260 Lord Rayleigh on the Remarkable Phenomenon 



then sensibly but one wave reflected at the first twin plane. 

 On the arrival of the transmitted wave at the hinder surface 

 of the twin stratum, a second reflexion ensues, similar to the 

 first, except for the reversal of phase due to the altered 

 circumstances. The relation to one another of the two 

 reflected waves is exactly the same as in the ordinary theory 

 of thin plates, and does not appear to admit of the produc- 

 tion of anything unusual. I think we may even go further, 

 and conclude that in conformity with our theory it is im- 

 possible to find an explanation of the brilliant and highly 

 selected reflexion, unless upon the supposition that there is a 

 repeated alternation of structure. 



The optical evidence in favour of the view that there are 

 a large number of twin planes thus appears to be very 

 strong ; the difficulty is rather to understand how such a 

 structure can originate. And yet if we admit, as we must, 

 the possibility of the formation of one twin plane, and of two 

 twin planes at a very small distance asunder*, there seems 

 nothing to forbid a structure regularly periodic, which may 

 perhaps be due to causes vibratory in their nature. 



It would undoubtedly be far more satisfactory to be able 

 to speak of the periodic structure as a matter of direct 

 observation, and it is to be desired that some practised micro- 

 scopist 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. 



In the absence of complete evidence it is proper to treat 

 the views here put forward with a certain reserve ; but it is 

 perhaps not premature to consider a little further what may 

 be expected to result from a structure more or less regular. 

 If the periodicity be nearly perfect, the bright central band 

 in the spectrum would be accompanied by subordinate bands 

 of inferior and decreasing brilliancy. If the angle of inci- 

 dence be small, so that the aggregate reflexion is but feeble, 

 each stratum may be considered to act independently, and 

 the various reflected waves to be simply superposed. The re- 

 sultant intensity will depend of course upon the phase 

 relations. At the centre of the band the partial reflexions 

 agree in phase, and the intensity is a maximum. As we 

 leave this point in either direction, the phases begin to 



* This is the simplest supposition open to us, when, as in most of the 

 coloured crystals, the parts on either side of a very thin lamina are 

 similarly oriented. 



