PHOTOGRAPHY 823 Daguerreotype. We have already noticed in the joint process of Daguerre and Niepce that polished silver plates were use( j j an( j we k now from the latter that amongst the chemical agents tried iodine suggested itself. Iodine vapour or solution applied to a silvered plate would cause the formation of silver iodide on those parts not acted upon by light. The removal of the resinous picture would leave an image formed of metallic silver, whilst the black parts of the original would be represented by the darker silver iodide. This was probably the origin of the daguerreotype process. Such shrewd observers as Niepce and Daguerre, who had formed a partnership for prosecut ing their researches, would not have thus formed iodide of silver without noticing that it changed in colour when exposed to the light. What parts respectively Daguerre and Niepce played in the development of the daguerreo type, which we shall shortly describe, will probably never be known with absolute accuracy, but in a letter from Dr Bauer to Dr Bennett, F.K.S., dated 7th May 1839, the former says : "I received a very interesting letter from Mons. Isidore Niepce, dated 12th March [about a month after the publication of the daguerreotype process], and that letter fully confirms what I sus pected of Daguerre s manoeuvres with poor Nicephore, but Mr Isidore observes that for the present that letter might be considered confidential." Dr Bauer evidently kneAv more of " poor Nicephore s " work than most people, and at that early period he clearly thought that an injustice had been done to Niepce at the hands of Daguerre. It should be remarked that Nicephore de Niepce died in 1833, and a new agreement was entered into between his son Isidore de Niepce and Daguerre to continue the prosecution of their researches. It appears further that Niepce communicated his process to Daguerre on 5th December 1829. At his death some letters from Daguerre and others were left by him in which the use of iodine, sulphur, phosphorus, &c., is mentioned as having been used on the metal plates, and their sensi tiveness to light, when thus treated, commented upon. We are thus led to believe that a great part of the success in producing the daguerreotype is due to the elder Niepce ; and indeed it must have been thought so at the time, since, on the publication of the process, life-pensions of 6000 francs and 4000 francs were given to Daguerre and to Isidore Niepce respectively. In point of chronology the publication of the discovery of the daguerreotype process was made subsequently to the Talbot-type process. It will, however, be convenient to continue the history of the daguerreotype, premising that it was published on 6th February 1839, whilst Talbot s process was given to the world on 25th January of the same year. Daguerreotype pictures were originally taken on silver- plated copper, and even at the present day the silvered surface thus prepared serves better than electro-deposited silver of any thickness. An outline of the operations is as folloAvs. A brightly-polished silver plate is cleaned by means, first of finely-powdered pumice and olive oil then of dilute nitric acid, and a soft buff is employed to give it a brilliant polish, the slightest trace of foreign matter or stain being fatal to the production of a perfect picture. The plate, thus prepared, is ready for the iodizing operation. Small fragments of iodine are scattered over a saucer, covered with gauze. Over this the plate is placed, face down wards, resting on supports, and the vapour from the iodine is allowed to form upon it a surface of silver iodide, which is the sensitive compound. It is essential to note the colour of the surface-formed iodide at its several stages, the varying colours being due to interferences caused by the different thicknesses of the minutely thin film of iodide of silver. 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 "solar- ization," but which was in reality due to the oxidation of the iodide, itself altered by prolonged exposure to light. As a matter of history it may be interesting to remark that the development of the image by means of 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. This operation in volved 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 of mercury, the vapour of which attached itself to the altered iodide of silver. Proof that such was the case Avas subsequently afforded by the fact that the mercurial image could be removed by heat. The developing box was so constructed that it AA r as possible to examine the picture through a yelloAv glass AvindoAV Avhilst the image was being brought out. The next operation Avas 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 AA ill not bear the slightest handling, and has to be protected from accidental touching. The first great improvement in the daguerreotype pro cess Avas the resensitizing of the iodized film by bromine vapour. Mr Goddard published his account of the use of bromine in conjunction with iodine in 1840, and M. Claudet employed a combination of iodine and chlorine vapour in 1841. In 1844 Daguerre published his improved method of preparing the plates, Avhich is in reality based on the use of bromine Avith iodine. That this addition points to additional sensitiveness will be readily understood Avhen Ave remark that so-called instantaneous pictures of yachts in full sail, and of large size, have been taken on plates so prepared, a feat Avhich is utterly impossible Avith the original process as described by Daguerre. The next im provement to be noticed in the process AA as toning or gilding the image by a solution of gold, a practice introduced by M. 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 Aiiich 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 shoAvn by Grove and others. These repro ductions are admirable in every Avay, and furnish a posi-
