The first person to notice that chloride of silver was darkened by light may have been J. H. Schulze, who made the discovery in 1732. It is probable, however, that this had been observed by others. In 1737 Hellot in Paris, was trying to make sympathetic inks, that is, inks that would be invisible when put on paper but which could be made visible afterwards. He found that if he wrote on paper with a solution of silver nitrate, the writing would not be visible until the paper was exposed to light, at which time it would turn dark and could be read. However, no use was made of these discoveries for the purpose of making pictures until 1802, when Wedgwood published a paper entitled "An Account of a Method of Copying Paintings on Glass and on Making Profiles by the Agency of Light upon Nitrate of Silver."
This reference to making profiles is a reference to one of the forms of portraiture which preceded photography. Before portrait photography was discovered, there were people who made what were called "silhouettes", which were profile pictures cut out of black paper and stuck on to white paper. Some of these silhouettists were very clever indeed. Others who had not great ability arranged their sitter so that they got sharp shadows thrown by a lamp onto a white screen and this gave them the profile to copy. Wedgwood thought that instead of cutting out the silhouette he might print this profile on the screen by using paper treated with silver nitrate, which would darken in the light. Wedgwood not only used his new process to record these silhouettes, but he tried to take photographs in what was then called the "camera obscura", which was the forerunner of the Kodak of to-day.
The camera obscura consisted of a box with a lens at one end and a ground glass at the other, just like a modern camera. It was used by artists to make a picture of anything they wanted to draw, as by observing the picture on the ground glass they could draw it more easily. Wedgwood tried to make pictures in his camera obscura by putting his prepared paper in the place of the ground glass. His paper however, was too insensitive to obtain any result; but Sir Humphrey Davy, who continued Wedgwood's experiments, using chloride of silver instead of nitrate, succeeded in making photographs through a microscope by using sunlight. These are apparently the first pictures made by means of a lens on a photographic material.
Fig. 1. Silhouette Picture from Old Print.
But all these attempts of Wedgwood and Davy failed because no method could be found for making the pictures permanent. The paper treated with silver chloride or silver nitrate was still sensitive to light after part of it had darkened, and if it were kept it soon went dark all over and the picture was lost. Davy concluded his account of the experiments by saying: "Nothing but a method of preventing the unshaded parts from being coloured by exposure to the day is wanting to render this process as useful as it is elegant."
This much needed method, however, remained wanting from 1802 until 1839, when Sir John Herschel found that "hypo", which he had himself discovered in 1819, could dissolve away the unaltered chloride of silver and enable him to "fix" the picture, as the process has been called ever since Herschel made the discovery, and from that time to this hypo has been the mainstay of the photographer, enabling him to fix his pictures after he has obtained them.
Fig. 2. Crystals of Thiosulphate of Soda or "Hypo".
In the meantime, Niepce in France had been working on an entirely different process, depending on the fact that such substances as resin or asphalt became insoluble when exposed to light, and he had succeeded in producing results by taking advantage of this property. In France also, Daguerre was working on various methods by which he hoped to make photographs, and entered into partnership with Niepce, but in 1839 Daguerre published the method of photography which was named for him - Daguerreotype. This was the first portrait process and became very popular. It depends upon the sensitiveness of plates of metallic silver which have been fumed with iodine so that the surface is converted into a thin layer of silver iodide. The plates so treated are exposed to light, and after a very long exposure, as we should consider it now, the plate in the dark is exposed to the vapor of metallic mercury, which deposits itself upon the image and produces a positive image of mercury upon silver.
The results were very beautiful, but these early processes of photography required very great exposures so that at first the unfortunate subject had to sit for as long as ten minutes in the full sun without moving in order to impress the plate sufficiently. Although many experiments were made in an attempt to find substances more sensitive to light so that the exposure could be reduced, the only real solution was to find some method by which light had to do only a little of the work and the production of the image itself could be effected by chemical action instead of by the action of the light.
A great step in this direction was taken by Fox Talbot in 1841. He found that if he prepared a sheet of paper with silver iodide and exposed it in the camera he got only a very faint image, but if after exposure he washed over the paper with a solution containing silver nitrate and gallic acid, a solution from which metallic silver is very easily deposited, then this solution deposited the silver where the light had acted and built up the faint image into a strong picture. This building up of a faint image or, indeed, of an image which is altogether invisible, into a picture is what is now called "'development". If we expose a film in the Kodak and then, after the shutter has allowed the light to act for a fraction of a second on the film, look at the film in red light, which will not affect it, we shall not be able to see any change in the film. But if we put the film into a developing solution, the invisible image which was produced by light, and which in photographic books is called "the latent image" will be developed into a black negative representing the scene that was photographed.