248 FRESNEL. 
experiments of Fresnel destroy entirely all the arguments 
which had been relied on in the phenomena of diffrac- 
tion to establish the materiality of light. 
The important branch of optics which treats of the 
intensity of reflected light, transmitted and absorbed by 
bodies, which is designated by the name of photometry, 
is but in its infancy; it at present consists of nothing 
more than isolated results, whose exactness may be open 
to much question. General mathematical laws are 
wholly wanting. Some attempts made a few years ago 
have, however, led to a very simple rule which, for 
every kind of transparent media, connects the angles of 
the first and second surface at which the reflexions are 
equal.* 
* The measures of intensity of light here alluded to are those of 
M. Poisson; which, however, were in a great degree anticipated by 
Dr. Young [Chromatics, Hncycl. Brit.], though Poisson calls his 
reasoning indirect, an opinion in which Sir J. Herschel says he can- 
not concur. Poisson takes the case of perpendicular incidence, 
and adopts the hypothesis of the vibrations being coincident with the 
direction of the ray ; he thus obtains expressions for the relative inten- 
sities of the incident, reflected, and transmitted rays; and thence, 
again, of the ray reflected at the second surface. These result in 
terms of the index of refraction. Arago applied this principle (as far 
as any photometrical measurements can be relied on) for the intensity 
of light reflected from Mercury to determine its refractive index. The 
formula of Young is derived from the analogy of the motion commnu- 
nicated from a portion of sether in one medium, to that in a different 
state of density in another, with that of the impact of unequal elastic 
bodies, and without any assumption as to the direction of the vibrations; 
the same principle on which the formulas of Fresnel are deduced in 
Mr. Airy’s Tract, (Art. 128.) See Sir J. Herschel on Light, Art. 
592; and Lloyd’s Lectures on the Wave Theory, p. 31. 
Mathematically, Young’s formula is deduced in this way. Ifm and 
m be the masses of two elastic bodies, m impinging on m/ at rest, by 
the principles of mechanics (the velocity of m being unity) it is well 
known that after impact m retains a velocity 
