RADIANT HEAT AND ITS RELATIONS 79 



who first unrolled the solar beam into the splendours of 

 the solar spectrum. At one end of this spectrum we 

 have red light, at the other, violet ; and between those 

 extremes lie the other prismatic colours. As we advance 

 along the spectrum from the red to the violet, the 

 pitch of the light if I may use the expression 

 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 of the visible spectrum is about 

 an octave. 



There is no solution of continuity in this spectrum ; 

 one colour changes into another by insensible gradations. 

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

 ally augmenting pitch, were vibrating at the same time. 

 But turning to another spectrum that, namely, ob- 

 tained from the incandescent vapour of silver you 

 observe that it consists 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 number 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 which 

 these bands were derived. This luminous stream is the 

 incandescent vapour of silver. The rates of vibration 

 of the atoms of that vapour are as rigidly fixed as those 

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



