xxiv.] TRIPLETS. 



bodies as sodium and potassium, lead, thallium, and the like ; 

 unlike them, again, it contains no one line which is clearly and 

 unmistakably reversed on all occasions. Compared, however, 

 with the spectrum of such bodies as cerium and uranium, the 

 spectrum is simplicity itself. 



My attention was first directed to the variation in the relative 

 intensity of the iron lines under different conditions by the 

 differences easily recognised in the case of some of the triplets 

 with which the spectrum is crowded. 



These give us beautiful examples of those repetitions of 

 structure which we meet with in the spectra of almost all bodies, 

 some of which have already been pointed out by Mascart, Cornu 

 and myself. In many photographs in which iron has been com- 

 pared with other substances, and in others again in which iron has 

 been photographed as existing in different degrees of impurity 

 in other substances, these triplets have been seen almost alone, 

 and the relative intensity of them, as compared with the few 

 remaining lines, is greatly changed. In this these photographs 

 resemble one I took ten years ago, in which a large coil and jar 

 were employed instead of the arc ; they necessitated an exposure 

 of an hour instead of two minutes. In this the triplet near G 

 is very marked ; the two adjacent more refrangible lines near it, 

 which are seen nearly as strong as the triplet itself in some of 

 the arc photographs I possess, are only very faintly visible, 

 while dimmer still are seen the lines of the triplet between H 

 and h. In the spark photograph, then, the more refrangible 

 triplet is barely visible, while the one near G is very strong. 



Fig. 109, which is copied from a photograph, will show 

 another difference. The lines at wave-lengths 4325 '0, 43007, 

 427 I/O, composing the less refrangible triplet, are three of 

 the strongest iron lines in the arc spectrum, and those 

 at 4071-0, 4063-0, 4045'0, composing the more refrangible 

 ones, are less strong than the others. It will be seen 

 that in the solar spectrum the last triplet is much more 



