218 FRAGMENTS OF SCIENCE. 



succession of impulses than that which produces the im- 

 pression of red. The vibrations of the violet are about 

 twice as rapid as those of the red ; in other words, the 

 range of the visible spectrum is about an octave. 



There is no solution of continuity in this spectrum ; 

 one color changes into another by insensible gradations. 

 It is as if an infinite number of tuning-forks, of gradually 

 augmenting pitch, were vibrating at the same time. But 

 turning to another spectrum that, namely, obtained from 

 the incandescent vapor of silver you observe that it con- 

 sists of two narrow and intensely luminous green bands. 

 Here it is as if two forks only, of slightly different pitch, 

 were vibrating. The length of the waves which produce 

 this first band is such that 47,460 of them, placed end to 

 end, would fill an inch. The waves which produce the 

 second band are a little shorter ; it would take of these 

 47,920 to fill an inch. In the case of the first band, the 

 number of impulses imparted in one second to every 

 eye which now sees it, is 577 millions of millions ; while 

 the number of impulses imparted in the same time by the 

 second band is 600 millions of millions. I now cast upon 

 - the screen before you the beautiful stream of green light 

 from which these bands were derived. This luminous 

 stream is the incandescent vapor of silver. The rates of 

 vibration of the atoms of that vapor are as rigidly fixed as 

 those of two tuning-forks; and to whatever height the 

 temperature of the vapor may be raised, the rapidity of its 

 vibrations, and consequently its color, which wholly de- 

 pends upon that rapidity, remains unchanged. 



The vapor of water, as well as the vapor of silver, has 

 its definite periods of vibration, and these are such as to 

 disqualify the vapor, when acting freely as such, from 

 being raised to a white heat. The oxyhydrogen flame, for 

 example, consists of hot aqueous vapor. It is scarcely 

 visible in the air of this room, and it would be still less 



