Physics. — “The Optical Investigation of Surface Layers on 
Mercury and a More Refined Method of Observation with 
Bapinet’s Compensator.” By C. A. Reeser and Prof. R. SrssiNGH. 
(Communicated by Prof. H. A. Lorentz.) 
(Communicated at the meeting of June 25, 1921). 
1. This research was made with the same apparatus as Haak 
used’). It will, therefore, only be mentioned here, what modifi- 
cations have been made in it. In order to obtain a greater light- 
intensity and to have greater certainty, that the wave-length of the 
light used was not modified during the observations, the mono- 
chromator, placed before the goniometer, was omitted. The light, which 
must be monochromatic as much as possible, was now obtained 
by means of colour-filters or a mercury lamp of Heranus (3,5 A, 35 V). 
As source.of light we finally returned to the self-regulating arc-lamp 
of 18 Ampéres?). The collimator-axis was in the right direction of 
incidence, so that the mirror behind the collimator could be omitted. 
2. As is known, the nicols with oblique end-planes give, besides 
a displacement, also a change of direction to the beam of rays in 
consequence of errors in the construction. It is about '/,° in the 
nicols used. This gives rise not only to a change in the direction 
of incidence, but, in using a monochromator, also to a shifting of 
the compensator line. The collimator slit takes out of the spectrum 
a small portion, the wave-length of which varies in the direction of 
the width of the slit. In consequence of the deviation by the polariser 
not always the same part nor light with the same wave-length falls 
between the threads of the compensator. In the most unfavourable 
case a shifting of the line took place in the compensator in conse- 
quence of this, corresponding to a phase-difference of 0,02 > 2. 
In order to eliminate this difficulty Mr. Reeser applied the following 
expedient. Two glass wedges with refractive angles of 1° were 
placed before the polariser. The angle between the planes, perpen- 
1) J. J. Haak. Proefschrift Amsterdam, 1918; These Proc., Vol. XXI., No. 5, 
p. 678 (1918). 
2) Compare for further particulars. G. A. Reeser Proefschrift. Amsterdam 1921. 
