Physics. — “An Evtension of the Theory of Basinet’s Compen- 
sator.”’ By C. A. Reeser and Prof. R. Sissincu. (Communicated 
by Prof. H. A. Lorentz.) 
(Communicated at the meeting of June 25, 1921). 
1. For an examination of elliptically polarized light, BABINET's 
compensator must satisfy the condition that the principal planes of 
the two wedges are at right angles to each other. Besides, if this 
elliptically polarized light arises from reflection, one of the principal 
planes must coincide with the plane of incidence of the mirror. For 
this purpose one of us has used the dark line in the field of polari- 
sation of the nicol, which was first observed as band by LanNpoLr, 
and afterwards studied by Lrepicn *). In an experimental investigation 
on tbe true optical constants of mercury, carried ou. by Reeser, it 
appeared, however, that phenomena, which had not been observed 
before in the compensator, can successfully be used in the adjustment 
of the compensator, which leads. to greater accuracy *). The phe- 
nomena in question have first been experimentally studied, and then 
theoretically confirmed. 
2. The above mentioned phenomena are obtained by means of a 
cylindrical beam of rays, which is not perfeetly achromatic. For 
‚this purpose a spectrum is thrown on the slit of a collimator, and 
the beams of light, issuing from it, are made to traverse a compen- 
sator placed on a goniometer between two nicols. Let the compen- 
sator at first contain only one wedge. The front plane of this wedge 
is at right angles to the beam of rays. 
The analyzer is placed behind the wedge and also a telescope, 
which is adjusted for parallet-rays:-Fhe change of direction, which 
the rays of light undergo in their passage through the polarizer, has 
been for the greater part neutralized by the aid of two glass wedges’). 
The polarisation-planes of the two nicols are about normal to each 
other and the wedge is adjusted so, that the illumination of the 
1) Liepicu, Wiener Sitz, Ber., 85, 1882; 91, 1885. Stssinex, Proefschrift, Leiden, 
1885; Arch. Néerl., 20. 1886 
3) C. A. Reeser. Proefschrift, Amsterdam. 1921. 
