160 Professor Thomas Preston [May 12, 



another and are produced by another ion, and so on. This grouping 

 of the spectral lines has been noticed in the case of several sub- 

 stances, and it has been a subject of earnest inquiry amongst spectro- 

 scopists for some time past. All such grouping, however, up to the 

 present, has had to depend on the judgment of the observer as to 

 certain similarities in the general character and arrangement of the 

 lines, and similarities which indeed may or may not have any specific 

 relation to the mechanism by which the lines are produced. In fact, 

 such grouping has been effected by guess-work, or by empirical 

 formulas, and we need not be surprised if it is found that the groups 

 so far obtained are more or less imperfect. 



I introduce this grouping of the spectral lines to your notice in 

 order that we may attack the problem of reducing to order the so far 

 apparently lawless magnetic effect. As I have already mentioned, 

 the lines in the spectrum of any given substance are not all resolved 

 into triplets by the magnetic held, but some are resolved into triplets 

 while others become sextets, etc. ; and further, the magnitude of this 

 resolution, that is the interval 8 A between the lateral components 

 does not appear at first sight to obey any simple law. 



According to the prediction of the simple theory the separation 

 8 A should be proportional to A 2 , and although this law is not at all 

 obeyed, if we take all the lines of the spectrum as a single group, 

 yet we find that it is obeyed for the different groups if we divide the 

 lines into a series of groups. In other words, the corresponding 

 lines A l5 A 2 , A 3 , etc. have the same value for the quantity e/m* or* 

 as we may say, they are produced by the motion of the same ion. 

 The other corresponding lines, B x , B 2 , B 3 , etc. have another common 

 Value for e/m, and are produced therefore by a different ion, and so 

 on. We are thus led by this magnetic effect to arrange the lines of a 

 given spectrum into natural groups, and from the nature of the effect 

 we are led to suspect that the corresponding lines of these groups are 

 produced by the same ion, and therefore that the atom of any given 

 substance is really a complex consisting of several different ions, 

 each of which gives rise to certain spectral lines, and these ions are 

 associated to form an atom in some peculiar way which stamps the 

 substance with its own peculiar properties* 



In order to illustrate the meaning of this, let us consider the 

 spectrum of some such metal as zinc. The bright lines forming the 

 spectrum of this metal arrange themselves to a large extent in sets of 

 three — that is, they group themselves naturally in triplets. Denot- 

 ing these triplets in ascending order of refrangibility by Aj B 1; C l5 

 A 2 , B 2 , C 2 , etc. we find that the lines A l5 A 2 , etc. show 7 the same mag- 

 netic effect in character, and have the same value of e/m, so that they 

 form a series obeying the theoretical law deduced by Lorentz and 



* The quantity e is the electric charge of the ion, and m is its inertia, and 

 the ratio e/m determines the precessional frequency, or spin, of the ionic orbit 

 round the lines of niHgnetic force in a given field. 



