5i8 SCIENTIFIC APPARATUS. 



variations in the absorption of light of various colours, according 

 to the directions of their vibration in traversing a crystal. The 

 application to this instrument of the methods of spectroscopic 

 investigation has yet to be carried out. 



But the recognition that in its physical characters the crystal 

 obeys similar laws of symmetry to those which control its form 

 was not confined to the case of its action on light or heat, or the 

 changes it undergoes with change of temperature. One by one 

 each of the recognised manifestations of force, from the ruder 

 forms of mechanical impact and strain to the subtle influences 

 induced by magnetism and electricity, fell under the law, until it 

 came to be seen that, so far, the symmetry of the crystal belongs 

 to it in all its attributes, as that a plane of symmetry to the form 

 of the crystal was a plane of symmetry to all symmetrically corre- 

 sponding directions in it for physical characters ; that is, as regards 

 the modifications the crystal-matter induces or itself receives under 

 the influence of any force soliciting ic. And some of the most 

 remarkable aberrations from the formal principles of symmetry 

 characteristic of a system, in the abeyance and suppression of 

 particular morphological features, have been found to be echoed, 

 as it were, in corresponding peculiarities in crystallo-physical 

 characters. Among these may be instanced the phenomena of 

 pyro-electricity and of rotatory polarisation of light. For estab- 

 lishing these properties of the crystal, observations of the most 

 subtle kind, and physical apparatus proportionately delicate, have 

 had to be employed. They comprise instruments for determining 

 with exactitude changes in the thermic, electric, and magnetic 

 conditions of the crystal, its hardness in different directions 

 (sclerometers), its cohesive power, its mechanical elasticity, and 

 the magnitude and direction of its axes of elasticity. And much 

 of this work has yet to be done, particularly in the more mechanical 

 sections of it. 



Hence in passing from optical to other forms of crystallographic 

 research in the domain of physics, such as those dealing with 



