78 HABIT AND INTELLIGENCE. [cHAP. 



are arranged symmetrically around it, like the petals 

 and other parts of a regular, or circular, flower around 

 its axis, 

 Ehombo- In the rhombohedral system, the parameters are equal : 



system. ^^^ ^^^^ ^^® ^^^ ^^ right angles, but each of them is in- 

 clined to the other two at the same angle. There is an 

 axis of symmetry, which however is not coincident with 

 any of the crystallographic axes, but equidistant from the 

 three ; it is parallel to the line connecting those two 

 opposite angles of the rhombohedron, which are similar to 

 each other and unlike all the rest. 

 Optical In the square prismatic and rhombohedral systems, the 



proper les. ^p^^gg^]^ elasticity is represented by a spheroid of rotation, 

 with its axis parallel to the crystal's axis of symmetry : 

 this is consequently the optic axis, and is the same in 

 position for rays of all colours.^ Biit for different colours 

 it is generally, though not in all cases, different in mag- 

 nitude ; the physical interpretation of which is, that the 

 amount of dispersion or angular separation of the rays 

 in double refraction is different for rays of different 

 colours. 

 Right In the right prismatic system, the axes are all at right 



system."' angles, and the parameters unequal. The axes of elasticity 

 are parallel to the crystallographic axes, and, like the 

 parameters, are all unequal : consequently the optical 

 elasticity is represented by an ellipsoid which is not a 

 spheroid of rotation, and there are two optic axes, neither 

 of which coincides with any axis of elasticity. The axes 

 of elasticity are the same in position for all colours, but 

 different in relative magnitude : as a consequence of which 



1 We must apply some qualification to the statements in the text, that 

 cubic crystals have no polarising structure, and that square prismatic and 

 rhombohedi-al ones are uniaxial. Sir David Brewster has found that many 

 cubic crystals have traces of polarising structure (Philosophical Transac- 

 tions, vol. ciii.) ; and Professor Breithaupt has found most square, prismatic 

 and rhombohedral crystals to be biaxial ; but he is of opinion that this is 

 due to want of symmetry in the crystals, and that the statements in the text 

 (which are those made in all works on mineralogy) are true of perfectly 

 formed specimens, which, however, are rare in nature. (Philosophical 

 Magazine, June 1860.) 



