681 



The positions of the movable compensator wedge, in wliicli the 

 difference of pliase foi- the narrow beam of light, which passes 

 between the threads before the fixed wedge, amounts successively 

 to — 2, 0, -f- i, are : 



63.86; 49.52; 35.10. 



As the displacements of the wedge are always reduced to those 

 between 49.52 and 63.86, a displacement of 14.34 mm. corresponds 

 with a phas-edifference of ^ or n. ^) 



The angle of incidence on the mercury surface is derived from 

 the position, in which the line of sight of the telescope of the goni- 

 ometer is horizontal. This position is found halfway between the posi- 

 tions, in which that line of sight runs successively parallel to that of an 

 incident beam of light and of the corresponding beam of light, which is 

 reflected by a liquid surface in the centre of the goniometer. These 

 positions are 81°36' and 104°24', so that the axis of the telescope 

 of the goniometer runs horizontal in the position 81°36' -|- (104°24' — 

 — 81°360 : 2 = 93°0'. As reflecting surface is used that of thick 

 machine oil, because this gives rise to a pure image of the slit, which 

 is not the case with mercury. ') The goniometer circle is vertical, 

 when the axis of the incident beam coincides with that of the tube 

 in which later the polarizer is placed and after reflection with the 

 axis of the telescope. 



5. Freliininary investigation of the optical injiuence of a condensed 

 layer of air. When, as was already communicated in § 2, it 

 became evident from the observations, that the layer of air condensed 

 on the mercury surface was optically active, the angles of principal 

 incidence / and the principal azimuth H were determined on the 

 mirror of pure mercury, immediately after the formation. This took 

 place by the determination of the phase-difference and the restored 

 azimuth for two angles on both sides of the angle of principal 

 incidence from extensive series of observations. From these values 

 those of / and H were obtained by interpolation. ') 



It was found that /= 79°J8', ^=: 35°45'. 



This determination could be made within three hours. 



On an earlier occasion we obtained /=: 78°23', H=:36°18', in 

 which the mercury surface was exposed to the air for some days, 



I) The phase-differences are given as phase-retardations with respect to the 

 light vector in the reflected liight, perpendicular to the plane of incidence. 



') For the mercury surface the compensator fringes remain however straight, 

 only slightly less sharply defined. The accuracy of tlie adjustment of this fringe 

 is somewhat less than with ,i solid surface, viz. 0.03 insleati of 0.02. 



') R. SissiNGH, loc. cit. 



