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XIV. — Note on Sir David Breivster's Line Y, in the infra- Red of the Solar 

 Spectrum. By C. Piazzi Smyth, Astronomer Eoyal for Scotland. (Plate 

 XXXVII). 



(Read 17th December 1883.) 



Of all known examples in physical science, of simplifying, and at the same 

 time " precisionising " some of its fundamental data, which might otherwise 

 fall to be entangled in high numbers, none has been happier than Fraunhofer's 

 application of the letters of the alphabet to certain chief lines in the solar 

 spectrum. Happy both in its conception by the inventor, and its universal 

 acceptance since then by the world. Whence it comes to pass now, that in every 

 country, whoever observes the solar spectrum at all, with whatever instrument, 

 large or small, diffracting or refracting, and whether he holds to the undulatory, 

 or any other theory of light, and catalogues spectral lines either in Wave-lengths 

 or Wave-numbers, or merely in terms of the brass scale screwed to his instru- 

 ment by a maker, — yet whenever he- speaks of the line A, or B, or C, or 

 any other so named by Fraunhofer, he singles out thereby from among 

 thousands, exactly the same identical line which any and every other spectro- 

 scopist alludes to under the same simple letter. 



Hardly less happy was the extension of the system made by our great 

 specialist in optical physics, Sir David Brewster, when, having discovered 

 several lines in the infra-red of the solar spectrum, beyond or before Fraun- 

 hofer's commencing line " great A " — he named them with the later letters of 

 the alphabet, whose stock of symbols had not been more than half used up 

 by Fraunhofer in reaching toward the further violet end of the spectrum. 

 Hence, without disturbing any one of Fraunhofer's lettered lines from red 

 through green, to blue and violet, Brewster called his new line next beyond, 

 or before great A in the "infra red," by the letter Z; the next before and 

 outside that, Y ; and the next before that again, X. 



In so far, Brewster's proceeding was quite as happy as Fraunhofer's ; and 

 if his assigned letters have been lately misused or omitted in certain high 

 quarters, that is not his fault, and perhaps not intentional on the part of those 

 who have done so, but has arisen firstly from the difficulty that many observers 

 have in seeing his lines in the ultra-red, on account of their exceeding faint- 

 ness; and secondly, from some of them being Solar, and others Telluric, to a degree 

 that even he himself had not fully anticipated. It would seem, therefore, to be 

 high time, in Brewster's own Society and Country, to come to a clearer under- 

 standing on the facts of his nomenclature, touching at least those three chief 



VOL. XXXII. PART II. 2 Q 



