(583 



After overflowing 56.45 43°37' 



After one dav 56.68 43°44' 



After overflowing 56.45 43°5()' 



The polarizing action of the side walls of the glass receptacles 



not having been examined, (hese vahies have no absolnte valne, 

 bat they represent the change by the adsorbed air layer no donbt 

 very accurately. 



6. Change of the, thiclxuess and the injiuence of the layer 



inith the time. 



There is no donbt bnt the thickness of the adsorbed layer of air 

 and so also its optical intlnence increases with the time. In order to 

 examine this intlnence the adjustments mnst be effected in a short time. 

 Those of the polarizer have been omitted, because it apjiears from 

 § 5, that the influence of the layer of air on the restored azimuth 

 is very small and about of (he order of magnitude of the errors of 

 observation. In order to be able to execute the observations in a 

 short time, only observations at one angle of incidence, viz. the angle 

 79°46', which is very near the angle of principal incidence, were 

 made, the analyzer also always being set in the same quadrant. The 

 determination of the phase-difference, (hat arises at metallic reflection 

 between the components vibrating perpendicular to and in the plane 

 of incidence, took place only by annulling this phase difference'). 



The shifting of (he movable compensator wedge was therefore 

 exclusively from 49.52 -> 63.86 (see § 4). In this way the errors in 

 consequence of the deviation of the ligh( in the polarizer and the 

 inaccurate position of the planes of polarization of the compensator 

 wedges continue to exist, but their influence on the slight change 

 in the phase-difference, (hat is to be determined, may be considered 

 as of the second order of magnitude. Care should, however, be 

 particularly taken, that the incident beam of light consists always 

 of the same part of the spectrum and keeps the same direction, 

 i. e. always falls on the middle of the slit of the collimator 

 of the goniometer. A slight shifting of the spectrum, which 

 was not even so much as the heiglit of the spectrum, already 

 modified the compensator-reading by 0.06. This is to be ascribed 

 to the cliauge in (he angle of incidence. In (he observations (he 

 mercury was placed in a shallow iron dish, attached to the bo((om 

 of a bronze cylinder. Two side-tubes, closed by plane parallel 

 glass plates, (he axes of which lie in a same meridian plane 



1) SissiNGH. Thesis fur Ihe doctorale, p. 79: Arcli. Néerl., 20, 196. 1886. 



