CHAP, viii.] HELIOGRAPHY PHOTOLITHOGRAPHY. 323 



the red negative is obtained by extinguishing the red rays by means 

 of a green glass. The yellow proof is obtained by the intervention 

 of a violet glass. 



"These three negatives each serve to produce a positive proof, 

 which may be obtained by the mixture of gelatine and bichromate of 

 potash, with the addition of the necessary colouring matter, either 

 red, yellow, or blue. The gelatine surfaces being prepared with trans- 

 parent colouring matters, are printed under their corresponding nega- 

 tives. That obtained with the blue-violet glass is placed on the 

 yellow film, and by washing, a monochrome yellow proof is obtained ; 

 the negative obtained with the green glass is put on the red gelatine ; 

 that which resulted from the interposition of orange-coloured glass is 

 placed on the blue gelatine. After exposing, developing, and drying 

 the images, they are superposed, and give a coloured proof with the 

 whole series of different phades and tints." 



The proofs obtained by M. Ducos du Hauron show the correctness 

 of his theory as carried out in his process. It is therefore an interest- 

 ing result, but it still leaves Unsolved the problem of fixing the 

 colours. 



IV. APPLICATION OF PHOTOGRAPHY TO THE ARTS AND TO THE 

 NATURAL AND PHYSICAL SCIENCES. 



Such, in their most essential features, are the processes of this 

 new art, one of the most original applications of the laws of physics 

 combined with those of chemistry. Such are the chief advances 

 made since Daguerre's time. We have only given an idea, be it 

 understood, of the different methods which constitute practical photo- 

 graphy, by trying to connect them with the principles of science ; but 

 there still remains much to be elucidated as to the reactions deter- 

 mined by the influence of the luminous waves, and it is on physi- 

 cists and chemists more than on professional photographers, however 

 talented, that the task of dissipating the obscurity which still reigns 

 on this point devolves. 



Photography, as it exists at present, has rendered the most eminent 

 services to the arts and sciences. In a certain point of view it is an 

 art which requires, in those who cultivate it, independent faculties of 

 technical skill. The choice of subjects, in portraits as in landscapes, 



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