186 W. D. HALLIBURTON. 



test, with the result that no depolarisation of the light can be 

 detected, when they are examined with the apparent basal 

 plane perpendicular to the axis of the instrument and rotated ; 

 nor when a quartz plate is inserted do they produce any modi- 

 fication of the tint, as the stage is turned. The instrument 

 used was a Zeiss polarising microscope. 



Hence the presumption is that they belong to the hexagonal 

 system, as rhombic crystals with hexagonal habit or rhombic 

 twins would produce some double refraction examined in this 

 way. 



I submitted the question as to whether this was conclusive 

 to Professor Lewis, of Cambridge, and he kindly wrote to me 

 in answer as follows : 



" The observation under the microscope between crossed 

 nicols, so far as it goes, is rather in favour of the crystals 

 being hexagonal, that is, presupposing that the field remains 

 dark when the crystal is rotated in the field of view. However, 

 this is not quite conclusive, and in such cases greater certainty 

 would be obtained if the crystals were placed under a Ber- 

 trand's polarising microscope, to see the shape of the inter- 

 ference rings and cross/' 



It should be here stated that uniaxal crystals in the direction 

 of their optic axis exhibit a symmetrical cross and circular 

 rings ; in biaxal crystals the rings are oval, or at any rate not 

 circular, and the cross is not symmetrical. This is the case, 

 because the resistance to displacement in the three cardinal 

 directions called the axes of elasticity are all unequal in biaxal 

 crvstals. This is true, not only for the crystalline substance 

 itself, but also for the luminiferous ether that pervades it. 1 



Acting on Professor Lewis's advice, I submitted the crystals 

 to Professor Judd, who with Mr. Fletcher's co-operation 

 examined them, and gave me the following report, for which I 

 am much indebted to him : — "I have every reason to believe 

 the crystals belong to the hexagonal system from their form, 

 and their extinction between crossed nicols. I regret, however, 



1 The cardinal directions are, however, believed not to be the same for the 

 ether as for the material of the crystal. 



