Part II.—Magnetie Perturbations of the Spectral Lines. 9 
resolved into a doublet or a triplet; or A may be a doublet, while B and @ 
are, or are not, further resolved. It may even happen that the constituents 
of A become, in some cases, more widely separated from each other than B is 
from @, so that the centre A of the triplet, as it were, contains the sides 
within it, and thus we become prepared to consider the case, if any such 
exists, in which 6 and C are not appreciably separated, or coincide, while 
the components of A are at a considerable distance apart. 
In order to examine A separately from 6 and C, advantage may be taken 
of the fact, that the plane of polarization of A is perpendicular to that of B 
B Al oO 3B Ak O 
BAC 13 Zl O 
Fic. 1.—Normat TRrIPLet. Fig. 2.—QUARTET. 
and C, so that if a Nicol’s prism be properly interposed, B and C can be 
extinguished, while A remains alone in the field of view, or A may be extin- 
guished while B and @ remain in the field of view. Instead of using a Nicol’s 
prism, however, it is better to employ a double-image prism—such, for example, 
as a Wollaston’s prism made of quartz. With this piece of apparatus, properly 
placed before the slit of the spectroscepe, two images of the source may be 
cast upon the slit, one above the other, and completely separated, or overlapping 
to some extent, as may be desired. One of these images contains the light 
A A. 
B  @ B 6 
Fig. 3. Hie. 4. 
(Showing effect of double-image prism.) 
vibrating parallel to the lines of force (A), while the other contains the light 
vibrating in the perpendicular direction (6 and (). These two images produce 
two parallel spectra, one above the other, in the field of view, one of which 
coutains all the middles (A) of the lines as modified by the magnetic field, while 
the other contains all the sides (band @). As a consequence, the appearance 
presented in the field of view is somewhat like that shown in figs. 8 and 4, in 
D2 
