Notices respecting JSew Books. 217 



it will be seen that it is not altogether easy for an ignorant person 

 to obtain readily the information he may want. We have found 

 information as to the nature of Alternating Currents under a great 

 number of heads with much unnecessary repetition, in many cases 

 at a considerable length and sometimes in a very misleading form. 

 It would be rash to express an opinion as to the value of this 

 book. Scientific men may be interested in knowing what an 

 Oerstedt was and what a Henry may be ; they may be so interested 

 in seeing three definitions of a Gauss as to try to verify one of 

 them, said to be Lord Kelvin's, which is obviously quite vague and 

 which therefore cannot be Lord Kelvin's. Practical engineers may 

 be able to obtain from it the sort of theoretical knowledge which is 

 supposed to be sufficient for them. At all events there seems to 

 be a reference to every conceivable electrical term or contrivance 

 and the book is well got up. 



The Optical Indicatrix and the Transmission of Light in Crystals. 



By L. Fletcheb, M.A., F.B.S., Keeper of Minerals in the British 



Museum. Oxford University Press. Pp. 112. 

 This is a remarkably clear application of geometry to the phenomena 

 of double refraction and the passage of light through crystals in 

 general. It is written with the view of simplifying optics for the 

 benefit of students of crystallography, and this aim it achieves in 

 a successful manner. 



But, as usual when a science comes to be applied, even though 

 it be only applied to the needs of another branch of science, the 

 benefit is reciprocated and the whole subject gains in clearness and 

 simplicity. So it is here. Mr. Fletcher observes that every detail 

 about a ray of light — its direction, its velocity, and its polarization- 

 plane, can be represented by a single finite straight line and a 

 point anywhere off the line. The point is naturally a Huyghenian 

 centre of disturbance, and the direction of a ray is specified by the 

 perpendicular from the point on to the line. The velocity is a 

 function of the length of the line (simply inversely as the length, 

 as in momental ellipsoids, ellipsoids of elasticity, and other similar 

 representations) and the plane of polarization is normal to the line. 



This being premised, it is shown that in crystals there is a 

 characteristic surface, or " indicatrix," to which the finite straight 

 lines characteristic of the rays are all normal; and that in uniaxal 

 crystals this surface is a spheroid concentric with the centre of 

 disturbance. This spheroid is indeed the customary extraordinary 

 wave-surface of Huygbens ; but the strength ot: Mr. Fletcher's 

 treatment comes out when it is extended to biaxal crystals. The 

 wave-surface in biaxal crystals is known to be a complicated 

 affair ; admiration has been felt at its discovery by Fresnel, and 

 some wonder as to the way in which he arrived at it. In whatever 

 way arrived at, it certainly corresponds to fact, having always 

 served as a reliable guide, and led to verified prediction. But 

 it is not a simple matter, and the ordinarily recognized mode of 

 arriving at it is seriously difficult. 



But Mr. Fletcher generalizes his characteristic surface or 

 Phil. Mag. S. 5. Vol. 34. No. 207. Aug. 1892, Q 



