caused by the different thicknesses of the minutely thin film of
iodide. The stage of maximum sensitiveness is obtained when
it is of a golden orange colour. In this state the plate is withdrawn
and removed to the dark slide of the camera, ready for exposure.
A plan frequently adopted to give an even film of iodide was to
saturate a card with iodine and hold the plate a short distance above
the card. Long exposures were required, varying in Paris from three
to thirty minutes. The length of the exposure was evidently a
matter of judgment, more particularly as over-exposure introduced
an evil which was called “solarization,” but which was in reality
due to the oxidation of the iodide by prolonged exposure to light.
As a matter of history it may be remarked that the development
of the image by mercury vapour is said to be due to a chance discovery
of Daguerre It appears that for some time previous to the
publication of the daguerreotype method he had been experimenting
with iodized silver plates, producing images by what would now be
called the “printing out” process. T is operation involved so
long an exposure that he sought some means of reducing it by the
application of different reagents. Having on one occasion exposed
such a plate to a camera-image, he accidentally placed it in the dark
in a cupboard containing various chemicals, and found after the
lapse of a night that he had a perfect image developed. By the
process of exhaustion he arrived at the fact that it was the mercury
vapour, which even at ordinary temperatures volatilizes, that had
caused this intensification of the almost invisible camera-image.
It was this discovery that enabled the exposures to be very considerably
shortened from those which it was found necessary to give in
mere camera-printing.
The development of the image was effected by placing the exposed plate over a slightly heated (about 75° C) cup of mercury. The vapour of mercury condensed on those places where the light had acted in an almost exact ratio to the intensity of its action. This produced a picture in an amalgam, the vapour of which attached itself to the altered silver iodide. Proof that such was the case was subsequently afforded by the fact that the mercurial image could be removed by heat. The developing box was so constructed that it was possible to examine the picture through a yellow glass window whilst the image was being brought out. The next operation was to fix the picture by dipping it in a solution of hyposulphite of soda. The image produced by this method is so delicate that it will not bear the slightest handling, and has to be protected from accidental touching.
The first great improvement in the daguerreotype process was the re sensitizing of the iodized film by bromine vapour. John Frederick Goddard published his account of the use of bromine in conjunction with iodine in 1840, and A. F. J. Claudet (1797-1867) employed a combination of iodine and chlorine vapour in 1841. In 1844 Daguerre published his improved method of preparing the plates, which is in reality based on the use of bromine with iodine. That this addition points to additional sensitiveness will be readily understood when we remark that so-called instantaneous pictures of yachts in full sail, and of large size, have been taken on plates so prepared-a feat which is utterly impossible with the original process as described by Daguerre. The next improvement in the process was toning or gilding the image by a solution of gold, a practice introduced by H. L. Fizeau. Gold chloride is mixed with hyposulphite of soda, and the levelled plate, bearing a sufficient quantity of the fluid, is warmed by a spirit-lamp until the required vigour is given to the image, as a consequence of which it is better seen in most lights. Nearly all the daguerreotypes extant have been treated in this manner, and no doubt their permanence is in a great measure due to this operation. Images of this class can be copied by taking electrotypes from them, as shown by Sir W. R. Grove and others. These reproductions are admirable in every way, and furnish a proof that the daguerrean image is a relief.
Fox-Talbot Process.—In January 1839 Fox Talbot described the first of his processes, photogenic drawing, in a paper to the Royal Society. He states that he began experimenting in 1834, and that in the solar microscope he obtained an outline of the object to be depicted in full sunshine in half a second. He published in the Philosophical Magazine full details of his method, which consisted essentially in soaking paper in common salt, brushing one side only of it with about a 12% solution of silver nitrate in water, and drying at the fire. Box Talbot stated that by repeating the alternate washes of the silver and salt-always ending, however, with the former—greater sensitiveness was attained This is the same in every respect as the method practised by Wedgwood in 1802; but, when we come to the next process, which he called “calotype” or “beautiful picture,” we have a distinct advance. This process Talbot protected by a patent in 1841.
