LINE Y, IN THE INFRA-RED OF THE SOLAR SPECTRUM. 237 



the Solar spectrum. One, and the fainter of these two lines, was an immense 

 distance further into visual darkness than any of the lines in my plate. It was 

 even beyond Captain Abney's and Colonel Festing's furthest photographic, 

 being at 23 130 Wave Number. But the other, at 31 010 W.N.— to be freely 

 taken as equivalent to our 30 860 — is no less than Brewster's Y, and is 

 honourably mentioned by M. Becquerel as being such. 



It is indeed so instructive, as well as encouraging, to find the line thus 

 alluded to in Paris as " Brewster's Y line," three years after that letter was 

 expunged in London from the Solar spectrum, that I beg to conclude with 

 M. Henri Becquerel's own words thus : — 



" La vapeur de sodium, qui est principalement caracte'ris6e dans le spectre 

 lumineux par la double raie D, pre'sente dans l'infra-rouge deux tres fortes raies 

 caracteVistiques dont les longeurs d'onde sont 819 ( = W.N.Br. 31 010) et 1098 

 ( = W.N.Br. 23 130). Ces raies sont les memes lorsqu'on volatilise dans Tare, 

 du sodium metallique ou clu chlorure de sodium; elles coincide avec deux fortes 

 raies du spectre solaire. 



" La raie X 819 (W.N. 31 010) que Ton peut voir a l'ceil nu avec un spectro- 

 scope ordinaire, coincide avec une des plus fortes raies du spectre infra-rouge 

 du Soleil que Brewster avait vue, et designee par la lettre Y. 



"Dans les conditions ou Ton de'double les raies D, je n'ai pu dedoubler 

 distinctement la raie Y." 



POSTSCRIPT. 



The above concluding remark of M. H. Becquerel is instructive to those 

 who would desire to see for themselves this salt representative of Brewster's 

 Y line ; for it shows that even in his "Arc " light, notwithstanding its necessary 

 brilliance, that particular line must have been too faint for neat physical 

 notation ; and, indeed, unless an arc light can be prepared as bright as, or 

 possibly still brighter intrinsically than, a high summer sun, such almost must 

 be the result. 



With the most powerful Bunsen gas burners, consuming any amount of 

 Chloride of Sodium, the trial is quite hopeless; and even with 1-inch induction 

 sparks, condensed by a half-gallon jar between platinum points, of which one 

 rises through moistened salt, with the effect of making the D lines painfully 

 bright, I have not succeeded in causing the same salt's Y line, or lines, to 

 certainly appear. 



I have, however, in the search found three air lines much further towards 

 the infra-red than any of the standard list of air lines entered in Dr Watt's 

 invaluable Index of Spectra, as compiled by him from the observations of the 

 greater spectroscopists. 



VOL. XXXII. PART II. 2 R 



