28 Mr. P. Smyth on Carbon and Hydrocarbon 



to be seen with all ordinary and some extraordinary forms of spec- 

 troscopes in any usual and convenient formula for preparing the 

 carbohydrogen-spectrum of a moderate intensity, I propose not 

 to take any further account of them now, except to say that, if I 

 ever do meet them in future spectroscopy, I shall probably call 

 them " linelets," or some such diminutive of lines, leaving there- 

 fore our present arrangement, and also Dr. Watts' s present-paper 

 arrangement, of bands and lines intact.) 



Of these last most proper lines, then (in any ordinary spectro- 

 scope with a fine slit), let us speak now touching their appear- 

 ance in Dr. Watts' s new-paper account of them for 1874. They 

 are given there as though they had never been observed or mea- 

 sured by any one else previously ; and no one is entitled to object 

 much to that, if they are now set forth in a better and completer 

 manner (especially for the practical use of astronomers) than 

 in all former accounts of them. Is that, however, the case ? 



No ! I am sorry to say ; for while thirteen distinct lines are 

 recorded, there is not the slightest indication as to their being 

 of any but equal visibility ; yet one of them is an actual Sirius 

 for brilliance, the very brightest of all the lst-magnitude lines or 

 stars, and others are as faint as 15th- and 20th-magnitude stars, 

 seldom seen by any one. Yet of all that he has actually seen, 

 I doubt not that Dr. Watts's measured wave-length places are 

 always respectable for accuracy, though not perfect; and I can 

 quite enter into his statement that the best- determined of them 

 all are the lines 5165*5 and 5585*5. 



Why or how that result came about is not explained by him ; 

 I will therefore proffer two reasons of my own, which will not 

 decrease the interest or importance of the measures. 



(1) The line 5165*5 is the first of the green band, the bright- 

 est line of the whole carbohydrogen-spectrum, the one which 

 should have been decorated with an a ; and it was much enlarged 

 on, no less than nineteen years ago, by Professor Swan for its 

 extreme beauty, its brilliance, its definition, and its capacity for 

 being accurately measured. And 



(2) The line 5585*5, not the first, but, strange to say, the 

 second of the citron band, was found in 1870 by Piazzi Smyth 

 to be so almost exactly coincident with the chief, the almost 

 only, the Angstrom-discovered aurora-line, that he has recom- 

 mended it repeatedly since then to all aurora-observers as afford- 

 ing a ready and instantaneous eye-proof whether there is any 

 variety of spectroscopic character in the chief part of the light 

 of successive auroras — he, too, having found none in nearly thirty 

 auroras spread through two or three years, and embracing the 

 widest external variations from arcs of bland light to needle- 

 shaped shooting rays, and from pale yellow-green or citron- 