It may be briefly described as the application of silver iodide to a paper support. Carefully selected paper was brushed over with a solution of silver nitrate (100 grains to the ounce of distilled water), and dried by the fire. It was then dipped into a solution of potassium iodide (500 grains being dissolved in a pint of water), where it was allowed to stay two or three minutes until silver iodide was formed. In this state the iodide is scarcely sensitive to light, but is sensitized by brushing “gallo-nitrate of silver” over the surface to which the silver nitrate had been first applied. This “gallonitrate” is merely a mixture, consisting of 100 grains of silver nitrate dissolved in 2 oz. of water, to which is added one-sixth of its volume of acetic acid, and immediately before applying to the paper an equal bulk of a saturated solution of gallic acid in water. The prepared surface is then ready for exposure in the camera, and, after a short insulation, develops itself in the dark, or the development may be hastened by a fresh application of the “gallo-nitrate of silver.” The picture is then fixed by washing it in clean water and drying slightly in blotting paper, after which it is treated with a solution of potassium bromide, and again washed and dried. Here there is no mention made of hyposulphite of soda as a fixing agent, that having been first used by Sir J. Herschel in February 1840.
In a strictly historical notice it ought to be mentioned that development by means of gallic acid and silver nitrate was first known to Rev. J. B. Reade. When impressing images in the solar microscope he employed gallic acid and silver in order to render more sensitive the silver chloride paper that he was using, and he accidentally found that the image could be developed without the aid of light. The priority of the discovery was claimed by Fox Talbot; and his claim was sustained after a lawsuit, apparently on the ground that Reade's method had never been legally published. Talbot afterwards made many slight improvements in the process. In one of his patents he recognizes the value of the proper fixing of his photogenic drawings by hyposulphite of soda, and also the production of positive prints from the calotype negatives. We pass over his a placation of albumen to porcelain and its subsequent treatment with iodine vapour, as also his application of albumen in which silver iodide was held in suspension to a glass plate, since in this he was preceded by Niepce de St Victor in 1848.
Albumen Process on Glass.—It was a decided advance when Niepce de St Victor, a nephew of Nicéphore de Niepce, employed a glass plate and coated it with iodized albumen. The originator of this method did not meet with much success. In the hands of Blanquart Evrard it became more practicable; but it was carried out in its greatest perfection by G. Le Gray.
The outline of the operations is as follows: The whites of five fresh eggs are mixed with about one hundred grains of potassium iodide, about twenty grains of potassium bromide and ten grains of common salt. The mixture is beaten up into a froth and allowed to settle for twenty-four hours, when the clear liquid is decanted off. A circular pool of albumen is poured on a glass plate, and a straight ruler (its ends being wrapped with waxed paper to prevent its edge from touching the plate anywhere except at the margins) is drawn over the plate, sweeping off the excess of albumen. and so leaving an even film. The plate is first allowed to dry spontaneously, a final heating being given to it in an oven or before the fire. The heat hardens the albumen, and it becomes insoluble and ready for the silver nitrate bath. One of the difficulties is to prevent crystallization of the salts held in solution, and this can only be effected by keeping them in defect rather than in excess. The plate is sensitized for five minutes in a bath of silver nitrate, acidified with acetic acid, and exposed whilst still wet, or it may be slightly washed and again dried and exposed whilst in its desiccated state. The image is developed by gallic acid in the usual way.
After the application of albumen many modifications were introduced in the shape of starch, serum of milk, gelatin, all of which were intended to hold iodide in situ on the plate; and the development in every case seems to have been by gallic acid. At one time the waxed-paper process subsequently introduced by Le Gray was a great favourite. Paper that had been made translucent by white wax was immersed in a solution of potassium iodide until impregnated with it, after which it was sensitized in the usual way, development being by gallic acid. In images obtained by this process the high lights are represented by metallic silver, whilst the shadows are translucent. Such a print is called a “negative.” When silver chloride paper is darkened by the passage of light through a negative, we get the highest lights represented by white paper and the shadows by darkened chloride. A print of this kind is called a “positive.”
Collodion Process.—A great impetus was given to photography
