248 FRESXEL. 



experiments of Fresnel destroy entirely all the arguments 

 which had heen 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. Poi*son : which, however, were in a great degree anticipated by 

 Dr. Young [Chromatics, Encycl. 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 intensit}'' 

 of light reflected from Mercury to determine its refractive index. The 

 formula of Young is derived from the analogy of the motion commu- 

 nicated from a portion of aether in one medium, to that in a different 

 state of density in another, with that of the impact of unequal elastic 

 bodies, and icithout 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. If m 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 



