Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/870

 834 PHOTOGRAPHY destroyed. In this process tough paper is coated with a fine layer of gelatin a-nd subsequently treated with alum or chrome alum, afterwards receiving another coating, as in the Southampton method. The printing too is carried out as in the Southampton method, but not so deeply. After withdrawing the prints from the printing-frame they are soaked in cold water, and a roller is passed over them charged with an ink made of 4 parts of best lithographic chalk ink mixed with 1 part of palm oil. A roller coated with velvet is said to be better than the ordinary composi tion rollers. The ink takes when the work is all clear ; the transfer is exposed to light, and is ready to be put down on stone or zinc. Photo-Engraving and Photo-Reliefs. Photo- This may be divided into two classes, one the production engrav- o f an engraved plate for printing by the copper-plate press, and the other for the production of cliches for printing with type. Niepce s process is still generally employed for the first when line engravings have to be reproduced. A copper plate is covered with asphaltum, a film negative placed in contact with it, and the necessary exposure given. After development with olive oil and turpentine the lines are shown as bare copper. The plate after being waxed at the back is next plunged into an acid bath and etched as are etched plates. When a half-tone negative has to be reproduced on copper Fox Talbot s method, described in his patents of 1852 and 1858, is still the simplest. A print on gelatin is transferred to a copper plate, and the surface etched by means of different strengths of ferric chloride, which renders the gelatin insoluble and imper meable ; hence it will be seen that a weak solution of ferric chloride is able to reach the copper through the gelatin more readily than a strong one. In order to be successful it is necessary to give a grain to the plate ; this is effected by sprinkling it with powdered resin, which is then warmed. Relief plates for printing with type are usually made on zinc. If an ordinary photo-lithographic transfer be trans ferred to zinc and then sprinkled with resin, the zinc may be immersed in weak acid and the uncovered parts eaten away. The regularity of the erosion is much increased by previously immersing the plate in a weak solution of copper sulphate. The particles of metallic copper deposited on the zinc form with it and with dilute acid galvanic couples, which rapidly eat away the zinc. The etching bath should be kept in motion. The depth of the erosion is increased by littering the surface again with powdered resin, which adheres to the lines, and then heating the plate. The warmed resin runs down the eroded lines and protects them from under-cutting when again placed in acid. This process is applicable to line-engravings. Niepce s bitumen process is also applicable, but in that case a posi tive must be applied to the plate to be etched. There exist several methods by which half-tone negatives may be reproduced for working off in the printing-press. They depend principally on breaking up the whole surface by means of lines. Thus, if, between the surface on which the printing is to take place (and which has been coated with some sensitive medium) and the positive, a film on which a network of lines has been photographed be inter posed, it is evident that the resulting print will consist of the half-tone subject together with an image of the net work of lines. This can be etched in the manner described above. Most of these processes are secret, but it is be lieved that this is the one most generally practised. PhotograpJts in Natural Colours. The first notice on record of coloured light impressing its own colours on a sensitive surface is in the passage already quoted from the Farbenlehre of Goethe, where Photo. Seebeck of Jena (1810) describes the impression he ob-S ra phy tained on paper impregnated with moist chloride of silver. , In 1839 Sir J. Herschel (Athenaeum, No. 621) gave a somewhat similar description. In 1848 Edmond Becquerel succeeded in reproducing upon a daguerreotype plate not only the colours of the spectrum but also, up to a certain point, the colours of drawings and objects. His method of proceeding was to give the silver plate a thin coating of silver chloride by immersing it in ferric or cupric chlor ides. It may also be immersed in chlorine water till it takes a feeble rose tint. Becquerel preferred to chlorinize the plate by immersion in a solution of hydrochloric acid in water, attaching it to the positive pole of a voltaic couple, whilst the other pole he attached to a platinum plate also immersed in the acid solution. After a minute s subjection to the current the plate took successively a grey, a yellow, a violet, and a blue tint, which order was again repeated. When the violet tint appeared for the second time the plate was withdrawn and washed and dried over a spirit-lamp. In this state it produced the spectrum colours, but it was found better to heat the plate till it assumed a rose tint. At a later date Niepce de St Victor chlorinized by means of chloride of lime, and made the surface more sensitive by applying a solution of lead chlor ide in dextrin. G. W. Simpson also obtained coloured images on silver chloride emulsion in collodion, but they were less vivid and satisfactory than those obtained on daguerreotype plates. Poitevin obtained coloured images on ordinary chloride of silver paper by preparing it in the usual manner and washing it and exposing it to light. It was afterwards treated with a solution of bichromate of potash and cupric sulphate, and dried in darkness. Sheets so prepared gave coloured images from coloured pictures, which he stated could be fixed by sulphuric acid (Comptes Rendus, 1868, vol. Ixi. p. 11). In the Bulletin de la Societe Franqaise (1874) St Florent describes experiments. which he made with the same object. He immerses ordi nary or albuminized paper in silver nitrate and afterwards plunges it into a solution of uranium nitrate and zinc chloride acidulated with hydrochloric acid ; it is then ex posed to light till it takes a violet, blue, or lavender tint. Before exposure the paper is floated on a solution of mer curic nitrate, its surface dried, and exposed to a coloured image. It is supposed though it is very doubtful if it be so that the nature of the chloride used to obtain the chloride of silver has a great effect on the colours impressed ; and Niepce in 1857 made some observations on the relationship which seemed to exist between the coloured flames pro duced by the metal and the colour impressed on a plate prepared with a chloride of such a metal. In 1880 (Proc. Roy. /Soc.) Abney showed that the production of colour really resulted from the oxidation of the chloride that was coloured by light. Plates immersed in a solution of hydro- xyl took the colours of the spectrum much more rapidly than when not immersed, and the size of the molecules seemed to regulate the colour. He further stated that the whole of the spectrum colours might be derived from a mix ture of two or at most three sizes of molecules. In 1841, during his researches on light, Robert Hunt published some results of colour-photography by means of fluoride of silver. A paper was washed with nitrate of silver and with sodium fluoride, and afterwards exposed to the spectrum. The action of the spectrum commenced at the centre of the yellow ray and rapidly proceeded upwards, arriving at its maximum in the blue ray. As far as the indigo the action was uniform, whilst in the violet the paper took a brown tint. When it was previously exposed, however, a yellow space was occupied where the yellow