REFLEXION AND REFRACTION OF LIGHT. 47 



the wave-theory, on the other hand, the phenomenon is easily 

 comprehended. As the vibrations of the air excite those of sound- 

 ing bodies, and communicate to them a motion which continues 

 for some time after the exciting cause has ceased to act, so it 

 must also be with the undulations of the ether. When the body 

 is in unison with the incident light, their vibrations will continue 

 isochronous, and the undulations of the ether excited by the body 

 will be of the same length as those by which it is itself excited. 

 In the other case, the period of vibration, and consequently the 

 length of the wave, will be altered, and the excited and exciting 

 lights will be of different colours. The fact observed by Canton 

 is indeed not so easily explained. Young supposed that the vibra- 

 tions of the body may be abruptly suspended by cold, and may 

 proceed anew when released from this restraint, like a string 

 which has been stopped and detained in any part of its vibration 

 on either side of the centre. 



The fixed lines in the solar spectrum first noticed by Wollaston, 

 and afterwards more minutely traced by Fraunhofer, have lately 

 been examined with great care, and with his usual success, by Sir 

 David Brewster ; and he has observed a remarkable coincidence 

 between these lines and the dark bands of the spectrum of the 

 nitrous-acid gas.* Sir David Brewster has also studied, in' con- 

 nexion with the same subject, the definite absorbing effects of the 

 earth's atmosphere. This has been effected by examining the 

 solar spectrum, when the sun was near the horizon ; and it has 

 been found that most of the dark bands thus developed belonged 

 to the fixed lines of Fraunhofer, which were thus, as it were, 

 widened and brought out by the absorptive action of the atmo- 

 sphere. A similar result has been arrived at in other cases, and it 

 has been found that the points of the spectrum on which absorbing 

 bodies exert the strongest specific actions are generally coincident 

 with the deficient rays of solar light, t This singular connexion 

 gives considerable weight to the speculations of Sir David Brew- 

 ster respecting the latter phenomena. + 



The observation of the fixed lines in the solar spectrum led 

 Fraunhofer to examine the optical characters of the lights ema- 



* " On the Lines of the Solar Spectrum." Edin. Tram., vol. lii. 

 t "On the Colours of Natural Bodies." Edin. Traits., voL xii. 

 J Report on Optics. 



