in the Magnetic Field. 



167 



supposed to represent the triplet into which a spectral line of 

 the standard type becomes resolved by the action of the 

 magnetic field, and fig. 2 in the same way represents a 

 quartet produced by the magnetic field, or, if we may say so, 



A A 



B A 



B A C 



Fi<?. 1. 



Fiff. 2. 



B 



Fig. 3. 



B C 



Fig. 4. 



a triplet in which the middle line A has become a doublet. 

 These lines are all plane-polarized, the vibrations in the side- 

 lines B and C being parallel to their length, while the 

 vibrations in the central constituents A are in the per- 

 pendicular direction, when the light is viewed across the lines 

 of force. 



Hence, if a double-image prism be placed before the slit of 

 the spectroscope in the path of the beam of light, the two 

 plane-polarized parts can be separated so that one part (say 

 A) forms one image on the slit, while the other part (B and 

 C) forms another image on the slit. These two images, being 

 separated, give rise to two spectra in the field of view of the 

 spectroscope which may be separated or may be caused to 

 partially overlap if so desired. As a consequence a triplet or 

 a quartet which appears in the field of view, as shown in 

 figs. 1 and 2, without the double-image prism, becomes trans- 

 formed by the double-image prism into figs. 3 and 4. Thus 

 the light vibrating parallel to the lines of force (A) is 

 separated from that vibrating in the perpendicular direction 

 (B and C), and this facilitates observation in the case of small 

 separations and in the case of overlapping lines. This sepa- 

 ration of the two parts can of course be effected by a Nicol's 

 prism, but the double-image prism has the advantage of 

 showing the two parts simultaneously. 



The question now before us is — can the quartet shown in 

 figs. 2 and 4 be derived from the triplets figs. 1 and 3 by 

 mere reversal of the central line A. In answer to this 

 question, it is to be remarked that it is not the central line 

 A alone which shows as a doublet, for in some cases the 



N2 



