322 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



chloride of sodium, which gives a yellow flame in the alcohol, it will 

 be the yellow which will be most intensely developed ; with the chlo- 

 ride of copper it will be green, with the chloride of strontium it 

 will be red. Unfortunately these results, which are most interesting 

 in a scientific point of view, have not been of practical use in photo- 

 graphic art. These colours given by the light only remain on the 

 sensitized coating as long as they are in complete darkness ; they can 

 only be hastily examined, and they vanish in the light of day. 

 Every effort made as yet to fix them has failed. 



Among the attempts made in the same direction as that of M. 

 Ed. Becquerel and M. Niepce de Saint-Victor, we may mention those 

 of M. Poitevin, who obtained most of the colours of the spectrum, 

 chiefly red, orange and yellow, on a paper charged w T ith hyposulphite 

 of silver, and covered over with a coating formed by a solution of an 

 alkaline bichromate, mixed with a strong solution of sulphate of 

 copper, and a solution of five per cent, of chloride of potassium. 

 With the paper thus prepared and placed for ten minutes on a paint- 

 ing on glass, the colours were reproduced ; but they faded away in 

 the light. 



Some investigators, unable to solve the problem in its integrity, 

 have tried another plan. Inspired no doubt by the processes of 

 chromolithography, they sought to obtain the colours separately, 

 the combination of which would reproduce the colours of the objects. 

 With three proofs, one of which would give red, the second yellow, 

 the third blue, they hoped, by superposition or union, to obtain the 

 compound colours. Two photographers, Messrs. Cros and Ducos du 

 Hauron, severally pointed out this solution ; but the latter alone has 

 put it in practice. His process is thus described in M. Davanne's 

 Photographic Annual: 



First of all three negatives are struck off, one of which is to 

 serve as the red positive, the second the yellow, and the third the 

 blue. " To make the blue negative, all the simple and compound 

 blue tints must be extinguished in the subject to be reproduced, 

 that they may have no action on the sensitized coatings ; for this 

 the proof has to be obtained through an orange coloured glass. 

 After a very long exposure, an image is obtained in which the 

 blues have exercised a very feeble influence on the sensitive coating, 

 while the yellow is sufficiently prominent. The proof representing 



