28 Mr. T. Preston. On the Modifications of the Spectra of Iron 



distinct triplets, but into what one may provisionally term " quartets." 

 In these the two side lines contain nearly all the light, while the two 

 middle lines are weak, but still very distinctly marked. It may be 

 that these arise from a partial reversal of the middle line of the 

 triplet, and that they would consequently show as triplets but for 

 such reversal. The middle line, of course, is the one which would 

 naturally suffer reversal in the layers of colder vapour, as it belongs 

 to the period of vibration which is uninfluenced by the magnetic 

 field yet the appearance of the weak middle pair is not that which 

 one usually observes in the cases of reversal, nor in any case do the 

 photographs of the line free from the magnetic field show any sign 

 of reversal. 



It is interesting to notice in passing that the lines to which 

 attention has been chiefly directed by other observers, namely, the 

 D godium lines and the blue line of cadmium of wave-length 4800, 

 belong to this latter class, and no doubt the difficulty of deciding as to 

 what really takes place, and the difference of opinion which has existed 

 are to be attributed to this cause. These lines do not resolve themselves 

 into sharp triplets, and if one wishes to observe a distinct and well- 

 defined triplet the violet line of cadmium 4678 or the violet line of zinc 

 4680 should be chosen. Further, some lines show as doublets, or, 

 if we wish to state it so, as triplets from which the middle line has 

 completely disappeared the source of light being still viewed across 

 the lines of force. Whether this is due to complete absorption of the 

 middle line in the outside vapour of the spark, or not, remains for 

 further investigation. The observation of it, however, led me to the 

 subject of this communication, namely, the investigation of the 

 spectrum of iron. Finally, there are spectral lines which are only 

 slightly broadened, and others which are scarcely affected by the 

 magnetic field, even when this is strong enough to resolve other lines 

 of the same substance into triplets, of which the side lines are more 

 than half a millimetre apart, and visibly resolved on the photographic 

 plate to the naked eye. 



Viewed from the theoretical standpoint, we have no reason to 

 demand that all, or indeed necessarily any, of the spectral lines 

 should be resolved into sharp triplets when the source of light is 

 viewed across the. lines of magnetic force. For, in order that a 

 spectral line should exhibit itself as a characteristic triplet under 

 the influence of the magnetic field, it is necessary that the freedom 

 of vibration should be equal in all directions, and in this case the 

 intensity of each rectangular component will be the same. Hence 

 the middle line of the triplet will contain as much light as the two 

 side lines taken together. . If, however, the vibrations are not equally 

 free in all directions the foregoing result will not hold, and it 

 becomes possible to have a triplet with a weak middle line or with- 





