70 J. Waterliouse — The Application of ^holography [No. 2, 



negative image is obtained. For line subjects a blue glass may also be 

 used, but for half-tone work only a red glass can be employed. 



In some processes also, it is convenient to use either direct or reversed 

 transparent positives instead of negatives. These can be obtained either 

 by contact printing on dry collodion plates or gelatine tissue, or in the 

 camera, in the manner adopted for making transparencies, as described in 

 the text-books. 



Having now described the preliminary operations for preparing the 

 original and producing the negative, which are common to all processes, we 

 may proceed to the consideration of the different printing processes which, 

 as stated in the introduction, may be divided into 6 classes, viz. : 



I. Printing on sensitive papers. II. Photo-lithography or Photo- 

 zincography. III. Photo-collotype. IV. Woodbury-type. V. Helio- 

 -graphy or Photo-engraving. VI. Photo-typography. 



IV. Photographic Printing on Sensitive Papers, 



The processes under this head may be divided into three classes — 



First : — Those in which the sensitive papers are prepared with salts 

 of silver and the results are not permanent. 



Secondly : — Those in which the sensitive papers are prepared with the 

 salts of iron, platinum and other metals, and the prints though not abso- 

 lutely permanent are more so than silver prints. 



Thirdly : — Those in which coloured gelatine or other colloid mixed 

 with an alkaline bichromate forms the sensitive surface and yields prints 

 which, for all practical purposes, may be considered perfectly permanent. 



Silver-Printing. — Notwithstanding its expensiveness and the want of 

 permanence of the prints, silver printing has hitherto maintained the first 

 j)lace among photographic printing processes, and though very nearly 

 equalled, is as yet unsurpassed for the beauty and delicacy of its results. 

 It is the process in most extensive use for producing copies of portraits 

 and views, and although rapid advances are being made in more permanent 

 methods, it is likely to be a long time before the beautiful but perishable 

 silver print is entirely superseded. 



The following brief outline of the operations will be sufficient to show 

 the nature of the process.* 



A sheet of paper coated with albumen containing an alkaline chloride, 

 such as common salt, or pai)er w^hich has merely been immersed in a 

 solution of such salt and dried, is floated on a solution of nitrate of silver 

 and allowed to dry in the dark. It is then placed above the negative in a 

 copying frame, which is so constructed that the light may pass freely 

 through the negative, and at the same time may admit of the examination 



* For details, see Abney's Instruction in Thotoyraphy, p. 113. 



