294 E. Sissineh on Kerr's 



magnetic reflexion will not be extinguished by the analyser, 

 because the magnetic component of the light does not pass 

 through the analyser without being weakened. By rotating 

 one or both Nicol prisms, however, the intensity of the light 

 can be reduced to a minimum or zero. These rotations will be 

 called minimum- or zero-rotations, and will be designated by 

 $?p°\ $? a (0 \ <t>? P ( °\ <S (o) - Tn e indices m and o denote the 

 kind of rotation (m = minimum, o=zero), p and a indicate 

 which ISTicol prism is rotated, i and I refer to the plane of polari- 

 zation of the incident light, i meaning that the light is polarized 

 in a plane nearly parallel, and I nearly perpendicular, to the 

 plane of incidence. Prof, van der Waals has shown how, by 

 the help of these rotations, we can determine the amplitude //, 

 and the phase m of the magnetic component of the light *. 



For the exact determination of the quantities observed the 

 positive direction of rotation must first be defined, and a mode 

 of expressing differences of phase must be agreed upon. Let 

 the observer imagine himself looking along the direction 

 of the incident or reflected light, with his face turned towards 

 the mirror; then the rotations of the Mcol prisms are positive 

 if in the same direction as the motion of the hands of a watch 

 with its face to the observer. The difference of phase of two 

 waves of light whose oscillations are not parallel to each other 

 is zero, if the greatest distances (of the rether molecules) from 

 the mean position are reached at the same time for each wave. 

 These distances, which are in a certain sense arbitrary, I have 



Fig. 1. 



y co 



chosen to be as given by Oy, Oa, Ob in fig. 1 ; Oy for 

 incident and reflected light polarized parallel to the plane of 

 incidence, Oa for incident and Ob for reflected light, both 

 polarized perpendicularly to the plane of incidence. In this 

 figure AO is the incident ray, OB the reflected ray, and SS 

 the mirror. Oa and Ob lie in the plane of incidence ; Oy is 

 perpendicular to the plane of incidence. Oa is at right 

 angles to OA, and Ob to OB. The differences of phase are 



* Cf. P. C. Kaz, " Over te terugkaatsing van het Licht door magneten, 

 Dissertation, Amsterdam, 1884 ; Beiblatter, ix. p. 275 (1885). 



