212 Obituary Notices of Fellows deceased. 



here expressed his belief that the trae dynamical theory of double 

 refraction was yet to be found. 



In 1885 he communicated to the Society his observations upon one 

 of the most curious phenomena in the whole range of Optics a peculiar 

 internal coloured reflection from certain crystals of chlorate of potash. 

 The seat of the colour was found to be a narrow layer, perhaps one- 

 thousandth of an inch in thickness, apparently constituting a twin 

 stratum. Some of the leading features were described as follows : 



(1) If one of the crystalline plates be turned round in its own 

 plane, without alteration of the angle of incidence, the peculiar 

 reflection vanishes twice in a revolution, viz., when the plane of inci- 

 dence coincides with the plane of symmetry of the crystal. 



(2) As the angle of incidence is increased, the reflected light becomes 

 brighter, and rises in refrangibility. 



(3) The colours are not due to absorption, the transmitted light 

 being strictly complementary to the reflected. 



(4) The coloured light is not polarised. 



(5) The spectrum of the reflected light is frequently found to con- 

 sist almost entirely of a comparatively narrow band. In many cases 

 the reflection appears to be almost total. 



Some of these peculiarities, such, for example, as the evanescence of 

 the reflection at perpendicular incidence, could easily be connected with 

 the properties of a twin plane, but the copiousness of the reflection 

 at moderate angles, as well as the high degree of selection, were highly 

 mysterious. There is reason to think that they depend upon a 

 regular, or nearly regular, alternation of twinning many times repeated. 



It is impossible here to give anything more than a rough sketch of 

 Stokes' optical work, and many minor papers must be passed over 

 withoat even mention. But there are two or three contributions to 

 other subjects as to which a word must be said. 



Dating as far back as 1857 there is a short but important discussion 

 on the effect of wind upon the intensity of sound. That sound is 

 usually ill heard up wind is a common observation, but the explanation 

 is less simple than is often supposed. The velocity of moderate 

 winds in comparison with that of sound is too small to be of direct 

 importance. The effect is attributed by Stokes to the fact that winds 

 usually increase overhead, so that the front of a wave proceeding up 

 wind is more retarded above than below. The front is thus tilted ; 

 and since a wave is propagated normally to its front, sound proceeding 

 up wind tends to rise, and so to pass over the heads of observers 

 situated at the level of the source, who find themselves, in fact, in a 

 sound shadow. 



In a more elaborate memoir (1868) he discusses the important 

 subject of the communication of vibration from a vibrating body to 



