352 SCIENCE PROGRESS 



upon its original path, the waves travelling in opposite directions 

 will interfere with each other. It is clear that as they move in 

 opposite directions each wave will alternately coincide with and 

 oppose a wave of the series that it is meeting, and that where 

 they coincide their moving power on the particle concerned 

 will be added, and that where they are opposed the particle can 

 obey neither impulse, and therefore remains stationary. There 

 is thus produced a series of "standing waves," for although 

 their crests rise and fall or move to and fro at (practically) right 

 angles to the path of the disturbance, they move in neither 

 direction along the path. The alternate crests move in opposite 

 directions, and between each and the next there is a stationary 

 point. If these conditions are realised in light waves, the light 

 will be quenched at those points where the movement of the 

 particles (of the luminiferous ether) is prevented, and there will 

 result a series of layers of light with intervals of darkness half a 

 wave length apart. 



Dr. W. Zenker, of Berlin, in 1868 published a small volume, 

 Lehrbuch der Photochromie {Photographic in naiilrlichen Farben), in 

 which he examined in detail the methods, theories, and results of 

 the work that had been done up to then in connection with the 

 reproduction of colour. While satisfied that coloured light will 

 under certain conditions produce something that reflects the 

 same, or very nearly the same, colour as the incident light, he 

 points out the insufficiency of the theories of the change that 

 had been put forward. The colour of the incident light, though 

 reproduced more or less in all cases, is especially noticeable 

 when silver plates are used, as in the method of Becquerel. 

 He then explains the nature of standing waves, and advances 

 this as an explanation of the phenomenon. The silver chloride, 

 especially if violet (that is, slightly reduced), is so affected by 

 the light that metallic silver is produced in layers, with intervals 

 of no chemical change which correspond to the parts where 

 the light is quenched by interference. He supposed metallic 

 silver to be produced because it is necessary for the reproduc- 

 tion of the colour that the product of the change shall be a 

 good reflector. As to why reflecting laminae of this kind should 

 reflect only or chiefly light of the same wave length (or colour) 

 as the light which produced the laminae when they are illumi- 

 nated by white light, he explains that light of a greater or less 

 wave length than that of the original incident ray will be out 



