354 SCIENCE PROGRESS 



Poitevin, but with slight improvements, was investigated. Silver 

 chloride precipitated in paper was exposed to daylight in a 

 solution of stannous chloride until darkened. It was then 

 immersed in a solution containing potassium bichromate and 

 cupric sulphate and exposed slightly moist. This reproduces 

 all the colours more or less, but with a tendency towards a 

 yellowish brown tint. These colours also are pigmentary. 

 The colours produced by Becquerel's process, in which a silver 

 plate has its surface converted into chloride by the electrolytic 

 method, were found to be chiefly interference colours due to 

 the action of standing waves, but there was pigmentary 

 matter also present. 



The colour obtained in these older processes being due 

 entirely to pigmentary matter, except when a metal plate was 

 used, the natural question is, What is the character of the 

 pigmentary substance ? Wiener found that silver subchloride 

 was sensitive to all the colours of the spectrum as silver chloride 

 that has been exposed to light is, and as these two substances 

 are alike in other properties, there seems little doubt that silver 

 subchloride is the colour-sensitive substance in these processes. 

 Why it should be changed by coloured light into a substance 

 that has some approach to the colour of the light that falls 

 upon it is a problem still unsolved. But it seems exceedingly 

 unlikely that any useful method of colour photography will 

 be realised on this line of work, because the colours at their 

 best are only approximately similar to the colour of the light 

 that produces them ; they cannot be fixed because the pig- 

 mentary matter is decomposed by solvents of silver chloride ; 

 the subchloride does not bleach to white in white light, nor 

 is it itself black — two conditions that must be fulfilled in a 

 material that is to reproduce colours by the direct action of 

 light. Wiener suggested as the ideal material a black mixture 

 of three differently coloured substances which are bleached to 

 white by white light, and are sensitive in proportion to their 

 absorption. In the next section the practical realisation of this 

 principle will be described. 



We have already seen that " standing waves " were predicted 

 as a necessary result of the undulatory nature of light, and 

 it appears that several physicists sought to obtain experimental 

 evidence of their existence, but it was not until 1891 that 

 Prof. Gabriel Lippmann of Paris was successful. If these stand- 



