OF THE RAYS OF LIGHT. 153 



beautiful system of bands was visible, in every respect similar to one- 

 half of the system formed by the two mirrors in Fresnel's experiment. 



The first band was a bright one, and colourless. This was 

 succeeded by a very sharply defined black band ; then followed a 

 coloured bright band, and so on alternately. Under favourable 

 circumstances I could easily count seven alternations ; the breadth 

 of the bands being, as far as the eye could judge, the same 

 throughout the series, and increasing with the obliquity of the 

 reflected beam. The first dark band was of intense blackness. 

 The darkness of the succeeding bands was less intense, as they 

 were of higher orders ; and after three or four orders, they were 

 completely obliterated by the closing in of the bright bands. At 

 the same time the colouration of the bright bands increased with 

 the order of the band; until, after six or seven alternations, the 

 colours of different orders became superimposed, and the bands 

 were thus lost in a diffused light of nearly uniform intensity. 

 All these circumstances are similar to those observed in Fresnel's 

 experiment, and correspond exactly with the results of theory. 



These bands are most perfectly defined when the eyepiece is 

 close to the reflector. Their breadth and colouration increased with 

 the distance of the eyepiece, but remained of a finite and very 

 sensible magnitude, when the latter was brought into actual 

 contact with the edge a circumstance which distinguishes them 

 altogether from the diffracted fringes formed on the boundary of 

 the shadow. 



These fringes appear to me to possess some interest in a theo- 

 retical point of view, independently of that which attaches to 

 them as illustrations of an important general law. Depending on 

 the interference of two lights, one of which proceeds directly 

 from the luminous origin, while the other has undergone re- 

 flexion, they would seem to afford the means of detecting any 

 difference which might exist in their condition when they meet, 

 and therefore of tracing the modifications produced by reflexion. 



There are two circumstances which chiefly demand our atten- 

 tion in the case of reflected light namely : 1st, tlic amplitude of 

 the vibration, on which the intensity of the light depends ; and 

 2ndly, the phase. The facts before us seem, to a certain extent, to 

 bear on both these points. 



The reasonings of Fresnel, with respect to the intensity of 

 reflected light, are partly of an analogical nature, and very far 



