III. OK A NEW CASE OF INTERFERENCE OF THE RAYS 

 OF LIGHT. 



Transactions of the Royal Irish Academy, Vol. XVII. 



THE experiment of Fresnel, on the interference of the lights pro- 

 ceeding from the same origin, and reflected by two mirrors inclined 

 at a very obtuse angle, has been justly regarded as one of the 

 most important in the whole range of physical optics. The 

 principle of interference itself had, indeed, been stated broadly by 

 Young, and supported by the evidence of phenomena, which, to 

 the unbiassed inquirer, left little to desire. All these phenomena, 

 however, admitted of other possible explanations ; and the advo- 

 cates of the corpuscular theory of light had recourse to these, 

 rather than admit the truth of a law which afforded such strong 

 support to the undulatory theory. In most of these phenomena, 

 the light was in part intercepted by an obstacle, and it was con- 

 ceived that, in passing by the edge, the molecular action, which 

 might be supposed to exist between the particles of the body and 

 those of light, was sufficient to account for the facts observed. 

 But, in Fresnel's experiment, the two lights which interfere are 

 regularly reflected by the surfaces of the mirrors, according to 

 the ordinary laws, and are divested of every extraneous circum- 

 stance which could, by possibility, be supposed to influence the 

 result. This experiment, accordingly, has materially changed the 

 character of the controversy respecting the nature of light ; and 

 the advocates of the Newtonian theory, of the present day, are 

 forced to admit the principle thus rigidly established, and labour 

 only to show how the theory and that principle may be recon- 

 ciled. 



