Prof. Forbes on the Refraction and Polarization of Heat. 205 



a sphere of the same mass on an equidistant particle, but cos ju, 

 is nearly = sine of latitude ; hence the attraction along a line 



which makes with the axis an angle whose cos = — ==, is the 



same as that of a sphere of the same mass. 



8. When the particle is on the equator of the oblate sphe- 

 roid 8 = a and |!x, =90° ; hence A = -——'{ 1+ -rx ^^ k and 



if e be small e^ = 2 6 nearly ; hence A = — ^S 1 + -^ e (-• 



9. If the particle be at the pole of the oblate spheroid 

 8 = 6, and neglecting powers of e above the first. 



r-*-i^{'-i-}- 



10. Should the ellipsoid become a prolate spheroid round 

 the axis of a, the formula becomes 



^ /Mr,, 3/3cos^X-l. aol,, . 

 If the point be at the pole of the spheroid cos A =1, and 



8 = a, and the formula P' = 



4<7rfb 



{-i-l 



3 

 11. If at the equator 8 = Z> and cos A = and the formula 



becomes B = — ^ — J 1+ — s r, omitting the powers of e 



above the first. 

 Trinity College, Dublin, Nov. 12, 1834. 



XXXVI. On the Refraction and Polarization of Heat, By 

 James D. Forbes, Esq,^ F.R.SS. L. SfE., Professor of Na- 

 tural Philosophy iii the University of Edinburgh, 



[Continued from p. 142.] 



§ 2. On the Polarization of Heat by Tourmaline. 



18. T T is well known that two slices of tourmaline cut parallel 

 ^ to the axis of the crystal, as they are looked through 

 with their axes parallel or perpendicular to one another, 

 transmit a great portion of the incident light in the one case, 

 and almost wholly intercept it in the other. 



1 9. It occurred to me as a curious question, at an early pe- 

 riod of my researches, whether non-luminous heat would un- 



