in the Modern Spectroscope. 



29 



colour up to almost any variety of red*. During the present 

 year, indeed, Piazzi Smyth has been informed by Mr. Rand 

 Capron, of Guildford, Surrey, that the coincidence is never quite 

 perfect, the aurora-line being slightly more refrangible than the 

 second line of the citron band; and Piazzi Smyth is quite ready 

 to bow before this correction from the superior spectroscope with 

 much larger dispersion, and the evident practical ability of Mr. 

 Capron. But the correction still leaves the reference-use of the 

 line 5585*5, though the second of the band (which gradually 

 goes off into invisibility), more important than the first — and just 

 as accurate as before, if each observer will remember that a nor- 

 mal aurora-line reads on the comb-like natural scale of the citron 

 band of the carbohydrogen-spectrum not 2'0, but 2*2 — the 

 wave-lengths of all the lines in that interesting band reading 

 thus, according to various authorities : — 





Lines. 





Wave-length according to 



• 

















Dr. Watts, 



BAND 





Compara- 



Swan, 



Swan, in 







in his 



of carbo- 



JNumbered 



tive bright- 



Angstrom, 



1871, by 



Swan, in 



Dr. Watts, 



paper in 



bydrogen 



" from less- 



ness as 



Lielegg,&c. 



projection 



1871, by 



in his 



the Philo- 



spectrum 



refrangible 



with star 



collected 



of his an- 



calculation 



' Index of 



sophical 





side. 



magni- 



by Piazzi 



gular mea- 



otthe 



Spectra,' 



Magazine 







tudes. 



Smvth in 



sures of 



same. 



in 1872. 



for Novem- 









1870. 

 5630 



1856. 







ber 1874. 



r 



1 



1 



5630 



5629-8 



5622 



5634-7 





2 



1 



5579 



5570 



55767 



5582 



5585-5 



Citron I 



3 



3 



5535 



5525 



5532-9 



5534 



5542-3 





4 



6 



5497 



5488 



5495-1 



5495 



5503-5 



I 



5 



10 



5460 







5463 



5478-4 



See also Plate II. 



How, therefore, Dr. Watts came in 1874 to single out the 

 second line of the citron band for super-accurate measurement 

 of place by mere chance, and without knowing any thing more 

 than appears in his paper of my having used it for years as a 

 standard reference for an almost exactly coincident cosmical 

 phenomenon in the night sky, is a strange problem. But its 

 due investigation I must leave untouched on now, as there are 

 other and more world-wide important matters to be discussed in 

 the Doctor's last paragraph, where he states with no little posi- 

 tiveness both what the visible spectrum concerned is chemically 

 the spectrum of, and of what it is not. 



* When the red colour is decided to the eye, a red line appears in the 

 spectrum with wave-length 6290 nearly ; but the 5585 citron-line is still 

 always in its due place, and always brighter than the red line, even in the 

 very reddest parts of the aurora. 



