692 



tare falls below 0°, llie water freezes and contracts to a few ice 

 crystals, which lie spread with large interstices on the mercury. Then 

 we have again a meroirv surface and the normal adjustment of 

 the compensator-fringe. In the meantime the mercury is still liqnid. 



Attempts to prevent this deposition of the water vapour by careful 

 drying of the air proved unsuccessful. This was done thoroughly 

 however by exhausting the bionze cylinder. When an iron mirror is placed 

 in the cylinder, it appears, that the glasses on the side-tubes do not 

 become bi-refringent when the case is exhausted, but that they do 

 so through the one-sided cooling during the fall of the temperature 

 of the bronze cylinder by solid carbonic acid and ether. Then the 

 adjustment of the compensator-fringe changes by the constant amount 

 0.08. During the cooling of the mercury in vacuum no sudden 

 change in the position of the compensator-fringe is observed at the 

 moment of the freezing. The reading of the compensator-fringe 

 diminishes gradually, till this change reaches an amount of 0.08. 

 It appears from this that only the slight double refraction of the 

 glasses plays a j)art here and that neither during the freezing of 

 the mercury, nor during the further cooling down to — 80° the 

 phase-difference, hence also the angle of principal incidence of the 

 mercury changes perceptibly. 



At first the position of the polarizer presented a change during 

 the freezing, which could amount to as much as 3°. This, however, 

 mi]St be attributed to the influence of wrinkles, which make their 

 appearance with the freezing in consequence of inevitable vibrations 

 caused by the traffic in the streets or by tram cars. This is in 

 agreement with the observations of Fizeau, Drude, and Haak ') on 

 the diminution of the restored azimuth through grooves or scratches 

 in various directions on the reflecting surface. In order to prevent 

 these wrinkles as much as possible, the iron dish is filled brimful 

 with mercury and then the temperature is slowly lowered. When 

 the traffic in the streets is not too great, it is then sometimes possible 

 to get such a smooth mercury mirror, that a pure image of the collimator 

 slit can be observed. In this case the adjustment of the polarizer 

 does not change. It appears from this, that on freezing and cooling 

 of the solid mercury to — 80° neither the angle of principal incidence 

 nor the princii)al azimuth are subjected to any change. Optically 

 liquid and solid mercury behave in the same way ^), 



1) FizEAU, Ann. de Chim. et de Phys., 3, 373, 1861 ; Drude, Wied. Ann., 39, 

 497, 1890; J. J. Haak, Thesis for the doctorate, Amsterdam, 1918. 



2) Besides it follows from this that Lummee and Surge's supposition (Ann. der 

 Phys., 31, 325, 1910), according to which internal tensions would give rise to the 

 elliptical polarisation, cannot be valid for solid mercury. 



