114 Prof. S. P. Thompson on the 



ticity " (i. e. dielectric properties), but being frittered down 

 into electric currents (and ultimately into heat) when these 

 vibrations pass into conducting media. Good insulators 

 should therefore be transparent, and good conductors opaque, 

 to light. If it can be shown that tourmaline crystals are better 

 conductors of electricity in one direction than another, it can be 

 deduced as a consequence of MaxwelVs theory that they will be 

 better transmitters of light in one direction than in another, and 

 that they will absorb more of those rays of light whose vibrations, 

 consisting of electric displacements, lie in the direction of best 

 electric conductivity. 



The present paper consists of three parts: — 



I. A resume of the Optical, Electrical, and Magnetic pro- 

 perties of Tourmaline Crystals. 



II. A Mathematical Theory of the Opacity of Crystals 

 which are electrically or magnetically seolotropic. 



III. A discussion of the Electric Conductivity of Tourma- 

 line Crystals, and of the Experimental methods of observing it. 



I. Physical Properties of Tourmaline Crystals. 



5. Optical Properties. — Tourmaline belongs to the rhombo- 

 hedral system of crystals, and is therefore optically a uniaxal 

 crystal. Its refractive indices are (approximately) 



Ordinary ray 1*6406, 



Extraordinary ray 1*6212; 



and it is therefore a negative crystal. Tourmalines are of 

 various colours, occasionally colourless, still more rarely pink 

 (rubellite), but frequently bluish-green, green, olive, or brown. 

 The commonest of all kinds is of a brownish tint when cut 

 into very thin slices, but appears exteriorly jet-black. Those 

 of bluish-green or green hue are, according to M. Mascart, 

 the most strongly pyroelectric; and of these the most transpa- 

 rent are more highly pyroelectric than the opaque varieties. 

 In ordinary light a coloured tourmaline is always found to be 

 much more opaque to rays traversing it in a direction parallel 

 to its axis than to rays traversing it at right angles to its axis, 

 equal thicknesses being considered. Tourmaline possesses the 

 property of polarizing partially or wholly the light which it 

 transmits. Its action on light is usually explained by saying 

 that a ray of ordinary light incident upon the crystal is divided 

 into two portions, the ordinary and the extraordinary ray, 

 which in passing through the crystal are unequally absorbed. 

 If a slice of the crystal be taken whose parallel faces are prin- 

 cipal planes of section, and which is of a suitable thickness, 

 the ordinary ray will be virtually suppressed, whilst the extra- 



