5 4 Intelligence and Miscellaneous Articles. 



In like manner the law given by our author, that reflection takes 

 place in a plane parallel to the boundary surface of transparent 

 substances, is not new. Assuming the change of phase on the 

 passage through the boundary surface of transparent substances to 

 be insensible, as all investigations hitherto appear to prove, that 

 law was established more than twenty years since, by Stokes *, 

 with the aid of the principle of reversion, without any special 

 assumption of a dynamic theory of light under the presupposition 

 that the effective forces depend merely on the situation of the 

 particles in motion. 



In accordance with this law is the fact which I have established 

 by experiment, " that with the same reflecting boundary surface of 

 two transparent substances, for corresponding angles of incidence 

 (which are mutually related as angles of incidence and refraction), 

 whether the reflection take place in the one or the other medium, 

 the observed values of the difference of phase of the two ray-com- 

 ponents polarized parallel and perpendicular to the plane of inci- 

 dence are equal and contrary, and the amplitudes of the two 

 components are in the same ratio. 



" If the reflection is positive in the one medium, it is negative 

 in the other, and vice versa " t. 



The distinction, originated by JaminJ, of transparent substances 

 into those with positive and those with negative reflection (that is, 

 with which the component polarized parallel to the angle of inci- 

 dence is accelerated or retarded in comparison with the perpen- 

 dicularly polarized component), cannot any longer be maintained, 

 although it has recently been so in German manuals of physics. 



That the phase of the reflected light in the case of grazing incidence 

 undergoes no change cannot be decided by the method made use 

 of by our author. By interference of the direct and in the most 

 decided manner grazingly reflected rays, I have proved § that the 

 change of phase is not, as M. Potier states, =0, but corresponds 

 to half a wave-length, both for the light polarized parallel and that 

 polarized perpendicular to the plane of incidence. 



For other incidence-angles than 90° the difference of the altera- 

 tion of phase in two differently reflected rays, e. g. from glass and 

 metal, can be only relatively determined ; and I have determined 

 by experiment, both with the method made use of by our author 

 and with some others, this difference for reflection in different 

 substances, and have discussed in detail how far they deviate from 

 the variety of change of phase required by the theory ||. Abstrac- 

 tion made of the uncertainty (there, p. 226, fully discussed) of the 

 magnitude of half a wave-length, the observations generally, but 

 not always, give a maximum of difference of phase of two dissimi- 

 larly reflected rays when J=0. The ray reflected from a metal 

 was, relatively to that reflected from glass, accelerated when the 

 * Camb. and Dubl. Math. Journal, vol. iv. p. 1 : 1849. 

 t Pogg. Ann. vol. cxxviii. p. 369 : 1866. 

 J Ann. de Chim. vol. xxix. p. 303 : 1850. 

 § Pogg. Ann. vol. cxli. p. 223: 18/1. 

 l| Pogg. Ann, vol. cxli. pp. 196-232,384-388: 1871. 



