453 
1909-10.] Mapping of Grating Spectra. 
freedom ; but to account for the complex cases which arise, couplings between 
the electrons are postulated. A fairly common type of resolution is the 
quartet in which the vibrations parallel to the lines of force do not preserve 
their period, but change into two vibrations, one of higher and one of lower 
frequency. In this case the vibration along the lines of magnetic force is 
associated with changes in directions at right angles, and the magnetogyric 
effect of the latter reacts on the original vibration. In this instance we 
should expect all four components to be of the same intensity. 
Referring now to fig. 3, the first column shows a simple spectrum line 
and its resolution into a triplet or quartet with normal intensities of the 
X- 
X 
2800. 
3450. 
5500 
\J n re s o lve,d 
Li ne,. 
1 
1 
1 
1 
1 
1 
I 
Tri p l e C. 
1 
QuarOeC. 
Fig. 3. — Intensities of Zeeman Effect Components. 
components. Im Zeeman effect investigations it is usual to employ a quartz 
lens for focussing the light on the slit of the spectroscope. Unless this lens 
is constructed in a special manner it will be found to have a dextro- or 
lsevogyric action. Suppose that, in consequence of the optical activity of 
the lens, the polarisation planes of all the components of a resolved line are 
brought to an angle of 45° with the grating rulings when the wave-length 
of the line has some definite value, say 3450. The intensities of the com- 
ponents (as is shown in the third column of fig. 3) will have their normal 
ratios, which were illustrated in the first column. Now if the lens has a 
rotary action equivalent to that of a millimetre quartz plate which has 
been cut with its faces perpendicular to the crystalline axis, it will cause 
rotation of the polarisation planes for rays of light of wave-lengths 2800 
