113 
compared with FiscHer’s observations on mercury *) and Devaux’ 
observations on water’). By means of the compensator it can be 
shown, what Fiscuer suspected, that a small quantity of oil on 
mercury immediately spreads on the surface in a very thin layer. 
The thickness of the very thin layer is 1—2 ug. Fiscner calls it the 
“vorauseilende Schicht”. 
The thickness of the thinnest layers of smoke, air, and oil are 
1.6 uu: 2.05 uu, and 1.07 wu. The values obtained for the index of 
refraction in all layers always point to a very great density of the 
layer at the mercury surface. 
Whereas with capillary phenomena, as the cessation’ of the 
movements of the camphor particles on water, the thinnest layer 
that can be observed, is 1 uu, it appears that the presence of a 
layer of 0.3 uu can very well be verified with the compensator. The 
optical method is, therefore, more minute, which was also found 
by Rarrmen. In conclusion it should be pointed out that in the 
calculation of the thickness of the layers from the number of milli- 
grammes, the greater density of the layer at the surface is, as a 
rule, left out of account. 
All the observations have been made by the former of us. 
1) Fiscuer. Wied. Ann., 68, 436. 1899. 
2) Devaux. Journ. de phys. (5), Il, 699, 1912. 
d 8 
Proceedings Royal Acad. Amsterdam. Vol. XXIV. 
