( (fo) 
magnet (of the RuamKorrr type). The spectroscope was a Row1anp’s 
erating, for which I am indebted to the kindness of the Directors of 
the Dutch Society of Sciences at Harlem; it has a radius of 6.5 M., 
10.000 lines to the inch and a divided surface of nearly 14 em. 
The grating was mounted for parallel light in the manner indicated 
by Rurer and Pascuen'). The source of light was in most eases the 
electric are, in some the sun. 
Using this arrangement of the experiment we can deduce immediately 
from the deformation of the interference fringes in the neighbourhood 
of the absorption bands, when the sodium vapour is under the action 
of the magnetic field, the value of the rotation of the plane of 
polarisation for different wave lengths. Fig. 1 of the Plate gives an 
idea of the aspect of the fringes in absence of the field in the 
neighbourhood of the sodium lines, rather much sodium being present 
in the flame between the poles. The observations were made in the 
second order. 
3. In the experiments first to be described, the distance between the 
perforated poles was about 4 m.m. and the intensity of the field about 
15.000 ¢. g. 8. units. In this field was placed a gas flame fed with oxygen 
and a small quantity of. sodium introduced in it by means of a glass 
rod. After removal of the polarisator and of the FRrSNer prism the 
two doublets, in which the sodiumlines are separated, in the inverse 
magnetic spectral effect were observed. Between the components of 
the doublet were seen the very narrow reversed sodiumlines due to 
the are light itself. 
The polarisator and the prism were now introduced in their proper 
places. The field of view was then crossed by the above mentioned 
(2) dark, nearly horizontal interference fringes. 
IT now wished to ascertain the deformation of the fringes by 
increasing continuously the quantity of sodium vapour, the field 
remaining constant. This method must be preferred for obvious 
reasons to the other which might: have been followed also, viz: the 
examination of a flame with constant percentage of sodium under 
varying magnetic intensities. 
The following observations refer to D, : 
If the quantity of sodium in the magnetic field was only extremely 
small, the interference fringe exhibited at the place of the reversed 
sodium line a protuberance — let us say downward — the lines 
of the doublet being somewhat stronger just above the interference 
fringe. In fig. 1 this behaviour is represented schematically. 
C 
