and ike Resonance Radiation of Electrons. 519 



fixed in position. What actually happens appears to be a 

 redistribution of intensity among the individual lines. 



The yellow-green-blue fluorescence spectrum can be 

 divided roughly into three parts, which we will designate 

 A, B, and C. Region A, comprised between wave-lengths 

 571 and 500, is made up o£ regularly spaced lines which 

 collectively form a system of bands, owing to the periodic 

 fluctuations of intensity. Region B, between 500 and 477, 

 is made up of irregularly spaced lines, often hazy on one 

 side, and of varying width. This region contains the first 

 three bands of the group already alluded to, though special 

 precautions are necessary to bring them into view. With 

 dense vapour, and illuminating light of the proper wave- 

 length (about 460), the region is seen to break up into three 

 very broad patches ; in other words, there is a periodic 

 fluctuation of intensity as in region A. . Region C between 

 wave-lengths 477 and 468 is made up of very broad lines, 

 spaced with a fair amount of regularity and arranged for the 

 most part in pairs. 



These three regions are indicated by brackets in fig. 1, 

 Pi. VII. 



The positions of the bands, which apparently have their 

 origin in the periodic fluctuations of intensity of the lines, 

 are recorded in the following table : — 



4791 5415 



4882 5465 



4971 5510 



5050 5551 



5120 5585 



5185 5620 



5246 56-J5 



5316 5665 



5361 



The centre of the bands cannot, of course, be very exactly 

 located, especially in the case of the first three which are 

 found in region B. 



These bands apparently obey the same law as the oxygen 

 bands in the solar spectrum, i. e. if we plot their wave-lengths 

 as abscissae, and take ordinates which increase by equal 

 amounts, we obtain a curve which is approximately a parabola. 

 In this way we can calculate the position of the head of the 

 group, which was found to fall at wave-length 5715. Two 

 bands were therefore missing, one of which at 5676 was sub- 

 sequently found in the spectrum obtained with blue-violet exci- 

 tation, dense vapour, and long exposure : still longer exposures 

 would probably record a trace of the first band at 571. 



As has been said, these bands shift their position as the 

 wave-length of the exciting light changes. The shifts are 



