336 FRAGMENTS OF SCIENCE. 



were first noticed by Dr. Wollaston, but were multi- 

 plied and investigated with profound skill by Fraun- 

 hofer, and named after him Fraunhofer's lines. They 

 had been long a standing puzzle to philosophers. The 

 bright lines yielded by metallic vapours had been also 

 known to us for years; but the connection between 

 both classes of phenomena was wholly unknown, until 

 Kirchhoff, with admirable acuteness, revealed the se- 

 cret, and placed it at the same time in our power to 

 chemically analyse the sun. 



We have now some difficult work before us. Hith- 

 erto we have been delighted by objects which addressed 

 themselves as much to our aesthetic taste as to our sci- 

 entific faculty; we have ridden pleasantly to the base 

 of the final cone of Etna, and must now dismount and 

 march through ashes and lava, if we would enjoy the 

 prospect from the summit. Our problem is to connect 

 the dark lines of Fraunhofer with the bright ones of 

 the metals. The white beam of the lamp is refracted 

 in passing through our two prisms, but its different 

 components are refracted in different degrees, and thus 

 its colours are drawn apart. Now the colour depends 

 solely upon the rate of oscillation of the atoms of the 

 luminous body; red light being produced by one rate, 

 blue light by a much quicker rate, and the colours be- 

 tween red and blue by the intermediate rates. The 

 solid incandescent coal-points give us a continuous 

 spectrum; or in other words they emit rays of all possi- 

 ble periods between the two extremes of the spectrum. 

 Colour, as many of you know, is to light what pitch 

 is to sound. When a violin-player presses his finger on 

 a string he makes it shorter and tighter, and thus, 

 causing it to vibrate more speedily, heightens the pitch. 

 Imagine such a player to move his fingers slowly along 

 the string, shortening it gradually as he draws his bow, 



