PHOTOGRAPHY. 



two or three minutes, pin it up to dry in a moderately 

 warm darkened room. It will keep good in the 

 dark for about two days, but it is better to use it 

 within twenty-four hours. To produce prints on 

 the sensitised paper so prepared, it is cut into 

 pieces of the size required. The negative is laid, 

 face up, on the glass of the printing-frame; on this 

 the paper is placed, with its albuminised side 

 next the negative. The hinged board, technically 

 called the back, is then laid on the paper, and the 

 whole pressed down by the cross-bars and springs. 

 The frame is then exposed to the light, and the 

 progress of printing examined from time to time 

 till completed, which is known by the print being 

 just a little darker than the finished result is 

 intended to be, as it loses a little of its intensity 

 in the operation of toning. The print is then 

 removed from the frame, and placed in a dish of 

 common water, which is changed several times, 

 or until it ceases to have a milky appearance. It 

 is now of a brick-red colour, and requires to undergo 

 the operation of toning, to bring it to the desired 

 purple brown. A good toning-bath may be made 

 by dissolving 100 grains of acetate of soda, and 

 four grains of chloride of gold, in 20 ounces of 

 water. This must stand for at least twelve hours 

 before being used, and will keep indefinitely. The 

 washed print is placed in this bath, and while 

 kept in constant motion, is carefully watched 

 till the operation is complete. First, the red will 

 become brown, then dark brown, a purple brown, 

 a slaty blue, and ultimately it will get bleached 

 out nearly altogether. The action must be stopped 

 when the proper colour has been attained, by 

 removing the print from the bath, and washing 

 it well in several changes of water. It is then 

 fixed, by being immersed for ten minutes in a 

 solution of soda hyposulphite, five ounces to the 

 pint of water. 



When fixed, the prints are to be subjected to a 

 most thorough and careful washing, in as many as 

 twenty changes of water, over a period of from 

 eight to twelve hours, so as to remove, if possible, 

 all trace of the soluble compounds produced 

 during the process. Imperfect washing is sure to 

 cause the fading of the prints in a longer or shorter 

 time, according to the treatment they receive. 

 When sufficiently washed, they are pinned up to 

 dry. They should then be subjected to pressure, 

 or rubbed on the back between two hard sub- 

 stances, so as to render the surface as smooth as 

 possible, after which they may be trimmed to any 

 desired shape. 



A solution of dextrine in water is the best ad- 

 hesive substance to use in mounting transfers. 

 Gelatine or starch may be employed, but pre- 

 cautions must be used against their becoming sour 

 or mouldy. 



Although thorough washing after fixing will 

 remove all tendency to fading from the action of 

 sulphur compounds, there has long been a sus- 

 picion that no print having silver for its basis can 

 be relied on as permanent ; and for a number of 

 years experimenters were hard at work with a 

 view to replace it by the more permanent carbon. 

 This has now been satisfactorily accomplished, and 

 carbon-printing has become a large commercial 

 enterprise. The process is based on the discovery 

 by Mungo Ponton, that soluble organic matter, in 

 presence of some of the alkaline bichromates, is 

 rendered insoluble by the action of light ; and the 



following may be taken as a type of the various 

 modifications at present in use : A sheet of paper 

 is coated with a mixture of gelatine, bichromate of 

 potass, and lampblack, or other carbon, in very 

 fine powder, and dried. This carbon tissue is 

 then printed under a negative, and developed by 

 immersion in warm water, which removes all the 

 soluble parts on which the light has not acted, 

 and which being the shadows in the negatives, 

 become the lights in the print ; while the parts 

 corresponding to the lights in the negative, and 

 on which the light has acted, remain insoluble, 

 and represent the shadows. 



Carbon prints of considerable beauty are also 

 produced in the ordinary printing and lithographic 

 presses. A bichromatised gelatine film is spread 

 on a thick plate of glass, and printed under a 

 negative. It is then soaked in cold water, which 

 causes the soluble parts to swell, producing a 

 picture in relief. This is hardened by alum, or 

 chrome alum, and inked by rollers in the usual 

 way; and by using inks of different consistency, 

 beautiful half-tone is procured. 



Instead of printing from the gelatine relief, its 

 surface may be made to conduct electricity by the 

 reduction of silver by phosphorus, and a copper- 

 plate produced therefrom by the ordinary electro- 

 type process. This has been called photo-galvan- 

 ography, and is of much value for many purposes. 



Photo-lithography is extensively employed, and 

 is well adapted for reproducing maps or drawings. 

 A sheet of paper coated with bichromatised gela- 

 tine is exposed under a negative, as in silver 

 printing. A lithographic stone is inked all over 

 on the surface sparingly with strong re-transfer 

 ink, the paper transferred taken quickly out of the 

 frame, and placed on the inked surface of the 

 stone, so as to gather the ink, and passed two or 

 three times through the lithographic press. After 

 that, the transfer is taken and dipped into clean 

 cold water, and then gently sponged over the 

 blackened surface, when the transfer lines will 

 be found to come out clear and sharp ; after the 

 transfer has dried, it is transferred in the ordinary 

 way to stone. 



Photo-zintography is probably one of the most 

 useful applications of the above principle. It was 

 introduced by Colonel Sir Henry James, of the 

 Ordnance Survey, in 1859, for the purpose of 

 reducing the 25-inch maps to the 6-inch scale ; 

 and this it did so successfully, that a government 

 committee reported that the greatest deviation 

 from the original plans did not amount to the 

 T tath part of an inch, and that its use would be 

 the means of saving to the country ^32,ooa The 

 process has since been successfully employed in 

 reproducing the original manuscript of Domesday 

 Book, and the 1664 folio edition of Shakspeare. 

 The sensitive tissue is made by coating a sheet of 

 paper with gum-arabic, or gelatine, or a mixture 

 of both, to which has been added some bichromate 

 of potass, or ammonia. When this is dry, it is 

 exposed under a negative made from the map or 

 manuscript in the ordinary way. Lithographic 

 ink is then freely applied to the whole surface, 

 and the inked tissue exposed to the action of a 

 stream of warm water, which dissolves and washes 

 off the parts not acted on by light, while the ink 

 adheres to the lines on which the light has acted. 

 The copy thus obtained is transferred to a suit- 

 ably prepared plate of zinc, from which any number 



