236 C. PIAZZI SMYTH ON NOTE ON SIR DAVID BREWSTER'S 



By dint of Captain Abney's really wonderful processes of changing the colour 

 of silver for transmitted light, he was enabled to photograph not only all that 

 part of the infra-red end of the solar spectrum discovered with so much pain 

 and labour by Brewster, but to procure records of other lines, some of them 

 very grand ones too, extending nearly three times as far away, and into what 

 is, to the human eye, absolute, unmitigated darkness. There is, therefore, not 

 the slightest intention here to compete with him in spectral range ; and I have 

 purposely left his spectrum strip bright and of full height up to the extreme left 

 hand end of my paper, to indicate that his view extends very much further still 

 in that same direction. The only point of difference in fact which I have with 

 him and his distinguished fellow-labourer, or the Central Metropolitan Society 

 which publishes their work, is, — that the very strong line, which from its place 

 in the spectrum can be no other whatever than Y, he calls Z ; and the letter Y 

 he gives no place to. 



Apparently Captain Abney and Colonel Festing had not seen the real Z line 

 at all ; and with little doubt because they worked in a too high Sun for it, 

 though excellent for their other, and chief, objects. For Strip 6 shows the result 

 of three observations which I had the fortune to make during an unusually long, 

 bright sun-shiny afternoon on the 30th of May last at the house No. 15 Eoyal 

 Terrace, Edinburgh. The apparatus was moderate in power ; there was 

 no attempt to resolve bands into their very thin component lines ; but only to 

 note the main features of Y and Z, " Great A" being given in as a necessary 

 mile-stone. 



At 5 h 50 m p.m. then, of distinct lines, Y alone was visible outside Great A. 



At 6 h 40'" p.m., with a lower Sun, besides Y, there was a suspicion of Z. 

 But 



At 8 h m p.m., with a very much lower Sun, there, besides Y nearly as before, 

 stood out Z as quite a strong line, accompanied too with bands, and proving 

 itself to be Telluric without a doubt. 



Finally, Strip 7 represents what the ancient Greeks might have called the 

 apotheosis of line Y, in its glorious identification at last by M. Henri Becquerel, 

 with a bright emission line of the same Solar Sodium (Na), which produces that 

 grand turning key to all the modern developments of Spectrum analysis, viz., 

 the Solar lines D 1 and D 2 . 



The fullest account of this final confirmation of the Solar character of Y 

 that I have yet seen is that contained in the Comjries Rendus for July 9, 1883, 

 pp. 71-74, by M. Henri Becquerel himself. He had been researching the 

 infra-red spectrum of chemistry by his celebrated Father's method of the pheno- 

 mena of Phosphorescence, and found two new distinct and widely separated 

 salt lines to exist therein. He next proved the correspondence of both of them 

 with'two extra strong and equally widely separated lines at the same points of 



