688 



has been calculated. Tt cannot be inferred from Brewster's records, 

 whether his observations have been made on a free mercury surface 

 or on mercury against glass. In the latter case scratches in the glass 

 may be responsible for the too low value of the principal azimuth. 

 Also the too low value of the principal azimuth determined by Quincke 

 should be attributed to scratches '). Des Coudres' observations have 

 been made on a free surface. The deviations seem to be owing to 

 inaccuracies in the observations. Drude obtains a clean mercury 

 surface by the aid of two funnels. No further particulars are given 

 about this. The method is probably similar to Röntgen's method of 

 overflowing. The observations wei'C made within two hours after the 

 formation of the mercury mirror. Quincke and Meyer used a mercury 

 surface against glass. A surface layer between mercury and glass 

 has undoubtedly caused the too low value of the angle of principal 

 incidence. Meese also made observations on mercury against glass 

 and demonstrates the existence of surface layers, that are then present. 

 The values of 1 and // given here, have been calculated from his 

 observations. In how far a condensed layer of air has also exerted 

 an influence cannot be ascertained. From the values determined by us 

 and those of Drude, Meese, and Meyer, whose value for /, which is 

 certainly too low, has not been taken into account, the following 

 values of the optical constants for mercury may be assumed as the 

 most probable : 



i = 79°27' 7/=r35°43'. 



§ 9. llie thickness of the adsorbed layer of air. Both Van Run 

 Van Alkemade (see § 1) and Drude have given equations, from 

 which the thickness of the surface layer may be derived. According 

 to Drude ") the change in the phase-difference between the compo- 

 nents of the reflected light, parallel to and normal to the plane of 

 incidence, brought about by the surface layer is: 



L! — L = ^^ ^^^y^^'^^'y {a—Cos\) r/ _ 1 I ^/ 



w. 



X {a—Cos\f)- + i 



a 



In this (f is the angle of incidence, L the thickness of the surface 



layer, n the index of refraction in this layer at the distance / of 



the reflecting metal surface : 



Cos 4 H Sin 4 H 



SinUt^f I ' SinUtg^ /. 



I) J. J. Haak, Thesis for the doctorate, Amsterdam, 1918, p. 47. 

 «) Dkude, Wied. Ann., 39. 481, 1890. 



