AS EXPLAINED BY THE HYPOTHESIS OF FINITE INTERVALS. 16? 



Which, both on account of F^r) being negative, and of every ^z being 

 greater than the corresponding ^x or hj, is greater than the former • 

 whence we conclude that a wave is transmitted with the greatest velo^ 

 city when it passes in the direction of the pressure. 



The piece of glass wUl consequently be analogous to a negative 

 crystal, (Vid. Ency. Metrop. Light. Art. 803.) the direction of pressure 

 coiTespondmg to that of the axis of the crystal. 



In nice manner, had the glass been dilated the velocity would have 

 been least along the axis, and the properties of a positive crystal would 

 have been exhibited. Now by reference to the Transactions of the 

 Royal Society for 1816, p. 158, Sir David Brewster informs us that 

 he arrived at the following conclusion by experiment: 



"When a piece of glass is under the influence of a compressing 

 force, its structure is the same as that of one class of doubly refracting 

 crystals, including calcareous spar, beryl, &c. (negative); but when it 

 IS under the influence of a dilating force, its structure is the same as 

 that of the other class of doubly refracting crystals, including sulphate 

 of hme, quartz, &c. (positive)." 



Here then our formula gives results coincident with experiment. 



This can, however, be considered only as a mere test of the general 

 accuracy of our deductions; and a more satisfactory mode of exami- 

 nation will be obtained when we apply them to those substances of 

 which we know accurately the refractive indices for different colours. 



We have seen that the square of the velocity is equal to 



T7<i . Sin- — - 



i2{,^(r)+£A^^^^} 2l- 



r it' 



\= 



