one side, and this was overcome by coating the other side with plain gelatine, thus producing the non-curling (NC) film.