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III. — Note on the Little b Group of Lines in the Solar Spectrum and the New 

 College Spectroscope. By C. Piazzi Smyth, Astronomer-Royal for Scot- 

 land. (Plate XV.) 



(Read June 1883). 



Every spectroscopist is perfectly aware that the group of dark Fraunhofer 

 lines in the Solar Spectrum, known as " little b," is composed of the biggest, 

 broadest, most colossal lines in all the brighter part of any and every spectrum 

 depending on Sunlight, whether direct from the Sun or reflected from the earth's 

 atmosphere, the Moon, or any of the planets. 



How the apparent misnomer came about, was not on the principle of the 

 biggest gun ever made by our military, being termed "the Woolwich infant ; " 

 but because after Fraunhofer had spaced out the spectrum into nearly equal 

 lengths, so far as the majority of the chief lines allowed, and called them by 

 capital letters, beginning with great A in the ultra red and ending with great 

 H in the ultra violet, — he then began again at the red end, and marked all the 

 notable intervening lines by small letters. Whence it came about that those 

 sometimes very imposing bands of telluric water- vapour lines " little a " are 

 found between great A and great B ; and those grand and truly solar lines 

 in the green, little b, are found, by accident as it were of Nature, between great 

 E and great F. 



In the smaller class of pocket spectroscopes and on the faint light of the 

 Sky, observers merely recognise two strong lines ; the first from the red end is 

 b 1 , and the second, considerably thicker, is b 2 * A very little increase of power, 

 however, easily shows b 2 to be composed of two lines, b 2 and b 3 ; and Fraun- 

 hofer himself had announced it. But that such b 3 was still further composed 

 of two lines was I believe first discovered by Professor Swan, and published in 

 our Transactions as part of his now classical spectrum paper of 1855 and 1856. 

 For therein (vol. xxi. p. 427) he mentions most clearly — though calling our 

 b\ b 2 , b 3 by the names b, b\ b 2 that " on the 20th of May, about 7 h 10 m p.m. 

 when the sun was rather low on the horizon, but free from clouds, he observed 

 with a magnifying power of 21 (on his large theodolite telescope, directed to a 

 single prism of 60°) the line b 2 (our b 3 ) to be very finely but distinctly double ; 

 so that," he adds, " the group consists of 4 lines " (our b 1 , b 2 , b 3 , and b 4 ). While 

 on page 426 he had already expressed his admiration for the group " as being 

 one which, whether we regard the singular configuration or the strength of the 

 lines which compose it, is perhaps the most notable in the solar spectrum." 



In the Philosophical Transactions of the Royal Society (London) for 1860, 



VOL. XXXII. PART I. G 



