70 Mr Basset, A Provisional Theoi^y of iTerr's [May 1, 



The table given on § 383 of my book was taken from 

 Eisenlohr's paper* referred to below, from which it appears that 

 throughout the visible spectrum, the value of a for steel is less 

 than 45°. I now find that his results are wrong, and that he 

 has published a corrected table in a subsequent paper -f*, in which 

 he finds that for steel a is about equal to 56°. This result 

 destroys the hopes which I lately entertained of constructing 

 a satisfactory electromagnetic theory of Kerr's experiments by 

 taking into account the conductivity combined with Hall's 

 effect. 



As a good deal turns on the values of the constants R, a and 

 u, I have calculated their values independently by means of the 

 formulae (2) and (3). 



Since the source of light in Kerr's experiments was the flame 

 of a paraffin lamp, I shall calculate the values of the quantities 

 for line D when the metal is steel. For this line Jamin j gives 



^,= 16° 48', 7=76° 40', 



from which I find, (since tan 6^ = tan^ /3,), 



^ = 28° 48', a = 55° 53', u=r 43', 



logi2 = -577749, i?= 3-7823. 



In the paper in Wied. Ann. Eisenlohr gives the following 

 values, viz. for line D in the case of steel, 



a = 56° 5', log i? = -6084. 



Sir John Conroy§ gives the following values for the principal 

 incidence and azimuth for steel 



/ = 76° 48', /3 =- 27° 53', 

 from which I find 



a = 54° 16', M = l°30', 



log J? = -6182602, i^ = 4-1045. 



Sir J. Conroy states (p. 32) that a soda flame was used as 

 the source of light ; but it appears from Jamin's experiments 

 that R and a are subject to chromatic variations. There is how- 

 ever a fairly close agreement between the three sets of numbers. 



The value of u of course depends on the angle of incidence; 

 the foregoing results give its value at the principal incidence. 



* -Fogg. Ann. Vol. civ., p. 368. 



t Wied. Ann. Vol. i., p. 205. His A, H, e and 6 are my T, (3, a and E. In this 

 paper there appears to be a misprint, viz. tg (or tan) ought to be log. 



X Ann. cle Chim. et de Phys. (3), Vol. xxii., p. 311; Mascart, Traite d'Optique 

 Vol. II., p. 639. 



§ Proc. Bmj. Soc. Vol. xxxv., p. 33. 



