O. N. Rood on Photometric Haperiments. 7 
of common light reflected at a perpendicular incidence was first 
given by Thomas Young; I -Gie ; the intensity of the 
incident beam being equal to unity, and » being the index of 
refraction of the reflecting substance. The same formula was 
afterwards reached by Poisson in a rigid and learned analysis 
of the subject, and it was again deduced by Fresnel.* As is 
well known Fresnel’s formulas were subsequently modified by 
| the celebrated Cauchy, but in Cauchy’s formula for reflection as 
: soon as the incidence of the light differs considerably from the 
polarizing angle, the small quantities which depend on e, the 
coefficient of ellipticity, become so much reduced that they can 
be omitted from the numerator and denominator, and Cauchy’s 
formula becomes identical with Fresnel’s.+ It hence appears 
that the formula above quoted is as well sign tie es- 
tablished, as any which has been deduced under the guidance 
of the Undulatory Theory, and one would naturally sup 
that it had been often tested carefully by experiment. This 
does not seem to have been the case, and tac not know that it 
has ever been rigidly tried by a delicate photometric method. 
On this account I have made a series of hietyaticia on plates 
of colorless glass which are detailed below. 
Mode of experimenting. 
* Pogg. Annalen, Bd. xxii, p. 98. Jamin in Poge. ta ee we 
tPogg. Annalen, Bd. ci, p. 241. t 08, Ee ey. Pee 
