and other Substances radiating in a strong Magnetic Field. 27 



that some doubt should exist at first as to what it is that actually is 

 presented to the observer ; and that discrepancies should occur in the 

 statements of different observers regarding the phenomenon now 

 under consideration is not surprising. Thus while Zeeman dis- 

 tinctly states that he obtained a tripling of the spectral lines when 

 the source of light is viewed across the lines of magnetic force, and 

 a doubling when it is viewed along the lines of force; yet these 

 effects were obtained only after the theory of the phenomena pointed 

 in this direction, and other reliable observers investigating the same 

 lines have given expression to the opinion that when viewed across 

 the lines of force a doubling (like a reversal) or a broadening com- 

 bined with a doubling occurred. So far as I am aware most 

 observers have failed to obtain distinct triplets when the light is 

 viewed across the lines of force, that is, they have not succeeded in 

 separating the constituents sufficiently to enable them to decide what 

 the exact composition of the modified spectral line is. By placing a 

 nicol's prism in the path of the light it has been determined that 

 the central part of the modified line is plane polarised, and that the 

 edges are also plane polarised, but in a plane at right angles to the 

 plane of polarisation of the middle part. Thus with the nicol in one 

 position the middle may be cut out, leaving the two border lines, and 

 with the nicol turned through a right angle the two border lines may 

 be removed, leaving the middle portion alone. It is to be remarked,, 

 however, that observations of this nature, although they are in 

 accordance with the supposition that the modified line is a triplet, 

 yet they do not absolutely prove tripling pure and simple. For this 

 purpose it is necessary to obtain a magnetic field strong enough to 

 separate completely the constituent parts of the modified line, so that 

 each can be observed separately, without the complications attending 

 the overlapping of the others. 



By way of illustration and explanation of the foregoing remarks it 

 should be mentioned here that the observations which I have made 

 in a general survey of the spectra of several substances indicate the 

 existence of four types of effect,* which, although they easily har- 

 monise as particular cases of one 'general type of effect, yet deserve 

 to be noted, inasmuch as they are the results actually presented to 

 observation. Thus when the source of light is viewed across the 

 lines of magnetic force some of the spectral lines are resolved into 

 sharp triplets, in which the constituent lines are distinctly separated 

 by clear spaces on the photographic plate. In these the middle line 

 of the triplet is stronger than the side lines, and agrees with the 

 expectation that it should contain as much light as the two side lines 

 combined. On the other hand, some lines are not resolved inta 



* These matters are treated in fuller detail in a paper communicated to the 

 Eoyal Dublin Society in December, 1896. 



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