256 FRAGMKNTS OF SCIENCE. 



might compare with those produced by our terrestrial 

 metals, and prove either their identity or difference? No. 

 The solar spectrum, when closely examined, gives us a 

 multitude of fine dark lines instead of bright ones. They 

 were first noticed by Dr. Wollaston, but were multiplied 

 and investigated with profound skill by Fraunhofer, and 

 named after him Fraunhofer's lines. They had been long 

 a standing puzzle to philosophers. The bright lines 

 yielded by metallic vapors had been also known to us for 

 years; but the connection between both classes of phenom- 

 ena was wholly unknown, until Kirchhoff, with admi- 

 rable acuteness, revealed the secret, and placed it at the 

 same time in our power to chemically analyze the sun. 



We have now some difficult work before us. Hitherto 

 we have been delighted by objects which addressed them- 

 selves as much to our aesthetic taste as to our scientific 

 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 colors are drawn apart. 

 Now the color depends solely upon the rate of oscillation 

 of the atoms of the luminous body; red light being pro- 

 duced by one rate, blue light by a much quicker rate, and 

 the colors between 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 possible 

 periods between the two extremes of the spectrum. Color, 

 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 aplayer 

 to move his fingers slowly along the string, shortening it 

 gradually as he draws his bow, trie note would rise in pitch 

 by a regular gradation; there would be no gap intervening 

 between note and note. Here we have the analogue to 

 the continuous spectrum, whose colors insensibly blend to- 

 gether without gap or interruption, from the red of the 

 lowest pitch to the violet of the highest. But suppose the 

 player, instead of gradually shortening his string, to press 



