100 Mr. Maskelyne on the Crystal Molecule [April 1, 



But it is to Light that we are to look as the most subtle instrument 

 for aiding our reason in scrutinizing the inner nature, or most intimate 

 structure of the molecular system, which we call a crystal. It seems 

 a true prerogative of light to do this. The frequent transparency, 

 and the varied colours in different directions of so many crystals ; and 

 the changes which the light is subject to within the crystal, its polarisa- 

 tion, absorption, fluorescence, and the other modifications it undergoes, 

 all point to this most subtle agency as a discriminative power the best 

 adapted for our purpose. 



In any of the systems under consideration, the light, on entering a 

 crystal, is, except along certain directions, divided and polarised. The 

 two polarised rays into which it becomes thus divided pursue new and 

 different paths in the crystal, each ray differing from the other in the 

 velocity of its propagation, the crystal retarding the progress of the 

 ray vibrating in one plane, more than it does the ray vibrating in a 

 plane polar to this. The spheroids, or ellipsoids of elasticity before 

 alluded to present admirable geometrical expressions for the degree 

 and relative amounts of the retardations effected, and for the direc- 

 tions of the planes of vibration thus induced by the crystal on the 

 waves of light. Without entering into an explanation of the polarisa- 

 tion of light, or its precise relations to the directions of optical elasticity 

 in the crystal, it was deemed enough to remark that parallel to one 

 direction the sections of the spheroid are circles, and that the ( locus 

 of, or) line formed by the consecutive centres of these circles is the 

 morphological as well as the optical axis of all crystals belonging to 

 those systems, whose elasticity can be represented in magnitude and 

 direction by a spheroid (the pyramidal — rhombohedral.) In the pris- 

 matic, (and approximately in the clinohedric systems,) whose elasticity 

 is represented by an ellipsoid, there are two circular sections that may 

 be made through the centre of the ellipsoid of elasticity. 



In either case lines perpendicular to these circles are the " optic 

 axes " of these systems, that is to say, are directions along which a ray 

 goes with only one velocity, and is, therefore, not broken up and 

 polarized ; as can be demonstrated by a very simple geometrical con- 

 struction. The spheroidally elastic systems have, therefore, one optic 

 axis — one only direction along which light passes unchanged {uniaxial 

 systems). The systems whose optical elasticity is represented by ellip- 

 soids have two such directions {biaxial system s). Moreover, in the 

 latter the plane of these can be readily shown to lie necessarily in the 

 same plane as contains the greatest and least axis of the ellipsoid, i.e. 

 the greatest and least axis of elasticity. 



The ^rst mean line of the optic axes in biaxial crystals, is the line 

 bisecting the acute angle formed by the optic axes. In prismatic 

 crystals it is, according as the mean elastic axis is proportionately 

 small or large, either the greatest axis of optical elasticity (optically 

 negative crystal), or the least (an optically positive crystal). The 

 second mean line is the axis bisecting the obtuse angle, formed by the 

 optic axes: The law of the prismatic system regarding the position of 

 the optic axes for different colours, appears to be that the first mean 



