Mr. Lubbock on the Wave-surface in Double Refract ion, 47 

 General Observations on the Year. 



Barometer, Greatest height, corrected and reduced, 30-664; 

 least ditto, 29*414; greatest range, 1*25: highest with 

 NE. and SE. winds; lowest, with NW. and SW. 



Thermometer. Greatest observed power of common black 

 bulb, 106°; greatest observed heat of ground, thermo- 

 meter barely covered, 142°; sun's greatest force about 

 1 p.m. ; hour of greatest heat 4 to 4^ in the hot season, 

 and 3 to 3^ at other times. Dew-point, accords with 

 the latter. Clear days, proportion 1 to 4 or 5. Halos, 

 rare. A. Emmett, 



Lieut.-Col. Royal Engineers. 



XIII. On the Wave-siirface in the Theory of Double Refraction. 

 By J. W. Lubbock, Esq.^ F.R,S.* 



■jl/FR. TOVEY remarks, p. 524 of the last Number, that I 

 "'-*-■- have taken for granted that the differential equations of 

 molecular attraction may be reduced to the form 



I confess that the reasoning of Fresnel connected with this 

 matter is to me by no means clear, but I presume the reduc- 

 tion will at all events be admitted to be possible in the man- 

 ner pointed out by M. Cauchy in the Nouveaux Exercises, 

 p. 11, the axes of elasticity being the principal axes of the 

 curve of the second order given by the equation (36.), p. 12. 



Instead of taking 



I might have taken, in the manner of Mr. Kelland and of 

 M. Cauchy, 



q = Acos {nt-kr) -^|- = - F^. 



A^= —Aco^{nt — kr){\ -cos (A: A r)) +^sin (/z^— A:;)sin(A: A r) 



= — 2 sin^ ( ] ^-\- Asm (n t — k r) sin {k A r). 



Neglecting the .terms multiplied by sin (k Ar), for the rea- 

 sons given by M. Cauchy, p. 10, if 



{^ sin^( ■ 

 <pr + vKr) A.r- U ^ 



* Communicated bv the Author. 



(^0 



