REPORT ON PHYSICAL OPTICS. 345 



wards showed that the result was independent of the expression of 

 the intensity of the reflected light ; and by the aid of the property 

 discovered by M. Arago, namely, — that the light is reflected in 

 the same proportion at the first and second surfaces of a trans- 

 parent plate, — he extended the conclusion to all incidences*. 

 The general expression of the intensity of the light in any part 

 of the reflected or transmitted rings has been given by Professor 

 Airy f. 



Here, then, we have reached a point with respect to which the 

 two theories are completely opposed. According to both, a cer- 

 tain portion of light is reflected from the first surface of the 

 plate. This in the Newtonian theory is left in all cases to pro- 

 duce its full effect ; while in the wave- theory it is, at certain 

 intervals, wholly destroyed by the interference of the other pen- 

 cil ; and the dark rings should be absolutely/ black in homoge- 

 neous light. The latter of these conclusions seems to accord 

 with phenomena, while the former is obviously at variance with 

 them. This is clearly shown by an experiment of Fresnel. A 

 prism was laid upon a lens having its lower surface blackened, 

 a portion of the base of the prism being suifered to extend be- 

 yond the lens. The light reflected from this portion, according 

 to the Nevrtonian theory, should not surpass in intensity that of 

 the dark rings. The roughest trial is sufficient to show that the 

 intensity of the light in the two cases is widely different, and 

 to prove that the dark rings cannot arise (as they are supposed 

 to do in the theory of fits,) from the suppression of the second 

 reflexion J. 



Mr. Potter has applied a new method of " photometry by com- 

 parison" to determine the relative intensities of the light in the 

 bright and dark rings of the transmitted system. In this method 

 the ratio of the intensities of the light reflected from two plane 

 glasses is varied, by varying the incidence, until it, is judged to 

 be equal to the ratio of the light in the bright and dark rings. 

 The former ratio is then deduced from the incidence by means 

 of an empirical formula. In this manner Mr. Potter concludes 

 that the ratio of the light in the rings, at a perpendicular inci- 

 dence, is 2-48 for green light, and 3-49 for red §. The ratio de- 

 duced from the principles of the wave-theory is about 1-20 in 



city of propagation witliin the plate is a mean propoitional to the velocities in 

 the bounding media. 



* jinnales de Chimie, tom. xxiii. p. 129. 

 . t Math. Tracts, p. 302, &c. 



X Memoire siir la Diffraction, p. 347. 



§ Lond. Sf Edin. Phil. Mag., 3vd Series, vol. i. p. 174. 



