687 



some dust \b swept up from the floor and some of it is strewn 

 over the mercury, the displacement of tiie compensator fringe can 

 immediately he observed without any adjustment. No liquid is, 

 however, to be observed on the mercury either with the naked eye 

 or with a telescope. When the dust is taken trom a place, where 

 oil has been spilt, the compensator- fringe assumes a tortuous form 

 and liquid streaks are to be observed on the surface by means of 

 the telescope. The sinuous compensator-fringe indicates, that not 

 everywhere an equal quantity of liquid is spread over the mercury. 



That in this case really a liquid is spread over the mercury 

 surface, is also in agreement with the fact, that the mercury surface 

 is smoother, so that an image may be observed in the telescope of 

 the thread, stretched across the centre of the slit of the collimator of 

 the goniometer. This does not succeed with a clean mercury surface 

 in consequence of the vibrations caused by the traffic in the streets. 



8. On the values of I and H for mercury toithout surface layer. 

 It follows from § 6 that for a pressure of air of one atmosphere, 

 the curve for /; = 0.79 best represents the observations, so that the 

 compensator reading successively increases by 0.045, 0.08, and 

 0.11 in 1, 2, and 3 hours or on an average by 0.04 an hour. As 

 the determinations of / and //, mentioned in ^ 5, took up three 

 hours, / has been diminished by 6' on an aveiage in this time, the 

 principal azimuth H has, however, remained unchanged. The change 

 in H lies namely within the errors of observation. From this follows, 

 that for mercury without adsorbed layer of air: 

 / = 79°24' // = 35°45' 



We subjoin the values obtained by other investigators: 



In this the following points are noteworthy. Brewster gives 26°0' 

 as restored azimuth after two reflections under the angle of principal 

 incidence. From this the above given value of the principal azimuth 



1) Brewstkk, Phil. Trans., 287, 1830. 



2) Quincke, Pogg. Ann., 142, 202. 1871 



5) Des Goudres, Thesis for the doctorate, Berlin, 1887. 

 ^) Drude, Wied. Ann., 39, 511, 1890. 



6) Meyer, Ann. d. Phys., 31, 1017, 1913. 

 «) MEf^SE, Gött. Nachr., 530, 1913. 



