111 
6. The observations have been carried out with four light-filters. 
The wave-lengths of the maxima of the transmitted narrow spectral 
regions are successively 669 uu; 637 uu; 558 uu and 482 uu. Besides, 
the yellow, green, and blue mercury lines have been used by the 
aid of the quartz mercury lamp. Its wave-lengths are 578 uu, 546 uu 
and 436 ug. For the way in which the angle of incidence and the 
lowest compensator reading is obtained, compare the thesis for 
the doctorate of Mr. Reeser. Let us call the lowest compensator 
reading for a mercury mirror without surface layer c’, the 
observed reading c, Wp’. and w the corresponding values of the 
restored azimuth, then (y’-—y) : (c/—c) can be calculated with the 
formulae in § 8, so that w’ and ¢’ can be derived from the 
observations. 
7. First the optical constants, principal angle of incidence / and 
principal azimuth A, also n, and &,, index of refraction, and coéf- 
ficient of absorption in a direction normal to the boundary plane have 
been determined for a pure mercury surface. The phase-difference 
p and the restored azimuth w is determined with each of the colours 
for five or six angles of incidence. From this „cosa and k are 
calculated for every angle of incidence, in which nand& are index 
of refraction and coefficient of absorption corresponding to the angle 
of incidence, and « represeuts the angle of refraction. The mean is 
calculated from the different values of ncosa and & for the same 
angle of incidence, and thus / and A according to the relations 
k = n COS ct 
tg 20 = —— _ , to fl = ——____ 
n COS a cos 2H sin I 
The second equation gives / through successive approximations‘). 
Here follow the values of /, H, n, and &, for three colours, viz. 
the yellow, green, and blue mercury lines. 
yellow mercury line US 79°28' [ESS 35 00) nN, == 1,693 k= 4,934|4— 578 uu 
ere os FBONB | 86916! 1,538 4,696, — 546 ,, 
N 76°21' 38°2' 0,995 3,754, = 436 ,, 
The accuracy of / is for the different colours from 3’ to 8’, for 
H from 6’ to 8’, for n, from 1°/,—2°/,, for &, from */,*/; to 1°/,. 
The values here obtained are compared with those of earlier investi- 
gators, and it is shown that in most of them the mercury had a 
surface layer '). 
blue 
1) See for further information the thesis for the doctorate of C. A. Reeser, 
Amsterdam 1921. 
