156 Professor Thomas Preston [May 12, 



the broadened line is not really a triplet, but is merely a band of 

 light polarised in one plane along its edges and in the perpendicular 

 plane along its centre, and that increase of the magnetic field might 

 never separate it into distinct constituents, but merely continue to 

 broaden it. This contention, however, might be disposed of by a 

 careful study of the facts, even though we might not be able to pro- 

 duce a magnetic field strong enough to completely separate the 

 constituent lines of the triplet. 



But clearly the thing to be arrived at is to so arrange matters — in 

 fact, to so design our electro-magnet and to plan the conditions of 

 our experiment — that the magnetic field acting on the source of light 

 shall be strong enough to completely separate the members of the 

 triplet, if such exist. You will understand that this is no easy thing 

 to do when you remember that it was only after repeated efforts and 

 many failures that even a slight broadening of the spectral lines was 

 obtained. Nevertheless, in spite of the great difficulty which besets 

 this investigation, and which arises from our inability to obtain a 

 magnetic field of unlimited strength, yet, with a properly designed 

 magnet and other properly arranged conditions, it is possible to 

 obtain a magnetic field strong enough to completely separate the 

 constituents of the magnetic triplet, and thus to prove that the pre- 

 diction of theory is verified by the actual facts. [Slide shown.] 



But with a magnetic field of great strength the facts as shown by 

 these slides [photographs shown here] turn out to be more com- 

 plicated and more interesting than the simple theory led us to expect. 

 For while some of the spectral lines are split up into triplets as indi- 

 cated by theory, some on the other hand become resolved into sextets, 

 or octets, or other complex types. [Slides shown here.] Thus, when 

 the magnetic field becomes sufficiently intense, we realise to the full 

 all the "theoretical predictions and more. The reason of this surplus 

 of realisation over expectation lies in the fact that the theory in its 

 simplest form deals only with the simplest types of motion under the 

 simplest conditions, and the conclusions arrived at are of course of 

 corresponding simplicity. When more complicated types of motion 

 are contemplated, the theory furnishes us with the dynamical ex- 

 planation of the more complicated types of effect produced by the 

 magnetic field. That tripling pure and single should occur in the 

 case of every spectral line (as predicted by the simplest form of 

 theory) is not a result which we should expect from a broader con- 

 sideration of the problem. In fact, if we reflect on the subject, we 

 are forced to the conclusion that deviations from the pure triplet 

 type should be expected, and, as we have seen, such deviations 

 actually do occur. In this respect, therefore, the experimental in- 

 vestigation which yields more than the simple theory expected is not 

 to be taken as in any way discordant with that theory, but, on the 

 contrary, to be in harmony with it. 



In order that you may form some idea as to what it is that the 

 theory supposes to be in operation in the production of these pheno- 



