74 REPORT ON PHYSICAL OPTICS. 



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

 This, in the Newtonian theory, is left in all cases to produce its full 

 effect ; while in the wave-theory it is, at certain intervals, wholly 

 destroyed by the interference of the other pencil, and the dark 

 rings should he absolutely black in homogeneous 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 suffered to extend beyond the lens. The light re- 

 flected from this portion, according to the Newtonian theory, 

 should not surpass in intensity that of the dark rings. The rough- 

 est 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.* 



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 incidence, is 2'48 

 for green light, and 3*49 for red.f The ratio deduced from the 

 principles of the wave-theory is about 1-20 in the case of crown 

 glass. But, independently of the uncertainty connected with the 

 empirical law, which is taken by Mr. Potter as the basis of his com- 

 putation in these deductions, the photometrical method itself seems 

 to be open to objection. It appears to be assumed, in the appli- 

 cation of that method, that where the quantity of light incident 

 upon an irregularly reflecting surface is given, the quantity of 

 reflected light will be the same in its entire amount, and in all 

 directions, whatever be the incidence. This seems to be contra- 

 dicted by obvious facts. There is yet another difficulty in the 

 application of this method, which appears to leave room for some 



* Memoire sur la Diffraction, p. 347. 

 t Phil. Mag., 3rd Series, vol. i. p. 174. 



