﻿Determination of the Optical Constants of a Crystal. 29 



and sufficient to assume that the electron expands at a certain 

 slow rate (§ 4). 



(2) In order to satisfy the principle of the conservation of 

 energy it is necessary to assume that the expansion is 

 resisted hy an internal stress, which in the simplest case is a 

 hydrostatic pressure (§5). 



(3) In order to explain this pressure in accordance with 

 the spirit of the electron theory, that is, without action at a 

 distance, we mu?t assume that it is due to the action of the 

 aether on the electron (§§ 6, 7). 



(4) The reaction of the electron on the aether produces a 

 pressure in the a?ther at the surface of the electron ; if this is 

 not exactly balanced by the pressure at infinity the electron 

 expands, presses back the aether and acts like a source in a 

 liquid. Hence electrons gravitate towards each other, as do 

 sources in a liquid (§ 8). 



III. The General Determination of the Optical Constants of 

 a Crystal by means of Refraction through a Prism. B;i 

 G. F. Herbert Smith, M.A., Assistant in the Mineral 

 Department of the British Museum*. 



BY far the simplest method of obtaining the three principal 

 indices of refraction of a crystalline substance is that 

 of total-reflexion, since observations need be made on only 

 one face or section of the crystal; but this method is neces- 

 sarily restricted to substances of lower refractive power than 

 the constant medium (usually a hemisphere of dense glass) or 

 the liquid used to obviate the intervention of a film of air. 

 In other cases, however, recourse must be had to refraction 

 through a prism formed by a pair of natural or artificial 

 faces on the crystal, inclined to one another at not too large 

 an angle. If possible, it is arranged that the plane bisecting 

 the angle between these faces is one of symmetry in the 

 crystal, so that the method of minimum deviation and the 

 corresponding simple formula are available. But it is often 

 the case in natural crystals that advantage cannot betaken of 

 this simple method : the crystal may not have suitable natural 

 faces inclined at a small enough angle, and may be so small 

 itself that artificial faces cannot be ground on to it. TVe 

 propose to show that, even in such cases, it is possible by 

 taking observations sufficiently far round a zone to determine 

 the optical constants. 



* Communicated by the Author. The substance of the paper was 

 discussed by him at meetings of the Mineralogical Society, June 19, 1900. 

 and November 17, 1903. 



