THE PHOTOGRAPHY OF COLOUR 353 



of harmony with the reflecting laminae, so that the reflections 

 from the various laminae will be in dissimilar phases, and the 

 resulting interference will cause this light to be more or less 

 extinguished. But that part of the white light that corresponds 

 in wave length to the original ray will be reflected at each 

 lamina in such a manner that the difference in progression will 

 always be equal to exactly a whole wave length, and therefore 

 the light reflected from each surface will reinforce that from 

 every other. He considered that this explanation followed from 

 the wave theory of light, and required only one hypothesis, 

 namely, that the product of the light action should reflect light 

 strongly. 



Although Zenker was not entirely correct and his theory 

 did not pass unchallenged, his work eventually proved to be 

 of great importance. Still, it was hardly appreciated at the 

 time, and was apparently soon forgotten. Perhaps the method 

 of its publication militated against its being widely known. 



Lord Raleigh in 1887, and Prof. Otto Wiener of Aachen a 

 few years later, both independently and without knowledge 

 of Zenker's work, suggested that the production of standing 

 waves might account for at least some of the results of the early 

 colour photographs. But Wiener went a great deal further 

 than this, and in 1895 published the results of an exhaustive 

 examination of these methods, including the testing of pictures 

 produced by them, to determine whether the colours were due to 

 pigmentary matter or interference or both. He used a right- 

 angled prism of dense glass with its three faces polished, placing 

 the hypothenuse surface in contact with the spectrum photo- 

 graph to be tested, with a liquid of high refractivity (benzene) 

 between to exclude air, so that the refracting edge of the prism 

 crossed the colour boundary. The colour was then viewed, 

 with certain precautions, so that part of it was seen directly and 

 part through the prism. With a pigmentary colour both parts 

 remained of the same tint, but a colour due to interference 

 was changed by the presence of the prism, so that, for example, 

 yellow appeared bluish green. Wiener prepared some examples 

 by Seebeck's method (precipitated silver chloride darkened by 

 exposure to white light and then exposed to the spectrum), 

 and found the red a rose-red, the blue greyish, and the green 

 and yellow very poorly represented, if at all. He found these 

 colours to be pigmentary. A modification, due originally to 



