On a New Case of Interference of the Rays of Light. By the Rev. Humpurery 
Luoyp, A. M., M.R.I.A. Fellow of Trinity College, and Professor of Natural 
and Experimental Philosophy in the University of Dublin. 
Read January 27, 1834. 
Tue experiment of Fresnex, on the interference of the lights proceeding 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 Youne, 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 advocates 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 ob- 
stacle, and it was conceived 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 Fresnet’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 circumstance 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 reconciled. 
While examining this important experiment—the adjustment of which is a matter 
of some delicacy—it occurred to me that the fact of direct interference might be 
shown in a yet simpler manner, by the mutual action of direct and reflected light. 
An interference of this kind was assumed by Youne to account for some of the phe- 
nomena of diffraction; but Fresne~ showed that the explanation was incomplete, 
and that the phenomena in question were caused merely by the interference of the 
secondary waves, reflexion playing no part in their production. Under these circum- 
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