502 On Velocities of Normal Propagation, SfG, [June 20, 



On the whole, however, I prefer to regard Fresnel's theory as a close 

 first approximation to the truth, and to look to the phenomena of 

 dispersion to explain the variation from it. 



I may perhaps be allowed to close with a suggestion, which, judging 

 from the results of a few experiments I have already made, appears to 

 have some basis of truth. 



Let us suppose that in a crystal 



, b . c . 



* =a+ + • 



where a, b, c, &c, are functions of the directions of vibration and 

 propagation. 



Let us suppose that for waves of infinite length Fresnel's construc- 

 tion is true, so that a is a radius vector of Fresnel's surface of wave 

 slowness, and can therefore be calculated, and suppose we neglect the 



terms &c. Observing the values of /x in this direction for different 

 rays, we get 



b 



A.1 



{ _ = b_ 



So that 



A 3 



[io,—a \i 



The results of experiments on the rays C, D, and F, in two different 

 directions give 



H= 1-2403. 



V 



fi-Q—a f 1*2875 (1st experiment). 



=a ll-S 



ju c — a Ll-2770 ( 2nd direction) 

 X »= 1-46978. 



/tp— a _ f 1*47208 (1st direction). 

 /i D -a 1 1-47348 (2nd direction). 



These numbers, especially the last, are sufficiently close to make it 

 worth while continuing the observations. 



The Society adjourned over the Long Vacation to Thursday, No- 

 vember 21. 



