the Lines of various Spectra. 45 



This last line is a broad reversal; and 2980*8 is probably 

 triple. As usual, " r " means reversed ; and " d " double. 



This list of lines does not pretend to include all ; for a 

 very few faint ones could not be measured as accurately as I 

 wished. 



The two spectra, as will be noticed, consist mainly of 

 groups of three lines ; these triplets overlap in the cadmium 

 spectrum, but owing to the physical similarity of the lines 

 forming any one triplet it was a matter of perfect ease to 

 select them. Thus it is evident that line for line, group by 

 group, the two spectra correspond. The only possible excep- 

 tion is in group ?. In cadmium the first two Hues are plainly 

 double, while in zinc they are not resolved. The lines are 

 broad and nebulous ; and the zinc lines will, I think, be proved 

 double. The more closely the two spectra are examined the 

 more perfect does their agreement appear. 



If we observe either spectrum, it is at once evident that 

 groups /3, 8, ?, 7], 6 form a series. Taking the differences 

 of the wave-numbers of the last lines in each of these triplets, 

 we have for zinc : — 



581'0, 263*1, 141-1, 84-5; 

 for cadmium : — 



586*7, 263-7, 140-8, 84-3. 



The correspondence is again evident. These series resemble 

 greatly the series of hydrogen lines ; and, as so many spectra 

 have these series, it may be well to describe them in detail. 

 These series always run in the same direction ; that is, the 

 relative distances of the lines grow smaller as the wave- 

 lengths become smaller. The hues forming the series are 

 always nebulous ; they are strong towards the red end, but 

 rapidly become faint and hazy as we go down the series. 

 The spectrum of the substance has, in general, other lines than 

 those forming the series : the latter are the ones that always 

 appear in the solar spectrum, the former ones may not. Their 

 regularity is perfect. Eeturning to the zinc and cadmium 

 spectra, and taking differences in wave-numbers between the 

 first and second and the second and third lines of each triplet 

 in the two spectra, excepting and 8, we have : — 

 For zinc, 



39-0 38-8 39-0 38'9 39*2 38'7 

 19-0 19-0 18-8 18-8 18-9 19'3 

 For cadmium, 



117-1 117-1 117-0 116-5 116-6 117*0 

 54*2 54*1 54*3 54*0 54-1 53*8 



