INTERFERENCE OF LIGHT. 87 



Hence 



26 6 



But this difference is equal to n ^, A being the length of a 



wave ; we have, therefore, 



nb\ 



in which the even values of n correspond to the places of the 

 bright bands, and the odd values to those of the dark ones. 



The preceding formula enables us to compute the length 

 of a wave of light, when the distances b, c, and x have been 

 determined by accurate measurement. It has been found in 

 this manner that the length of a wave is -0000266 of an inch 

 for the extreme red rays ; -0000167 for the extreme violet ; 

 and -0000203, or about the 30000 of an incil > for ^e mean 

 rays of the spectrum. 



(106) But though the principle of Interference seemed to 

 be established by the experiments and reasonings of Young, 

 it was not freed from all question. It might be supposed 

 that the light passing by the edges of the apertures, in the 

 experiment last described, underwent modifications of some 

 kind or other which produced the observed effects. It was, 

 therefore, of importance to show that these effects were 

 wholly independent of apertures or edges ; and that any two 

 rays proceeding from the same luminous origin, and meeting 

 under a small obliquity, will interfere in the manner already 

 described, whatever be the attending circumstances. This 

 has been done by Fresnel ; and the experiment, which he de- 

 vised for the purpose, has been justly ranked among the 

 most important and instructive in the whole range of Physi- 

 cal Optics. 



