204 Lord Rayleigh on the Remarkable Phenomenon 



" reflect little light at small angles of incidence, but at all 

 angles greater than about 10° they reflect light with a bril- 

 liancy which shows that the reflexion must be almost total. 



When the plate is turned round in its own plane, two 



positions are found, differing in azimuth by 180°, in which 

 the crystal reflects no more light than an ordinary crystal 

 under the same conditions. In these cases the plane of inci- 

 dence coincides with the plane of crystallographic symmetry." 



Mr. Madan worked with comparatively thick (1 millim.) 

 plates, from which the associated twin had been removed by 

 grinding. In repeating his experiments I found it more con- 

 venient to use thin plates, such as may be obtained without 

 difficulty from crystallizations upon a moderate scale, and 

 which appear to be free from twinning*. There seems to be 

 little doubt that the altered crystals are composed of twinned 

 layers. Except in respect of colour, there is no difference 

 between the behaviour of these crystals and that of the bril- 

 liantly iridescent ones described by Stokes. If light be inci- 

 dent at a small angle, and be polarized in or perpendicularly 

 to the plane of incidence, the polarization of the reflected 

 light is the opposite to that of the incident. 



The only difference that I should suppose to exist between 

 the constitution of these crystals and that of the iridescent 

 ones is, that in the former case the alternations are irregular, 

 and also probably more numerous. Mr. Madan conceives 

 that there are actual cavities between the layers in the heated 

 crystals, comparing them to films of decomposed glass |. 

 It is, however, certain that no closeness of contact could ob- 

 viate the optical discontinuity at a twin plane; and there is 

 besides a marked experimental distinction between the cases in 

 question. It is easy to observe, and was, I think, observed by 

 Brewster, that the application of water to a film of decom- 

 posed glass destroys the effect. The water insinuates itself 



* It is not clear why composite crystals free from included mother- 

 liquor should suffer disruption upon heating. A line drawn on the twin 

 plane would tend to expand equally, to whichever crystal it be considered 

 to belong. 



t "Although a large amount of light must escape reflexion at a single 

 cavity, yet if the transmitted rays encountered a large number of precisely 

 similar and similarly situated cavities at slightly lower levels in the 

 crystal, the sura of the partial reflexions would produce an effect almost 

 equivalent to a total reflexion of the original incident ray, and a corre- 

 sponding deficiency in the amount of light transmitted through the whole 

 plate. The brilliancy of the colours in the light reflected from the well- 

 known films of decomposed glass is accounted for in precisely the same 

 way, and the successive separate films of glass can be easily seen under a 

 microscope at the edges of the compound film, where they only partially 

 overlap." 



