1899.] on Magnetic Perturbations of the Spectral Lines. 159 



Now, experimental investigation shows us that all the spectral 

 lines do not become triplets when viewed across the lines of force 

 in a magnetic field, for some lines show as quartets, or sextets, or 

 octets, or in general as complex triplets derived from the normal 

 triplet by replacing each component by a doublet or a triplet. We 

 conclude, therefore, tbat the ions which give rise to these complex 

 forms are not perfectly free in their motions through the magnetic 

 field, but are constrained in some way by association with each other 

 in groups, or otherwise, while they move in the magnetic field. 



And now we come to a very important point in this inquiry. 

 According to the simple theory every spectral line, when viewed 

 across the lines of force, should become a triplet in the magnetic 

 field, and the difference of the vibration frequency between the side 

 lines of the triplet should be the same for all the spectral lines of a 

 given substance. In other words, the precessional frequency should 

 be the same for all the ionic orbits, or the difference of wave-length 

 8 X between the lateral components of the magnetic triplet should 

 vary inversely as the square of the wave-length of the spectral line 

 under consideration. Now, when we examine this point by experi- 

 ment, we find that this simple law is very far from being fulfilled. 

 In fact, a very casual survey of the spectrum of any substance shows 

 that the law does not hold even as a rough approximation ; for, while 

 some spectral lines show a considerable resolution in the magnetic 

 field, other lines of nearly the same wave-length, in the same sub- 

 stance, are scarcely affected at all. This deviation is most interest- 

 ing to those who concern themselves with the ultimate structure of 

 matter, for it shows that the mechanism which produces the spectral 

 lines of any given substance is not of the simplicity postulated in the 

 elementary theory of this magnetic effect. 



Our previous knowledge of the line spectra of different substances 

 might indeed have led us to suspect some such deviation as this from 

 the results predicted by the simple theory. For if we view the line 

 spectrum of a given substance we find that some of the lines are 

 sharp while others are nebulous or diffuse, and that some are long 

 while others are short — in fact, the lines exhibit characteristic 

 differences which lead us to suspect that they are not all produced 

 by the motion of a single unconstrained ion. On closer scrutiny 

 they are seen to throw themselves into natural groups. For example, 

 in the case of the monad metals (sodium, potassium, etc.), the spectral 

 lines of each metal form three series of natural pairs, and again, in 

 the case of the diad group (cadmium, zinc, etc.), the spectrum of each 

 shows two series of natural triplets, and so on. 



Thus, speaking generally, the lines which form the spectrum of a 

 given substance may be arranged in groups which possess similar 

 characteristics as groups. Calling the lines of these groups A lf B x , 

 G x . . ., A 2 , B 2 , C 2 . . ., A 3 , B 3 , C 3 . . . we may regard the suc- 

 cessive groups as repetitions of the first, so that the A's — that is 

 A l5 A 2 , A 3 , &c, — are corresponding lines produced probably by the 

 same ion ; while the B's — namely, B 1? B 2 , B 3 , &c. — correspond to one 



