Studies of iridescent colour, and the structure producing it.—I. The colours of potassium chlorate crystals

It is known that in the crystallisation of potassium chlorate from solution, crystals are occasionally found which show a brilliant iridescence. The subject was investigated by Stokes and by my father, the late Lord Rayleigh.The latter was the first to conclude that the character of the reflexion required that it should come from a large number of reflecting surfaces at regular intervals. These hypothetical surfaces he regarded as twin planes. Only by the postulate of a large number of surfaces can we account for the highly selective character of the reflexion, limited in many cases to a narrow band in the spectrum. (See Plate 6, fig. I. The sodium and lithium lines are placed on the plate for comparison.) He wrote (loc. cit .): “It would undoubtedly be far more satisfactory to speak of the periodic structure as a matter of direct observation, and it is to be desired that some practised microscopist should turn his attention to the subject.Ex hypothesi we could not expect to see the ruled pattern upon a section cut perpendicularly to the twin planes, as it would lie upon or beyond the microscopic limit. I have tried to detect it upon a surface inclined to the planes at a very small angle, but hitherto without success.”