Intelligence and Miscellaneous Articles, 253 



of incidence, was situated at different distances from the common 

 boundary when, one of the two media being left constant, the 

 other was altered in such wise that the optical thickness of a thin 

 lamella varied with the nature of the substances adjacent to it. 

 AVith glass and air or sulphide of carbon, the difference between 

 these distances was found by experiment to be =yg- wave-length 

 for light polarized parallel to the plane of incidence. The phase of 

 the reflected light undergoes different variations with different 

 incidence-angles, being with grazing incidence and having a 

 maximum value with normal incidence. The change of phase 

 varies with the nature of the thin lamella between the lens and the 

 plane mirror. The ray reflected from metal is retarded when the 

 reflection is normal, the retardation amounting to ^ of a wave- 

 length for silver and air, to -J- for silver and an essential oil. This 

 retardation has a sensible influence when the thickness of a thin 

 layer of metal upon a glass plate is determined from the different 

 diameters of the Newton's rings formed beneath a lens when laid 

 upon a plate of metal and when laid upon a plate of glass. 



" Moreover, with a retardation of the reflected ray, there was a 

 corresponding change of phase in the transmitted ray ; and in 

 passing through a transparent lamella of silver, the phase must 

 have been accelerated y 1 ^ of a wave-length. This must be taken 

 account of in the determination of the velocity of light in the 

 interior of metals ; and this circumstance accounts for it if certain 

 experimenters have found the refraction-index of metals too small 

 or negative. The phenomena in metals were more complicated, 

 as there did not exist here, as in transparent substances, a plane, 

 parallel to the boundary, in which the incident and the reflected 

 ray, both polarized parallel to the plane of incidence, coincide, but 

 only a plane in which the two had a definite difference of path. 

 The situation of this plane varies with the nature of the substance 

 over the metal, since the extinguishing-force of the metal, and, in 

 a less degree, the nature of the substance above the metal, varied 

 the constants of the elliptic polarization produced by metallic 

 reflection — that is, the principal angle of incidence and the prin- 

 cipal azimuth." 



As, to my knowledge, no one except me * has investigated the 

 refraction-indices of transparent layers of metal, I must refer to 

 myself the expression " certain experimenters," and remark that to 

 the above statements, save so far as they contain what was already 

 known, I cannot assent. 



My optical experimental investigations published in the Annalen 

 contained, besides those methods which are described in the above- 

 mentioned communications, some others, which have concordantly 

 conducted to this result — " that, in order to be in accord with the 

 known facts, we have to make the assumption that reflection and 

 refraction take place in a transition-layer the thickness of which 

 can be measured by experiment " f. 



* Pogg. Ann. vol. cxx. p. 599 : 1863. 

 t Ibid. vol. cxli. p. 398 : 18/1. 



