218 Intelligence and Miscellaneous Articles. 



indicatrix as applied to uniaxal crystals, in the simplest possible 

 way, by assuming that in a biaxal crystal the surface to which the 

 finite straight lines specifying the rays have to be normal is an 

 ellipsoid with three unequal axes, instead of a spheroid. 



If this idea sustains itself it manifestly constitutes a beauti- 

 fully simple mode of stating every particular concerning rays in 

 a biaxal crystal. It does sustain itself, for it leads to Fresnel's 

 wave-surface in so straight and clear a manner that Mr. Fletcher 

 conjectures with some assuredness that this was the path by which 

 Fresnel really arrived at that famous result, and that his elaborate 

 but unsatisfactory quasi-dynamical structure was an afterthought 

 and backward development. 



It seems to the writer that Fresnel's version of an aether theory 

 was really based on his study of reflexion ; his treatment of double 

 refraction as ordinarily given must apparently have succeeded that ; 

 but it now seems exceedingly probable, as Mr. Fletcher surmises, 

 that the substantial truths in his double-refraction results were 

 arrived at, not by his dynamical hypotheses, but by the simple 

 geometrical method now rediscovered. 



Mr. Fletcher's treatment is independent of theories of light ; 

 nevertheless he points out how untenable the theory of vibration in 

 an incompressible elastic aether is, and that the optical properties 

 of crystals are not to be explained by different aetherial elasticities 

 in different directions ; he shows how superior the modern theory 

 of Lord Kelvin and Mr. Glazebrook is, that the aether may be a 

 very compressible elastic substance, with elasticity uniform in all 

 bodies and in all directions, but with a directional loading by 

 matter which can account for both single and double refraction. 



But although these matters are discussed, the essential part of the 

 book, the invention of the ellipsoidal indicatrix and deduction of 

 the optical behaviour of crystals from it, is entirely independent 

 of theories as to the nature of the vibration which constitutes 

 light ; so much so that it is possible to deal with rays alone and 

 to develop the subject in a simple geometrical manner. O. J. L. 



XXV. Intelligence and Miscellaneous Articles. 



ON THE ELECTRICAL RESISTANCE OF THE HUMAN BODY. 

 BY M. VON FREY. 



HPHE author has applied to the human body Kohlrausch's method 

 -*- of determining the resistance of electrolytes by means of alter- 

 nating currents and the telephone. Certain modifications were at 

 once found to be necessary. It was necessary to dispense with the 

 metre measuring-wire, since its resistance is so small compared 

 with that of the human body that, in order to hear any sound at 

 all, it is necessary to use induction -currents so strong as to be 

 painful to the subject of experiment. The author replaced the 

 ineasuring-wire by a circular groove not completely closed, filled 

 with solution of zinc sulphate, in which the part connecting it with 

 the telephone could be moved round like the hand of a clock. It 



