300 



DR. L. N. G. FILON ON THE 



where /IT denotes the index of refraction of the ray vibrating along the direction in 

 which the principal stress (tension being considered positive, as usual) is T. 



Since the direction of vihration if we take the electric force to be the light- vector 



is perpendicular to the direction of polarization, and the index of refraction is 



inversely proportional to the velocity, this means that a positive C implies a higher 



i 



-i-o 



-5 



-1-0 



-\-5 



-1-0 



1-0 



-5 -5 



VALUES OF(\-k p )/a. 



Fig. 11. Diagram of the curve y = [(X" - V) 2 - <* 2 V] [(* 2 - V) 2 + " 2 V]~ 2 - 



velocity in the ray polarized in the direction of greatest stress. Now this ray 

 corresponds to the ordinary ray if the glass be compared with a uniaxial crystal 

 whose optic axis is parallel to the direction of greatest stress. 



The glass, therefore, produces the same effect as a positive uniaxial crystal so 

 placed. 



This is, in fact, what does occur. This definition, therefore, agrees with the 

 earlier one. 



Now, since (for this glass) X, C, p. are all positive, it follows that if we are to have 

 a maximum followed by a minimum, as in fig. 10, dQ/dH < 0. 



Now, in Q = NeV/wiTr, since r does not change, the only quantity which can 

 change with the polarization is N, the number of electrons per unit volume. 



dQ/cTT is then eV(N 3 -N 1 )/7rw(T !1 -T 1 ), where N 2 , N t are the number of electrons 

 of period T which respond to vibrations in the directions of T 2 and TI. 



Our result will, therefore, signify that a tensional stress decreases the number of 

 electrons which respond to vibrations in the direction of the stress, relatively to the 

 number responding to vibrations in a perpendicular direction. In other words, tension 

 appears to tend to set the electrons vibrating in a plane at right angles to the line of 

 stress, pressure having the reverse effect. 



It seems probable that if this effect were due to an absorption band, a glass 



