•2:54 C. PIAZZI SMYTH ON NOTE ON SIR DAVID BREWSTER'S 



lines X, Y, Z ; and the case is all the more claimant just now, seeing that a 

 very grand chemical identification has just been made out in France for one of 

 them ; but one, unhappily of late called after one letter by some persons, and 

 another letter by others, a fruitful source of future trouble unless corrected 

 speedily. I propose, therefore, to inquire here, by help of a few recent obser- 

 vations, and reference to many old ones, which is the right letter to employ for 

 each of those three lines. 



Sir David Brewster's activities in Solar- spectrum observation were in full 

 force at his favourite Border seat of Allerly, in 1833, as evidenced by three 

 spectroscopic papers in our volume of Transactions for that year ; but the fullest 

 and most authoritative publication on his new lines in the infra-red is that con- 

 tained in his joint paper with Dr Gladstone in the Philosophical Transactions 

 of the Royal Society, London, in 1860. 



Of the longest spectrum- view contained in a plate accompanying that paper, 

 I submit a portion copied by myself, as Strip No. 1 of my own plate now pre- 

 sented, with very little alteration, except slightly expanding it to suit my scale ; 

 and freely crossing and recrossing the lines representing both shade and the 

 inevitable darkness at and about the very origin of spectrum light, which, 

 beginning on the left-hand side of the picture, rapidly increases in intensity 

 towards the right — Fraunhofer lines and bands therein always excepted. 



As an observer, I like Sir David's drawing much, for its truthful representa- 

 tion of the real and necessary degree of darkness, in midst of, or antagonistically 

 to, which the new lines had to be detected; a feature of Nature, this darkness 

 at either end of the spectrum, so rarely introduced in modern spectrum draw- 

 ings. And though the shade bands are rather too sharply defined on either 

 edge, I recognise, in spite of the depreciatory comments of M. Kirchoef, that 

 it is exceedingly like what appears at that end of the spectrum, when a spec- 

 troscope is under-prismed and over-telescoped. So too it must most eminently 

 have been in Sir David's case, when he seems to have employed but one simple 

 prism of not very heavy glass, and no less than a 5-foot achromatic telescope 

 to look into it. But then it was Brewster's eye that looked ; so no wonder 

 that he saw with it more than any of his predecessors, and most of his suc- 

 cessors as well. 



" The light less refrangible than A," say the conjoint authors at their page 

 1.30, " is red, but extremely faint, so faint indeed, that few observers of the spec- 

 trum have perhaps ever seen it; and the only drawing hitherto published of lines 

 in it appears to be in a map of the solar spectrum by M. Matthiesen of 

 Altona. He represents a few lines which, on comparison with fig. 1, may be 

 identified as the band anterior to Y, Y itself, and the band Y 1 . In order to map 

 the lines and bands in this portion of the prismatic image, Sir David Brewstek 

 was obliged to take extraordinary precautions. The telescope was lined with 



