112 
8. The surface layers of smoke, air, and oil upon mercury 
have been examined by means of the more delicate method of 
observation (see 2, 3, and 4). For a surface layer which is so thin, 
that the index of refraction may be considered as constant, the 
following equations may be derived from Drupr’s theoretical con- 
siderations *) : 
| 
nee sin 2 — (1 — n° cos’ 1) 
SE 
C,—C¢, wv cos*1— a 
400 z aka a cos?i — a 1 
Oeil ae rare Ree (coat ay ne (: — =) (1, — l,). 
In this w' and w are the restored azimuths for a clean mercury 
surface and one with a surface layer, c\ and c the corresponding 
compensator readings, rz the compensator displacement for a phase- 
difference +, 7 the angle of incidence, n the index of refraction of 
the surface layer, /,—/, its thickness, a and a’ are determined by: 
cos 4 j sin 4H 
lain nnn end tol 
Seven surface-layers of smoke have been examined. Two have 
been obtained by blowing more smoke on to an already existing 
smoke layer. Smoke is blown into a pipette, provided with a bulb, 
which ends above the mercury in a fine point. The determinations 
of phase-difference and restored azimuth with compensator and polarizer 
(the. azimuth of the analyzer is always 45°) are carried out successively 
on pure and smoked mercury under circumstances, which are the same 
down to the minutest particulars. The values of / obtained for the different 
colours with the same layer do not diverge too much. 10°/: 2 varied 
from 3,15 for the thinnest to 34,4 for the thickest layer. The values 
of n range for every colour with increasing thickness of the layer 
from about 4—5 to 22.5. The graphical representations point 
to the existence of a maximum for a definite value of /. For all 
layers / remains below the limiting value, about 0,03 2, for which 
the formula may still be applied. 
Two layers of air have been examined. In these circumstances the 
mercury was protected by a glass plate, so that particles of dust and 
fat are excluded. The thicknesses of the layers were 2.2 > 10-6 and 
3.4 « 10-6 mm., n ranges for the different colours from 2.6— 4.4 
to 3.4—5.5. 
Besides bone-oil has been communicated to the mereury-surface and 
the spreading of the oil has been examined. The results have been 
') Drupe, Wied. Ann., 39. 1890, 488. 
