76 



HABIT AND INTELLIGENCE. 



[chap. 



Polarisa- 

 tion. 



rectangular prism appears again. The changes take place 

 not by the addition of new crystals, but by changing the 

 growth of the original ones. 



Most crystals, except those of metals and their alloys, 

 are transparent ; and most transparent crystals polarise 

 light. 



In what follows, I take the wave theory of light as 

 proved : and it results from that theory that transparency, 

 or the power of transmitting light, depends on optical 

 elasticity, and that polarisation is caused by the optical 

 elasticity being unequal in different directions.^ It has 

 been proved by mathematical reasoning on the laws of 

 the propagation of luminous waves through media of 

 unequal elasticity in different directions (or, as they may 

 be called for brevity, unequally elastic media), that in every 

 such medium there are three directions, at right angles to 

 each other, in which the elastic force on which the propa- 

 gation of the waves depends, reacts in the direction of 

 the displacement. Two of these are also directions of 

 maximum and minimum elasticity. These three direc- 

 tions are called the axes of elasticity, and are distinguished 

 as axes of greatest, least, and mean elasticity. In polar- 

 ising crystals whereof the three crystallographic axes are 

 at right angles to each other, the axes of elasticity are 

 parallel to the crystallographic axes ; but this, of course, 

 is not the case where the crystallographic axes are not at 

 right angles. If now an ellipsoid is constructed with its 

 three axes proportional to the three axes of elasticity, 

 those diameters of the ellipsoid at right angles to which 

 the sections of the ellipsoid are circles will be directions 

 in which light passing along them will not be polarised : 

 while light passing through the crystal in any other direc- 

 tion will be polarised. The directions of no polarisation 

 Optic axes, are called optic axes. It follows from the properties of 

 ellipsoids, that when the three axes of elasticity are all 

 equal, the ellipsoid is a sphere ; consequently all diameters 



Axes of 

 elasticity, 



Ellipsoid 

 of elas- 

 ticity. 



^ My cMef authorities for what follows on the optical properties of 

 crystals are Dr. Lloyd, on the Wave Theory of Light ; Pereira's Lectures 

 on Polarised Light ; and Mr. Maskelyne's Lecture referred to above. 



