RADIANT HEAT AND ITS RELATIONS 85 



an electric lamp, and causing the light to pass through an 

 arrangement of prisms, it is decomposed. We have the 

 effect obtained by Newton, who first unrolled the solar 

 beam into the splendors of the solar spectrum. At one 

 end of this spectrum we have red light, at the other, vio- 

 let; and between those extremes lie the other prismatic 

 colors. As we advance along the spectrum from the red 

 to the violet, the pitch of the light if I may use the ex- 

 pression heightens, the sensation of violet being produced 

 by a more rapid succession of impulses than that which 

 produces the impression of red. The vibrations of the 

 violet are about twice as rapid as those of the red; in 

 other words, the range 6f 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 sees it, is 577 millions of millions: while the num- 

 ber of impulses imparted, in the same time, by the second 

 band, is 600 millions of millions. We may project upon 

 a white screen the beautiful stream of green light from 



