184 



Prof. G. G. Stokes. On a remarkable [Feb. 26, 



fact in some of the crystals which showed the more complicated 

 spectra, and which were broken across, I was able to make out under 

 the microscope the existence of a system of more than two twin plans, 

 close together. Restricting ourselves to what may be regarded as the 

 normal case, we have then to inquire in what way the existence of 

 two twin planes near together can account for the peculiar character 

 of the spectrum of the reflected or transmitted light. 



Section II. — Of the Proximate Cause of the Phenomenon. 



20. Though I am not at present prepared to give a complete explana- 

 tion of the very curious phenomenon I have described, I have thought 

 it advisable to bring the subject before the Society, that the attention 

 of others may be directed to it. 



That the seat of the coloration is in a thin twin stratum, admits I 

 think of no doubt whatsoever. A single twin plane does not show 

 anything of the kind. 



For the production of the colour the stratum must be neither too 

 thick nor too thin. Twin strata a good deal thicker than those that 

 show colour are common enough ; and among the crystals sent to me 

 I have found some twin strata which were a good deal thinner, in 

 which case the crystal showed no colour. 



The more complicated spectra which are frequently observed seem 

 referable to the existence of more than two twin planes in close 

 proximity. There is no reason to think that the explanation of these 

 spectra would involve any new principle not already contained in the 

 explanation of the appearance presented when there are only two twin 

 planes, though the necessary formulas would doubtless be more com- 

 plicated. 



Corresponding to a wave incident in any direction, in one com- 

 ponent of a twin, on the twin plane, there are in general two refracted 

 waves in the second component in planes slightly inclined to each 

 other, and two reflected waves which also have their planes slightly 

 inclined to each other, the angle of inclination, however, being by no 

 means very small, as chlorate of potash is strongly double refracting. 

 The planes of polarisation of the two refracted waves are approxi- 

 mately perpendicular to each other, as are also those of the two 

 reflected waves ; but on account of the different orientation of the 

 two components of the twin, the planes of polarisation of the two 

 refracted waves are in general altogether different from those of the 

 incident wave and of its fellow, the trace of which on the twin plane 

 would travel with the same velocity. In the plane of symmetry at 

 any incidence, and for a small angle of incidence at any azimuth of 

 the plane of incidence, the directions of the planes of polarisation of 

 the two refracted waves agree accurately or nearly with those of the 



