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ARTICLE XIII. 
Photometrical Method of estimating the Intensity of ordinary and 
extraordinary Rays, as also those of reflected light; with Re- 
marks on M. Cauchy’s increase in the Intensity of Light by 
total reflexion, and reproduction of Fresnel’s formula for total 
reflexion, &c.* By F. KE. NEuMANN. 
[From Poggendorff’s Annalen, vol. xl. No. 4, 1837.] 
ARAGO’S photometrical process essentially consists in the 
formation of a system of fringes by interference of each of the 
two rays, the intensity of which is to be ascertained; these two 
systems of fringes are allowed to fall upon one another in such 
a manner that the maxima of the one covers the minima of the 
other; one of the systems is now diminished by a known method, 
until both the systems of fringes are completely destroyed. 
This process is capable of almost universal application, if we 
allow that the law according to which the light impinging upon 
a perfectly transparent medium, is divided into reflected and re- 
fracted light, and when the refracting medium is a crystal, that 
the manner in which light is divided. between the ordinary and 
extraordinary ray is accurately known. The knowledge of this 
law forms the practical foundation of M. Arago’s general photo- 
metric method of proceeding, at least as far as we can ascertain 
from the imperfect notices which have been published of it. It 
will therefore not be uninteresting to give a process which 
allows us to estimate by observation the intensity of the reflect- 
ed, refracted, ordinary and extraordinary rays with great accu- 
racy. The principle of this process has not. hitherto been ap- 
plied to photometric purposes, but where the circumstances per- 
mit of its application, it has more advantages than any other 
with which we are acquainted. It consists in allowing the rays, 
whose intensity we wish to ascertain, to act upon one another. 
Weare acquainted with two reciprocal actions of luminous rays. 
Two rays arising from the same source, polarized in different 
azimuths, and proceeding in the same direction, produce a re- 
sulting condition of the plane of polarization, the azimuth of 
* Translated by Dr. J. W. Griffith. We are indebted to Sir D. Brewster 
for suggesting and revising the translation of this Memoir.—Ep, 
