1908-9.] Dissymmetrical Separations in the Zeeman Effect. 77 
To obtain the light, a thin layer of the salt to be used is put on strips of 
carbon, 1 mm. thick and 5 mm. long ; these are placed against the poles of 
the magnet, and a spark is passed between these electrodes. To focus the 
light of the spark on the slit a quartz lens is used, as glass absorbs the 
violet rays. The maximum thickness of the lens used is 6*6 mm. The 
light passing through the slit and falling on the grating comes from a 
portion of the lens 4 cms. in diameter, which, if spark, lens and slit are in 
a line, and the lens at the proper height, is at the middle of the lens, and 
then the difference between the maximum and minimum thicknesses used 
is about 2 mm. Zeeman has shown * that the intensities, when a diffraction 
grating is used, depend on the angle which the plane of the vibrations 
makes with the lines of the grating. By introducing quartz plates of thick- 
nesses to give different rotations, he was able to vary the intensities in the 
case of a triplet, from having a strong middle component with weak side 
components, to the reverse distribution of intensity. When the plane of 
vibration is parallel to the lines of the grating the intensity is a maximum, 
and when at right angles to this the intensity is a minimum. The influence 
of the magnetic field gives vibrations in two planes at right angles to each 
other, and by rotation first one plane of vibration and then the other can 
be brought parallel to the ruled lines, and so the maximum and minimum 
for each kind of vibration got. When the planes of vibration make 45° 
with the lines, then the vibrations are in the same relative positions to the 
rulings, and then it was found that in the case of triplets the intensities 
were as Lorentz had predicted, namely, the middle component twice as 
strong as the side components. In the apparatus used the quartz lens 
produces rotation. When the lens is placed as mentioned above it has the 
same effect as a quartz plate of about 1 mm. thickness. Thus, for approxi- 
mately A = 3450 the middle component was twice as strong as the side 
components, that is, the planes of vibration made 45° with the rulings. As 
the amount of the rotation varies with the wave-length for the region 
A = 2800 to A = 2900, the planes had been rotated through 45°, and now the 
middle component had a minimum intensity. The cycle of variations is 
passed through more quickly as one proceeds further towards the violet, on 
account of the greater variation in rotation there. Thus the relative 
intensities of the components in the Zeeman triplets give an indication of 
the amount of the rotation of the plane of polarisation produced by the 
quartz lens for the different parts of the spectrum. In the case of over- 
lapping spectra of different orders, it was quite easy, by means of this 
property, to distinguish the lines belonging to the same order, for the 
* P. Zeeman, K. Akad. v. Wet., Amsterdam, Oct. 1907. 
