362 KOC'RTH RKPORT — 1834. 



sure processes of niuthcmatical deduction ; and we arc therp- 

 fore unable to state how far the explanation offered is com- 

 petent to express even the general facts, — far less can we 

 calculate them numerically, and compare the results with those 

 of observation. 



The first attempt to connect the modifications of reflected 

 light with the theory of waves, was made by Dr. Thomas 

 Young. This sagacious philosopher succeeded in solving the 

 problem of reflexion in the case of perpendicular incidence, 

 and showed that the intensity of the reflected light in that case 

 was represented by a simple function of the refractive index*. 

 This formvda was afterwards reproduced as the result of a more 

 refined analysis by M. Poisson, in a memoir on the simultane- 

 ous motions of two elastic fluids in contact, read to the French 

 Academy in 1817 f. In that memoir, however, the author had 

 considered only the case of perpendicular incidence; or the law 

 of propagation of a plane wave parallel to the bounding surface 

 of the two media. In a subsequent memoir, to which I have 

 already alluded, and which was read to the Academy in the year 

 1823 I, he has resumed the problem generally, and examined 

 the modifications produced in the intensity as well as the direc- 

 tion of a wave, or series of waves, in passing from one fluid to 

 another of the same elasticity but of a diff^erent density. The 

 expressions obtained for the intensity of the reflected and re- 

 fracted waves, are functions of the angle of incidence and of the 

 ratio of the velocities of propagation in the two media. When 

 the wave is incident upon the surface of the denser medium, 

 the expression for the intensity of the reflected wave vanishes 

 at a certain angle, whose tangent is equal to the ratio of the 

 velocities of propagation. At this angle, which is the angle 

 of complete polarization, objects should therefore cease to be 

 visible by reflected light; — a result which is contradicted by 

 all experience, and is only true when the light is polarized in a 

 plane perpendicular to the plane of reflexion. When the wave 

 is reflected at the surface of the rarer medium, there are two ex- 

 pressions for the intensity, for incidences above and below the 

 limiting angle of total reflexion respectively ; there are also in 

 this case two angles of evanescence. These conclusions, which 

 apply to the case of sound as well as light, are sufficient to show 

 the physical inapplicability of the theory. 



• Encyc. Brit., Supp., Art. Chromatics. 



•(• Mem. Inst., torn. ii. 



X Only a portion of this memoir has heen printed in the Memoirs of the 

 Institute, under the title " MSmoire sur le Mouvement de deux Fluides 

 elastiques superposes," torn. x. 



