PHYSICS-NINETEENTH CENT.SPECTROSCOPY. 481 



purposes at Munich. He thus was able to observe a very great number 

 of fine dark lines crossing the spectrum transversely, and in 1814 he 

 published an account of these lines with a carefully drawn map, in 

 which the positions of 354 lines were laid down ; but he counted 

 altogether no fewer than 576 in the solar spectrum. These lines are 

 of different intensities and breadths, some of them appearing as deli- 

 cate as a spider's line does to the unassisted eye, while Others are 

 comparatively strongly marked. The most conspicuous lines (or 

 groups of lines) are indicated in Fraunhofer's map by the letters A, B, 

 c, D, E, F, G, H, and by these letters the lines are still designated, as 

 they constitute as many fixed points, to which it is often convenient 

 to refer the position in the spectrum of other lines ; but for accurate 

 purposes, any given line is now generally indicated by a number re- 

 presenting the wave-length (page 457) of the corresponding ray. 



Fraunhofer ascertained that these lines were always produced in. 

 identically the same position by sunlight, whether by the rays direct 

 or reflected from the moon or the planets. But when he examined 

 the light of fixed stars, he found that though these gave spectra con- 

 taining dark lines, the lines were very different in their number, order, 

 and position from those in the solar spectrum. Hence he concluded 

 that the dark lines, whatever their cause, did not depend upon our 

 atmosphere, but upon something special in the light of our own and 

 other suns. This observation may be considered the starting-point of 

 perhaps one of the most interesting and extraordinary series of re- 

 searches that belong to the present century. 



The course of discovery leads us now to the mention of certain re- 

 searches in coloured flames by Brewster, Fox-Talbot, and Sir John 

 Herschel. In 1822 Brewster proposed the use of spirit-lamps with 

 salted wicks as a means of obtaining monochromatic light for optical 

 experiments. The fact of compounds of sodium yielding a light con- 

 taining only yellow rays had, in fact, been announced long before 

 Brewster's time. About the same time Sir John Herschel examined 

 by the prism the lights given off by flames containing severally chloride 

 of strontium, chloride of copper, nitrate of copper, boracic acid, and 

 chloride of potassium. The first of these substances introduced into 

 the flame of a spirit-lamp causes an intense red coloration, the next 

 three tinge the flame green, and the last one makes it violet coloured. 

 In the article on Light in the "Encyclopedia Metropolitan" in 1827, 

 Herschel mentions the colours imparted to flames by compounds of 

 lime, strontia, lithia, baryta, and copper ; and he says that the colours 

 may readily be produced by placing salts of these substances, but 

 preferably the chlorides, in powder in the wick of a spirit-lamp. He 

 offers a distinct suggestion of the employment of flame colorations in 

 chemical analysis : " The colours thus communicated by the different 

 bases to flame afford in many cases a ready and neat way of detecting 

 extremely minute quantities of them." 



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