566 PHOTOGRAPHY 



silver -while wet .and electrotyped in copper, and from this plate with proper precau- 

 tions any number of copies can be printed off. In 1872 Mr. Swan informed the 

 writer that the process had been much improved, particularly in the transfer part of 

 the process. The tissue, after being impressed by the solar action in the printing- 

 frame, is now attached to the surface on which the development of the pictures is 

 effected without the aid of any cementiiuj material, and without the employment of a press. 

 The natural adhesiveness of the tissue itself, aided as it is by atmospheric pressure 

 and capillary attraction, obtains a very perfect attachment of the tissue to either a 

 plate of glass or metal or to insoluble gelatinised or insoluble coagulated albumenised 

 paper. In practice the tissue is transferred for development (by warm water) to a 

 zinc plate when it is required to be re-transferred to paper (to procure reversal), and 

 when the re-transfer is not required the tissue is at once transferred to (and the pic- 

 ture developed upon) paper faced with some insoluble material such as named, or 

 lac. Mr. Johnson, who has latterly done much in the improvement of the process in 

 connection with the points referred to, and who works in conjunction with the 

 Autotype Company, prefers to use a pnper faced with lac, decolourised of course. This 

 process has been largely developed as the ' autotype process ' and many very fine pro- 

 ductions are now being regularly published. 



Numerous modifications of these processes have from time to time been brought 

 under the notice of the public. Mr. Duncan Campbell Dallas, amongst others, 

 patented the following process, known as the Dallastype : 



The design is photographed or drawn upon a glass plate in a medium that inter- 

 cepts the actinic rays. Over the design a solution of bichromatised gelatine is poured, 

 and when the coating thus formed is sufficiently dry, the uncoated side is exposed to 

 the light. The parts not acted upon by light are then softened and caused to swell 

 by treatment with cold water ; this is poured off and the design is repeatedly washed 

 with warm water till the design is free from the gelatinous mixture and allowed to dry. 

 A mould is then taken of the surface by electrotyping, by casting, or by pressure. 



Further information relative to processes of this character must be sought in the 

 numerous Specifications of Patents which exist. The process of Photozincography will 

 be found under its special heading. See PHOTOZINCOGBAPHY. 



PHOTOGRAPHY. (From <><5s, phos, photos, light ; 7/>o<^, graphs, a writing or a 

 description.) The art of producing pictures by the agency of sunshine, acting upon 

 chemically-prepared papers. 



There are certain chemical compounds, and especially some of the salts of silver, 

 which are rapidly decomposed by the influence of the sunshine, and, though more 

 slowly, by ordinary daylight, or even by powerful artificial light. As the extent 

 to which the decomposition is carried on, depends upon the intensity of radiation 

 proceeding from the object, or passing through it, accordingly as we are employing 

 the reflected or the transmitted rays, it will be obvious that we shall obtain very deli- 

 cate gradations of darkening, and thus the photograph will represent in a very refined 

 manner all those details which are rendered visible to the eye by light and shadow. 



There are two methods by which photographs can be taken : the first and simplest 

 is by super-position, but this is applicable only to the copying of engravings or of such 

 botanical specimens as can be spread out upon paper, and objects which are entirely 

 or in part transparent. The other method is by throwing upon the prepared paper 

 the image obtained by the use of a lens fitted into a dark box the camera obscura. 



To carry out either of these methods certain sensitive surfaces must be produced; 

 these therefore claim our first attention : The artist requires 



1. Nitrate of silver. 2. Ammonia nitrate of silver. 3. Chloride of silver. 4. 

 Iodide of silver. 6. Bromide of silver. These five chemical compounds may be re- 

 garded as the agents most essential in the preparation of photographic surfaces. 



1. NITRATE OF SILVER. The crystallised salt should, if possible, always be pro- 

 cured. The fused nitrate, which is sold in cylindrical sticks, is liable to contamination. 

 A preparation is sometimes sold for nitrate of silver, at from 6d. to 9rf. the ounce less 

 than the ordinary price, which may induce the unwary to purchase it. This reduction 

 of price is effected by fusing with the salt of silver a proportion of some cupreous salt, 

 generally the nitrate, or nitrate of potash. This fraud is readily detected by observing 

 if the salt becomes moist on exposure to the air, a very small admixture of copper 

 rendering the nitrate of silver deliquescent. The evils to the photographer are, wiint 

 of sensibility upon exposure, and the perishability (even in the dark) of the finished 

 drawing. 



The most simple kind of photographic paper is that washed with the nitrate of silver 

 only ; and for many purposes it answers well, particularly for copying lace or feathers ; 

 and it has this advantage that it is perfectly fixed by well soaking in pure warm water. 

 The best proportions in which this salt can be used are 60 grains of it dissolved in 

 a fluid ounce of water, Care must be taken to apply it equally, with a quick but 

 steady motion, over every part of the paper. 



