104 Mr. Mashely?ic on the Crystal Molecule, [April 1, 



units of mass of the crystal, and would be arranged in the iuterior of 

 those molecules in a manner closely analogous to the arrangement of 

 the molecules in the crystal itself; which, as was before stated, may be 

 looked on (at any rate as an illustration to the mind of crystallo- 

 physlciil facts) as so co-ordinated in direction as that every plane in 

 which a series of the centres of gravity of the molecules lies, must be 

 (an actual or a possible) crystal plane. The mysterious ether assumed 

 and adopted by the undulatory theory plays its part within the mole- 

 cules and enshrouds its constituent chemical units of mass. 



On the one hand, then, axes, such as those of acoustic elasticity, 

 of magnetic induction, or thermic axes, belong to the crystal as a 

 whole, depending for their magnitude and direction on the statical con- 

 dition and the relative arrangement of the molecules inter se. The 

 axes of optical elasticity on the other hand will be the axes of elasticity 

 of the molecule itself, that is, of the ether within it. But thv^ elasticity 

 of tlie ether, as exhibited by the velocity with which a ray is propa- 

 gated in it along its different directions is controlled by the action of 

 tlie reposing ponderable units (or atoms at rest) on the ether particles 

 in motion. As long as the directions in which the ether is constrained 

 to vibrate, by reason of this perturbing action - its axes of elasticity — 

 are continuous throughout the crystal (as will be the case if the 

 neighbouring molecules are arranged inter se in the same symmetry as 

 the ether particles in any one of the molecules, so that the neighbour- 

 ing molecules do not distract those directions by a perturbing action 

 oblique to theirs,) so long will the influence of the material particles be 

 confined to the exercise of a constraint on the velocity of each wave, 

 whatever its length, i.e.^ colour ; but it will not cause any dispersion 

 of these. This condition of coincidence in direction of the crystal- 

 line and molecular axes occurs only in the rectangular systems ; 

 and in the biaxial ones, therefore, only in the Prismatic system. 

 But the coincidence is confined to direction, neither the magnitude of 

 the axes, nor their orientation (as had been shown) presenting any 

 identity. 



In the oblique systems there is no reason to assume the obliquity in 

 the molecule as of the same inclination as that of the inclined axes of 

 the crystal. Thus these two sets of oblique axes cannot coincide in 

 direction. Hence the perturbing influence of the ponderable matter 

 oj)erates, by reason of this want of coincidence, in directions other than 

 those of the axes of the individual molecule, and extends its results to 

 the directions of vihratiori, that is to say, effects the dispersion of these 

 directions for different colours. In fact, for any light-wave now to 

 traverse the crystal without being broken up, it must take directions 

 neither coincident with the axes of elasticity of the molecule, nor with 

 the crystalline axes. These directions will have different orientation 

 for different colours), and, not being coincident with the thermic 

 axes, will vary with the temperature. They will be rectangular axes 

 — axes of a polarisation -ellipsoid, different for each colour — and will 

 indicate the directions of the principal planes of polarisation, which 

 umst be normal to each other, and will also prescribe the limits of the 



