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Rh 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 In this state the plate is withdrawn and removed to the dark slide of the camera, ready for exposure.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.

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.

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