This section is from the book "A Manual Of Photography", by Robert Hunt. Also available from Amazon: A Manual of Photography.
On the 31st of January, 1839, six months prior to the publication of M. Daguerre's process, Mr. Fox Talbot communicated to the Royal Society his photographic discoveries, and in February he gave to the world an account of the process he had devised for preparing a sensitive paper for photographic drawings. In the memoir read before the Royal Society, he states—" In the spring of 1834,I began to put in practice a method which I had devised some time previously, for employing to purposes of utility, the very curious property which has been long known to chemists to be possessed by the nitrate of silver, namely, its discoloration when exposed to the violet rays of light." From this it appears that the English philosopher had pursued his researches ignorant of what had been done by others on the Continent. It is not necessary to enlarge, in this place, on the merits of the two discoveries of Talbot and Daguerre; but it may be as well to show the kind of sensitiveness to which Mr. Talbot had arrived at this early period, in his preparations; which will be best done by a brief extract from his own communication.
"It is so natural," says this experimentalist, "to associate the idea of labour with great complexity and elaborate detail of execution, that one is more struck at seeing the thousand florets of an Agrostis depicted with all its capillary branchlets, (and so accurately, that none of all this multitude shall want its little bivalve calyx, requiring to be examined through a lens), than one is by the picture of the large and simple leaf of an oak or a chestnut. But in truth the difficulty is in both cases the same. The one of these takes no more time to execute than the other; for the object which would take the most skilful artist days or weeks of labour to trace or to copy, is effected by the boundless powers of natural chemistry in the space of a few seconds." And again, "to give some more definite idea of the rapidity of the process, I will state, that after various trials, the nearest valuation which I could make of the time necessary for obtaining the picture of an object, so as to have pretty distinct outlines, when I employed the full sunshine, was half a second" This is to be understood of the paper then used by Mr. Talbot for taking objects by means of the solar microscope.
In the Philosophical Magazine, Mr. Fox Talbot published the first account of his Photogenic experiments. This term was introduced by this gentleman: and his experiments cannot be better described than in his own words. " In order to make what may be called ordinary photogenic paper, I select, in the first place, paper of a good firm quality and smooth surface. I do not know that anything answers better than superfine writing-paper. I dip it into a weak solution of common salt and wipe it dry, by which the salt is uniformly distributed throughout its substance. I then spread a solution of nitrate of silver on one surface only, and dry it at the fire. The solution should not be saturated, but six or eight times diluted with water. When dry, the paper is fit for use.
" I have found by experiment that there is a certain proportion between the quantity of salt and that of the solution of silver which answers best, and gives the maximum effect. If the strength of the salt is augmented beyond this point, the effect diminishes, and, in certain cases, becomes exceedingly small.
" This paper, if properly made, is very useful for all photogenic purposes. For example, nothing can be more perfect than the images it gives of leaves and flowers, especially with a summer sun,—the light passing through the leaves, delineates every ramification of their nerves.
" Now, suppose we take a sheet thus prepared, and wash it with a saturated solution of salt, and then dry it. We shall find (especially if the paper is kept some weeks before the trial is made) that its sensibility is greatly diminished, and, in some cases, seems quite extinct. But if it is again washed with a liberal quantity of the solution of silver, it becomes again sensible to light, and even more so than it was at first. In this way, by alternately washing the paper with salt and silver, and drying it between times, I have succeeded in increasing its sensibility to the degree that is requisite for receiving the images of the camera obscura.
"In conducting this operation, it will be found that the results are sometimes more and sometimes less satisfactory in consequence of small and accidental variations in the proportions employed. It happens sometimes that the chloride of silver is disposed to darken of itself without any exposure to light: this shows that the attempt to give it sensibility has been carried too far. The object is to approach to this condition as near as possible without reaching it, so that the substance may be in a state ready to yield to the slightest extraneous force, such as the feeble impact of the violet rays when much attenuated. Having, therefore, prepared a number of sheets of paper with chemical proportions slightly different from one another, let a piece be cut from each, and, having been duly marked or numbered, let them be placed, side by side, in a very weak diffused light for a quarter of an hour. Then, if any one of them, as frequently happens, exhibits a marked advantage over its competitors, I select the paper which bears the corresponding number to be placed in the camera obscura".
The increased sensitiveness given to paper by alternate ablutions of saline and argentine washes, the striking differences of effect produced by accidental variations of the proportions in which the chemical ingredients are applied, and the spontaneous change which takes place, even in the dark, on the more sensitive varieties of the paper, are all subjects of great interest, which demand further investigation than they have ever yet received, and which, if followed out, promise some most important explanations of chemical phenomena at present involved in uncertainty, particularly those which appear to show the influence of time, an element not sufficiently taken into account, in overcoming the weaker affinities. Few fields of research promise a greater measure of reward than these ; already the art of making sun-pictures has led to many very important physical discoveries, but most of the phenomena are yet involved in obscurity.