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XXVI. On the Remarkable Phenomenon of Crystalline Re- 

 flexion described by Prof. Stokes. By Lokd Kayleigh, 

 Sec. E.S* 



THE phenomenon in question is that exhibited by certain 

 crystals of chlorate of potash, consisting of a peculiar 

 internal coloured reflexion. The following, stated very 

 briefly, are its leading features as described by Stokes f : — 



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

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

 peculiar reflexion vanishes twice in a revolution, viz. when 

 the plane of incidence 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 polarized. It is produced 

 indifferently whether the incident light be common light or 

 light polarized in any plane, and is seen whether the reflected 

 light be viewed directly or through a Nicol's prism turned in 

 any way. 



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

 to consist almost entirely of a comparatively narrow band. 

 When the angle of incidence is increased, the band moves 

 in the direction of increasing refrangibility, and at the same 

 time increases rapidly in width. In many cases the reflexion 

 appears to be almost total. 



Prof. Stokes has proved that the seat of the colour is a 

 narrow layer, about a thousandth of an inch in thickness, in 

 the interior of the crystal ; and he gives reasons for regarding 

 this layer as a twin stratum. But the phenomenon remains 

 a mystery. " It is certainly very extraordinary and para- 

 doxical that light should sutler total or all but total reflexion 

 at a transparent stratum of the very same substance, merely 

 differing in orientation, in which the light had been travelling, 

 and that, independently of its polarization." 



From the first reading of Prof. Stokes's paper, I have been 

 much impressed with the difficulty so clearly set forth. It 

 seemed impossible that a combination of two surfaces merely 

 could determine either so copious or so highly selected a 

 reflexion. If light of a particular wave-length is almost 

 totally reflected, what hinders the reflexion when the wave- 



* Communicated by the Author, 

 t Proc. Roy. Soc. Feb. 1885. 



