1881.] the Electromagnetic Theory of Light. 167 



and 



tanO, = -ta D fl COS <t + £i + ■ 9 ?JT % w£^ X * ■•(»»• 



1 cos (<f) -(f>) sin2(<£ — <£) sin (<£ + <£) cos v y 



The first of these equations has been recently tested by me experi- 

 mentally, when the second medium was a piece of Iceland spar. 

 For angles of incidence between 40° and 55° the results of theory 

 and experiment agreed fairly. For higher angles there were 

 marked differences amounting to about 15 per cent, of the quanti- 

 ties measured, for angles below 40° differences also were found to 

 exist. An abstract of the paper giving details of the experiments 

 was read before the Royal Society on Nov. 17, 1881. 



The equations (24 — 27) are the same as those given by the 

 theories of Neumann, MacCullagh and Kirchhoff, to express the 

 relations between 0, 0', and 6 V 



