PHOTOGRAPHY 



6 1 25 



PHOTOGRAPHY 



dyes, plates so-called pan-chro- 

 matic can now be made sensi- 

 tive to every radiation from blue 

 to red. 



At the present day the sensitive 

 coating on dry plates consists of an 

 emulsion of silver bromide in 

 gelatin, the composition and pre- 

 paration of which vary according 

 as the plates are required to be of 

 high speed for instantaneous or 

 portrait photography, or slow for 

 copying or lantern-slide making. 

 The emulsion is made by mixing 

 weighed and dissolved quantities 

 of silver nitrate and potassium 

 bromide in warm gelatin solution. 

 It is caused to set, the jelly 

 shredded through canvas, washed 

 in water, and " cooked " for a 

 time and at a heat according to 

 the sensitiveness required. Glass 

 plates are machine-coated by 

 causing the warm fluid emulsion to 

 flow over them from a species of 

 weir, are allowed to set and then 

 dried, the operations being carried 

 out in a red or deep green light 

 according to the colour-sensitive- 

 ness of the emulsion. 



Printing papers, in the earliest 

 days of photography, were identical 

 with those used for negatives, but 

 with the introduction of the wet- 

 colloilion process albumenised or 

 silver paper came into universal 

 use. Paper was coated with 

 albumen and ammonium chloride, 

 and rendered sensitive by tlie 

 photographer himself by floating 



on a bath of silver nitrate. It 

 kept in good condition only a 

 few days, but in later years was 

 made commercially to keep several 

 months. 



Exposed to daylight under a 

 negative it yielded a print of ugly 

 reddish colour, which was converted 

 into one of pleasing purple by 

 toning in a solution containing gold 

 chloride. The prints were then 

 fixed in hypo, which dissolved out 

 the silver compounds undarkened 

 by light. This paper was the only 

 one in general use from about 1850 

 to 1892. In 1865 papercoated with 

 collodion emulsion for printing- 

 out and tonitig in this way was 

 devised, but was little used. In 

 1882 Captain (afterwards Sir Wm.) 

 Abney worked out a gelatin 

 emulsion to be used in the same 

 manner. Paper of this kind was 

 first made in Germany, but did 

 not displace albumen paper until 

 its manufacture in 1891 by the 

 llford Company, from whom it re- 

 ceived the name P.O.P. (print-out 

 paper). 



There are now manufactured 

 three chief varieties of such papers, 

 i.e. P.O. P., bromide paper pre- 

 pared with an emulsion akin to 

 that of a dry-plate and requir- 



ing to be developed, and gas-light 

 paper, which is a much less sensi- 

 tive variety of bromide, and 

 may be handled without a dark- 

 room, though it is rapid enough to 

 be impressed when exposed for a 

 few seconds behind a negative. A 

 fourth variety, self -toning paper 

 (q.v.), is P.O.P. with sufficient gold 

 in the coating to dispense with the 

 use of a separate toning bath, but 

 paper of this kind, introduced 

 about 1901, is usually made as a 

 collodion emulsion. Bromide paper 

 was made commercially as early as 

 1879, but its use and that of gas- 

 light paper only became general 

 some 12 years later. 



A chromium compound (sodium 

 or potassium bichromate) serves as 

 the basis of several processes in a 

 quite different way. It is not itself 

 sensitive to light, but is so when 

 mixed with substances such as gum 

 or gelatin. The action of the light 

 is to render the previously soluble 

 gum or gelatin insoluble in water, 

 as was first observed by Fox Talbot 

 in 1852. In 1855 a French engineer, 

 Poitevin. applied this fact to the 

 making of prints by mixing the 

 gelatin and bichromate with a pig- 

 ment which was held by the light- 

 affected gelatin whilst the re- 

 mainder might be removed, with 

 the soluble gelatin, by hot water. 

 Pouncy in 1858 further improved 

 on this pioc'css, but it was not 

 until 1864 that 

 Sir Joseph 

 Swan made 

 it commercial 

 by devising a 

 means for over- 

 coming the chief 

 difficulty, i.e. 

 the formation 

 of an insoluble 



Photography. 1. Focal plane shatter. 2. Folding roll-film camera. 3. Folding camera with local plane shutter; widely 

 used for press photography. 4. Between-lens or diaphragm shutter, containing thin leaf segments which rapidly open 

 and close: b, release; d, setting lever; g, setting pointer for diaphragm. 5. Reflex camera: A, position of sensitive 

 plate; B, shutter release ; C, knob for altering- width of slit; D, winding key!; E, adjustments for long and short 

 exposures; F, rotating back ; G, lens shade; H H, catches lor hood. 6. Diagram illustrating action of reflex camera. 

 The rays from the lens are reflected by a mirror, and the picture is seen, through the hood, on the ground glass. 

 7. Magazine hand camera for plates, with side opened to show how the exposed plates fall to the lower part of the camera 



