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VIII. On the Solar Spectrum, and the Spectra of the Chemical 

 Elements. Bij Professor H. E. Roscoe. 



To the Editors of the Philosophical Magazine and Journal. 



Owens College, Manchester, 

 Gentlemen, December 21, 1861. 



THE following extract from Professor Kirchhoff's interesting 

 memoir " On the Solar Spectrum, and the Spectra of the 

 Chemical Elements," just published, with magnificent maps of 

 the lines, in the ( Transactions of the Berlin Academy/ and about 

 to appear in English, may interest your readers as helping to 

 explain the appearance of the blue band in the spectrum of in- 

 tensely ignited lithium vapour, first noticed by Dr. Tyndall, and 

 referred to in your Number of last month by Dr. Frankland. 



"The position of the bright lines (or, to speak more precisely, 

 the maxima of light in the spectrum of an incandescent vapour) 

 is independent of the temperature, of the presence of other sub- 

 stances, and of all other conditions except the chemical compo- 

 sition of the vapour. The truth of this assertion has been 

 well tested by experiments made by Bunsen and myself with 

 special regard to this point, and it has been confirmed by 

 many observations which I have had occasion to make with 

 the extremely sensitive instrument above described*. Never- 

 theless the spectrum of the same vapour may, under different 

 circumstances, appear to be very different. Even the alteration 

 of the mass of the incandescent vapour is sufficient to give another 

 character to its spectrum. If the thickness of the column of 

 vapour whose light is being examined be increased, the luminous 

 intensities of all the lines increase, but in different ratios. In 

 accordance with a theorem which will be considered in the next sec- 

 tion, the intensity of the bright lines increases more slowly than 

 that of the less visible lines. The impression which a line produces 

 on the eye depends upon its breadth as well as upon its brightness. 

 Hence it may happen that one line, being less bright although 

 broader than a second, is less visible than the latter when 

 the thickness of incandescent gas is small, but becomes more 

 distinctly seen than the latter when the thickness of the vapour 

 is increased. Indeed if the luminosity of the whole spectrum be 

 so lowered that the most striking of the lines only are seen, it 

 may happen that the spectrum appears to be totally changed 

 when the mass of the vapour is altered. Change of temperature 

 seems to produce an effect similar to this alteration in the mass 

 of j the incandescent vapour. If the temperature be raised, no 

 deviation of the maxima of light is observed; but the intensities 

 of the lines increase so differently, that those which are most 

 plainly seen at a high temperature are not the most visible at a 



* With the magnificent instrument here referred to, Kirchhoff was able 

 to separate the two " D " lines by a width of 4 millimetres,— H, E. R. 



