Spectral Lines of Solar and Terrestrial Origin. 459 



vapours of the earth's atmosphere. We ought then to distin- 

 guish the solar from the telluric lines. 



Thus, of the eight principal Fraunhofer-lines, six are cha- 

 racteristic of metallic elements and are of solar origin (C and 

 F, hydrogen ; D, sodium : E, G, iron ; H, calcium) ; the 

 other two (A and B) are telluric. 



Fraunhofer had besides distinguished two complex groups, 

 namely a band a, very broad, in the extreme red, and a well- 

 marked triple line, b, in the green ; b is solar (magnesium), 

 and a of terrestrial origin (fig. 1). Brewster, on discovering 



Fig. 1. 



Solar spectrum, with principal lines marked. 



new bands of variable intensity in the spectrum, added new 

 designations ; it is sufficient here to mention the band a 

 situated in the orange, and the band 8 in the yellow. These 

 symbols have been adopted by Angstrom (Spectre normal du 

 Soleil). 



Up to the present time it has been considered a difficult, 

 and in any case a troublesome, matter to distinguish between 

 these two kinds of lines. It was necessary, in fact, to observe 

 the solar spectrum at two very different altitudes of the sun, 

 under various meteorological conditions, to be able to affirm 

 that the spectrum-lines do or do not change in intensity with 

 the thickness of the atmosphere or the quantity of water- 

 vapour traversed by the solar rays. 



The improvement of spectroscopes, in respect of the sharp- 

 ness and especially of the dispersion of the lines, has allowed 

 me to arrive at a method which renders the distinction between 

 the two kinds of rays in a certain sense intuitive. 



This method is founded on a principle due to M. Fizeau* — 

 the principle of the displacement of the spectral lines of the 

 light emitted by a source which is in absolute or relative 

 motion. We easily obtain the expression for the apparent 

 wave-length V of a radiation from a point in motion with a 



* Bulletin de la Societe Philomathique, de"cembre 1848; and Ann. de 

 Chim. et de Phys. 4 serie, t. xix. p. 211. 



