Chap XXVI.] 



THE SPECTRUM. 



321 



According to the undulatory theory, the different 

 colours are due to vibrations of different rates of 

 rapidity, vibrations whose periods and wave lengths 

 are different. The wave length of red light is greater 

 than that of violet, and the time of vibration of red 

 is also greater than that of violet. Thus the extreme 

 red rays vibrate at the rate of 395 billion times per 

 second, and their period of vibration is, therefore, one 

 395 billionth of a second ; the violet rays vibrate 763 

 billion times per second. But the wave length of the 

 ray changes in passing through different media, the 

 velocity of propagation changes. Some rays are re- 

 tarded, the violet more than the red. The rays are, 

 therefore, differently refracted, and dispersion is the 

 result. 



B>aa'k lines of the solar spectrum. If the 

 beam of light, which has been split up into its con- 

 stituents, be obtained from the oxy hydrogen lamp, or 



Fig. 149. The Dark Lines of the Solar Spectrum. 



The brackets below indu-ate the regions occupied by the different colours, in 

 the order, red, orange, yellow, etc, 



a gas flame, the band of colours is continuous, one 

 colour gradually merging into another. If, however, 

 sunlight has been used, tli3 spectrum is seen to be 

 interrupted by a series of dark bands crossing it 

 vertically. They are called Fraunhofer's lines, 

 because Fraunhofer first described them in 1814. 

 Fraunhofer counted a large number of these lines, and 

 marked their positions. The more prominent he 

 signified by letters of the alphabet ; thus, A, B, and c 

 lines are all in the red part of the spectrum, the D 

 line is in the border-land between orange and yellow, 

 v 7 



