28 REPORT ON PHYSICAL OPTICS. 



and magnitude of the space of greatest vibration from either of 

 the components. The magnitude of the resulting vibration may be 

 the sum, or difference, of those of the component vibrations, or it 

 may have any value intermediate to these limits. When the com- 

 ponent vibrations are equal, the resultant may even vanish altoge- 

 ther ; and two lights of equal intensity, when added together, will 

 produce darkness, provided that the interval of retardation of 

 one wave on the other is an odd multiple of the length of half a 

 wave. 



This important consequence of the theory of waves the princi- 

 ple of interference of the rays of light was first distinctly stated 

 and established by Dr. Thomas Young, although some of the facts 

 by which its truth is experimentally confirmed were known to 

 Grimaldi.* The general calculation of the intensity of the result- 

 ing light, for any relative position of the interfering waves, is due to 

 Fresnel, and has been followed out and developed by Sir John 

 Herschel in his valuable Essay on Light. When a beam of homo- 

 geneous light is transmitted through two small apertures in a 

 card, or plate of metal, the light will diverge from each as from a 

 new centre. If the two apertures are close together, and the 

 diverging pencils received on a reflecting surface, a series of 

 parallel straight bands is observed, perpendicular to the line con- 

 necting the apertures, and separated by intervals absolutely dark. 

 That these alternations of light and darkness are produced by the 

 mutual action of the two pencils, Young proved by the fact that 

 when one of the beams is intercepted, the whole system of fringes 

 instantly disappears, and the dark intervals recover their former 

 brightness. 



The experiment of Fresnel is still more satisfactory. In this 

 important and instructive experiment, the fact of interference is 

 placed beyond all question. The two pencils proceed from one 

 common origin, and are separated simply by reflexion at plane 

 surfaces, without any attending circumstance which can, by possi- 

 bility, be supposed to influence the result. The phenomenon is 

 thus divested of everything non-essential, and it becomes impos- 

 sible to hesitate about its nature. But the accordance of theory 

 and experiment is maintained, not only in the general features of 



* This ingenious philosopher even stated explicitly that an illuminated body may 

 be rendered darker by the addition of light, and adduced a simple experiment in proof 

 of it. Phyiico-Mathesis de Lumine. Bologna, 1665. 



