( 436 ) 
Grest has now extended these observations and will give a more 
detailed exposition of his results elsewhere’); I intend to give here 
a short explanation of them. 
The arrangement of the apparatus was for the most part the 
same as in our former experiments. Plane polarized light, under 
azimuth 45° to the vertical, falls on a Basiner’s Compensator with 
horizontal edges. The light then traverses a second Nicol with its 
plane of polarization perpendicular to that of the first. An image of 
the system of parallel imterference bands in the compensator, is 
thrown on the slit of the spectroscope. The light is then analysed 
by means of a large RowraNp grating mounted for parallel light. 
The greater part of the experiments were made with a compensator 
of which the prisms had angles of about 50’, but for the study of 
some details compensators were used with angles of 10’ or of 3°. In 
the spectroscope a few dark horizontal interference bands are observed 
as long as the magnetic field is off. The fine absorption lines of the 
vapour are then coincident with the reversed sodium lines due to 
the are light. As soon as the field is on, the bands become distorted. 
Their vertical displacements are, with the method used, proportional 
to the difference of phase between vibrations respectively parallel 
and normal to the field. 
For the simplest case of a line split by the field into a triplet, 
Vorer deduced a formula giving the difference of phase as a function 
of the wave length *). The sodium lines YD, and D, being split, 
however, by the magnetic field into a quartet and a sextet, it was, 
in order to compare theory with observation, necessary to deduce 
the formulae for these cases. Mr. Guest has made these calculations 
according to the method already indicated by Vorer *) on another 
occasion. According to his calculation, the difference of phase between 
vibrations normal and parallel to the field, the light having traversed 
a layer / of the absorbing vapour, is given by: 
woel 4d —d' po? 1 
Vv: | (4d?—d'? 90)? ARO Agde 
In this formula V indicates the velocity of light in the aether, 
fh the strength of the field, ¢,d,d' and ¢ being constants characte- 
ristie of the medium. Moreover 2273, =— tr, is the period of vibration 
and d=0%. The formula given applies to the case of the sextet; for 
the quartet, d' =o and for the triplet, moreover d= o0. Figs. 1—3 
1) Gerst, Thesis for the doctorate, Amsterdam, 1904. 
2) Vorer |. c. 
5) Vorer Wied. Ann. 68 p. 352. 1899, 
