336 FRAGMENTS OF SCIENCE. 



lines. They had been long a standing puzzle to philo- 

 sophers. The bright lines yielded by metallic vapours 

 had been also known to us for years ; but the connec- 

 tion between both classes of phenomena was wholly 

 unknown, until Kirchhoff, with admirable acuteness, 

 revealed the secret, and placed it at the same time in 

 our power to chemically analyse the sun. 



We have now some difficult work before us. Hither- 

 to we have been delighted by objects which addressed 

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

 tific 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 com- 

 ponents 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 lumi- 

 nous body ; red light being produced by one rate, blue 

 light by a much quicker rate, and the colours between 

 red and blue by the* intermediate rates. The solid in- 

 candescent 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. 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, the 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 

 colours insensibly blend together without gap or inter- 



