378 



C. G. KNOTT 



in the exquisite purity of tlie smaller ono, which forms the suhjc-i- of 

 the present note. 



The exact dimensions of the latter are as follows. 

 Diameter, 15.08 cms, or 5.94 inches. 

 Volume, 179n c. cms. or 109.6 c. inches. 



These numbers are correct certainly to the third significant 

 figure, with a probable error of ± 1 in the fourth. Within the 

 errors of observation, the crystal was foumed to be a ])erfect spliere. 

 The mass measurements are as follows: 



True Mass, 4650.8 grms. or 10.250 lbs. Specific flravity, 2.59. 



It is impossible in words fitly to describe the exquisitely clear 

 transparency of the crystal. It was absolutely colourless and free 

 from any surface flaw or internal blemish. Shining in all lights with 

 a peculiar lustre, it was an object fitted to charm every eye, and 

 more especially, because of its perfect sphericity, the eye of the mathe- 

 matician. The inverted image of the landscape as viewed through it 

 appeared beautifully defined, only a slight indistinctness arising from 

 the superposition of the two images due to double refi-ariioii. It was 

 this optical property indeed which alone enabled me to distinguish 

 definite directions in the crystal, the two images coinciding along one 

 unique diameter and also along every other diameter at right angles 

 thereto. 



Several distinct ex{)eriments gave as a good mean for the mean 

 refractive index, 1.56. 



Resting the sphere on a sheet of finely divided section paper, and 

 rolling it about until the two images of any line sliowed widest apart, 

 I w;is able to mahc a fairly good estinvate of the ratio of the indices 

 of refraction of the extraordinary and (ordinary rays It is doubtful 

 if as good a value was eyei- obtained by so simple a method. 



