﻿68 Prof. R. W. Wood on Interference Colours 



Lord Rayleigh has shown that the peculiarity o£ the 

 crystals consists in the occurrence of a large number of twin 

 planes, parallel to the surfaces of the flake, the narrowness 

 of the reflected band resulting from multiple reflexions, as in 

 the Fabry and Perot interferometer. 



Tn the present paper the behaviour of the crystals in the 

 infra-red and ultra-violet regions will be considered. 



Stokes observed that many crystals reflected two or even 

 three narrow bands in the visible spectrum, and inferred 

 that, in these cases, there must be several sets of multiple 

 twin planes. It seems curious that he was led to adopt this 

 Hypothesis, as the more obvious explanation would be to 

 refer the colours to different orders. 



A thin film, or a number of parallel thin films of equal 

 thickness D, reflect at normal incidence light of wave-lengths 

 given by the equation 2D = 0, 2D = X, 2D = 3\, 2D = 4\, &c, 

 if we consider no phase change to occur. If 2D = '0006 mm. 

 the film will reflect red light, and ultra-violet light of wave- 

 lengths -00030 and '00020. If, however, 2D 'is equal to 

 some wave-length in the infra-red region, say at "0012 mm. 

 (1*2 //-), it is clear that we shall have more than one reflected 

 colour in the visible region, the maxima occurring at wave- 

 lengths -0006, -0004, '0003, -00024, &c. Such a film should 

 appear purple, since it reflects red and violet light. If the 

 first-order colour is at 2 jjl, the higher orders will fall at 1 fi, 

 '66 At, *50 £t, '40/*, <fec, and the films will reflect narrow 

 regions in the red, green, and violet. 



We can moreover, by measuring the wave-lengths of the 

 bands in the visible and ultra-violet spectrum, calculate the 

 position of the first-order band in the infra-red. An ex- 

 amination of the crystals by transmitted light, derived from 

 a cadmium spark, with a small quartz spectrograph, showed 

 that they were perfectly transparent down to the very end 

 of the spectrum *. 



A number of the films were then mounted in glycerine 

 between two quartz prisms, by which device all light except 

 that reflected from the equidistant laminae is eliminated. 

 The colours appear much more saturated when the films are 

 thus mounted, for the light reflected from their uppsr and 

 lower surfaces is not destroyed by interference, and dilutes 

 the selectively reflected colours. This circumstance may be 

 due to the fact that the distances of the surfaces from the 

 neighbouring twin planes is not equal to the distances between 



* This property is worth remembering, as a thin lamina, or plate, 

 transparent to ultra-violet light is sometimes required in optical work ; 

 they are very fragile however, and require careful handling. 



