in the Modern Spectroscope. 27 



sources of most feeble intensity) of five bands, viz. the orange, 

 citron, green, blue, and violet, yet Dr. Watts, though from his 

 intense (and even oxygen-excited) spectrum, indicates the exist- 

 ence of only three out of those five. 



These three, which are really the citron, the green, and the 

 blue, he names 7, S, and e rather confusingly ; for in such case 

 any one will ask, " where are the a and /3 oi the system ?" They 

 are nowhere in the Doctor's paper of 1874 ; and even in his book 

 of 1872, a very useful little book (' The Index of Spectra'), and 

 where a does appear as the orange group, there is no (3, and the 

 violet band is most annoyingly called, among so many Greek 

 letters, by the italic k, and there are many other lines only seen 

 when oxygen and various other elements are used extravagantly. 



But letters, whether Greek or Roman, are sadly misplaced in 

 their application to coloured " bands." They are not needed, 

 because the names of the colours describe such bands, and 

 their places, better ; and Greek letters have been already ap- 

 propriated to the bright lines of luminous and discontinuous 

 spectra, in whose range they appear then consistently with the 

 application of the same Greek letters to the brighter stars in any 

 constellation, but never as a designation of a whole constellation 

 of stars. 



To make this point (of what are the bands and what are the 

 lines) clear, I beg to introduce my present working diagram of 

 the spectrum as a reference in astronomy (Plate I.) ; and 

 while it will be seen that the " bands " are distinguished by the 

 names of their colours, the lines in each band are distinguished 

 by the numbers of their wave-lengths, precisely as Dr. Watts 

 distinguishes them in his new paper of 1874; while they are 

 also distinguished further in nature, and my drawings, by the 

 brightest of each band being on the less-refrangible side, and 

 going off gradually into invisibility on the more-refrangible 

 side. 



(And here I crave permission to write a rather long paren- 

 thesis, on a tangle needlessly introduced into the printed ac- 

 counts of this spectrum, thus. As there is no allusion in the 

 paper of 1874 to the Doctor's more important work of 1872 

 being cancelled, it remains as an authority, a competing authority, 

 too, with the paper ; whence, on putting the two side by side 

 this singular discrepancy of terms comes out — viz. that the book 

 calls "groups" what the paper calls "bands," the book again 

 calling " bands " what the paper calls " lines," while finally the 

 book calls "lines" some finer and intercalated modifications 

 still, not mentioned in the paper at all. But the book does not 

 venture to give the wave-length places of these extra-fine and in- 

 terstitial lines belonging to it alone; and as they are certainly not 



