392 Professor Tyndall [June 7, 



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 analyze the sun. 



We have now some hard work before us ; hitherto we have been 

 delighted by objects which addressed themselves rather to our aesthetic 

 taste than to our scientific faculty. We have ridden pleasantly to 

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

 wearily 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 compo- 

 nents are refracted in different degrees, and thus its colours are drawn 

 apart. Now the colour depends solely upon the rate of oscillation of 

 the particles of the luminous body ; red light being produced by one 

 rate, blue light by a much quicker rate, and the colours between red 

 and blue by tlie 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. They 

 have particles oscillating so as to produce red ; others, to produce orange ; 

 others, to produce yellow, green, blue, indigo, and violet respectively. 

 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, augments the 

 pitch. Imagine such a player to move his finger 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 

 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 his finger on a certain point, and to sound the corre- 

 sponding note ; then to pass on to another point more or less distant, 

 and sound its note ; then to another, and so on, thus sounding particular 

 notes separated from each other by gaps which correspond to the 

 intervals of the string passed over ; we should then have the exact 

 analogue of a spectrum composed of separate bright bands with 

 intervals of darkness between them. But this, though a perfectly true 

 and intelligible analogy, is not sufficient for our purpose ; we must look 

 with the mind's eye at the very oscillating atoms of the volatilized 

 metal. Figure these atoms connected by springs of a certain tension, and 

 which, if the atoms are squeezed, together push them asunder, or if the 

 atoms are drawn apart, pull them together, causing them, before coming 

 to rest, to quiver at a certain definite rate determined by the strength 

 of the spring. Now the volatilized metal which gives us one bright 

 band is to be figured as having its atoms united by springs all of the 

 same tension, its vibrations are all of one kind. The metal which gives 

 us two bands may be figured as having some of its atoms united by 

 springs of one tension, and others by a second series of springs of a 



