Part II. — Magnetic Pertiirhations of the Spectral Lines. 



9 



resolved into a doublet or a triplet ; or A may be a doublet, while B and C 

 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 C, 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 B 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 B and C, advantage may be taken 

 of the fact, that the plane of polarization of A is perpendicular to that of B 



B A c 

 Fio. 1. — Normal Tkiplet. 



B A C 

 Fig. 2. — Qtartet. 



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- 

 guislied while B and C 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, properl}^ 

 placed before the slit of the spectroscope, two images of the source may be 

 cast upon the slit, one above the other, and completely separated, or overla2)ping 

 to some extent, as may be desired. One of these images contains the light 



B C II (■ 



Fig. 3. Fig. i. 



(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 [B and C). These two images produce 

 two parallel spectra, one above the other, in the field of view, one of which 

 contains all the middles {A) of the lines as modified by the magnetic field, while 

 the other contains all the sides {B and C). As a consequence, tlie appearance 

 presented in the field of view is somewhat like that shown in figs. 3 and 4, in 



D2 



