1889.] The Right Hon. Lord Bayleigh on Iridescent Crystals, 4:41 



WEEKLY EVENING MEETING, 

 Friday, April 12, 1889. 



Sir Frederick Bramwell, Bart. D.C.L. F.R.S. Honorary Secretary 

 and Vice-President, in the Chair. 



The Right Hon. Lord Eayleigh, M.A. D.C.L. LL.D. F.R.S. M.R.L 



PUOFESSOK OF NATURAL PHILOSOPHY, R.I. 



Iridescent Crystals. 



The principal subject of the lecture is the peculiar coloured reflection 

 observed in certain specimens of chlorate of j^otash. Reflection implies 

 » high degree of discontinuity. In some cases, as in decomposed 

 glass, and probably in opals, the discontinuity is due to the inter- 

 position of layers of air ; but, as was jJi'oved by Stokes, in the case of 

 3hlorate crystals the discontinuity is that known as twinning. The 

 5eat of the colour is a very thin layer in the interior of the crystal 

 and parallel to its faces. 



The following laws were discovered by Stokes : — 



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

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

 tion vanishes twice in a revolution, viz. when the plane of incidence 

 3oincides with the plane of symmetry of the crystal. [Shown.] 



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

 )ecomes brighter and rises in refrangibility. [Shown.] 



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

 )eing strictly complementary to tlie reflected. 



(4) The coloured light is not polarised. It is produced indifibr- 

 ntly, whether the incident light be common light or light polarised 

 ci any plane, and is seen whether the reflected light be viewed directly 

 r through a Nicol's prism turned in any way. [Shown.] 



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

 onsist almost entirely of a comparatively narrow band. When the 

 ngle of incidence is increased, the band moves in the direction of 

 acreasing refrangibility, and at the same time increases rapidly in 

 'idth. In many cases the reflection appears to be almost total. 



In order to project these phenomena a crystal is prepared by 

 .menting a smooth face to a strip of glass, whose sides are not quite 

 arallel. The white reflection from the anterior face of the glass can 

 len be separated from the real subject of the experiment. 



A very remarkable feature in the reflected light remains to be 

 oticed. If the angle of incidence be small, and if the incident light 



