Proceedings. 187 



Ordinary Meeting, April 1st, 1890. 



Professor Osborne Reynolds, M.A., LL.D., F.R.S., 

 President, in the Chair. 



Dr. Alexander Hodgkinson read anote on "Diamonds 

 and double refraction." He pointed out that a distinguishing 

 feature of the diamond, according to numerous popular 

 works on the subject, is said to be that it does not exhibit 

 double refraction or possess the power of polarising light. 

 Having been working with diamonds in connection with an 

 investigation of " Newton's rings," he had noticed that this 

 was not correct. By the kindness of Messrs. John Hall 

 and Co., of King Street, Manchester, he had had the 

 opportunity of examining a very considerable number of 

 large and small diamonds from various localities, which 

 showed that, so far from diamonds of undoubted authenticity 

 being necessarily single refracting, 63 per cent of the stones 

 exhibited marked double refraction. Thus, of 1 5 diamonds 

 from the Jagersfontein Mine, eight were double refracting ; 

 and of five specially large diamonds from the same mine 

 five exhibited the phenomenon. Of five ordinary sized 

 diamonds from the Kimberley Mine five were double 

 refracting, and of five from another mine four showed the 

 same characteristic. The phenomen was shown to have 

 some relation to the cavities common in diamonds. It 

 allies itself rather with the state of glass under pressure 

 than with the usual double refraction in crystals, and seems 

 .connected with inequality in the state of internal stress. 

 A similar condition can be shown, as was demonstrated, in 

 other naturally single refracting bodies, by rapid cooling or 

 by compression. The important practical inference is that 

 , a body which exhibits double refraction or polarisation is 

 M 



