434 M. G. Quincke on the Refractive Index of the Metals. 



observe the displacement of the bands of interference with a 

 NicoPs prism, of which the principal section is parallel to the 

 plane of incidence, so that only those rays reach the eye which 

 are polarized perpendicular to the plane of incidence of the 

 metal. 



From the nature of the case, no high degree of accuracy is 

 attainable in observations of this kind ; for, on account of the 

 great difference between the intensity of light which has passed 

 through air and the intensity of that which has passed through 

 metal, the interference-bands are only weak, and an error of 0*1 

 of the space between the fringes may be easily made. To this 

 must be added that the rays which are polarized in a vertical 

 plane, perpendicular to the plane of reflexion of the thick 

 plane-parallel glasses of the interference- apparatus, have alto- 

 gether only a low intensity. In the same way the rays polarized 

 perpendicular to the plane of incidence of the metal are less 

 weakened by the elliptical polarization of the light transmitted 

 through the metallic plate, than the rays polarized parallel to 

 the plane of incidence of the metal. In general, therefore, the 

 observations on rays which are polarized parallel to the plane 

 of incidence of the thick plane-parallel glasses, and perpendicular 

 to the plane of incidence of the metal plate, deserve greater 

 confidence on account of the greater intensity of the light ; and 

 thence, in order to alter the angle of incidence I, the metal 

 plate must be turned about a horizontal axis. However, for the 

 sake of control, observations by turning the metallic plate 

 about a vertical axis were instituted, as well as for light polarized 

 perpendicular to the plane of reflexion of the thick plane- 

 parallel glasses. 



The following Table gives the observations on a plate of blue- 

 violet silver, which, according to Foucault's process*, was 

 formed and polished on a plane-parallel glass plate. The thick- 

 ness was 0-000122 to 00001788 of a millimetre. The light 

 falling perpendicularly, for 1 = I observed a displacement of 

 the interference-bands equal to —0*2 or — 03 of the distance of 

 the interference-bands in the spectrum in the neighbour- 

 hood of Fraunhofer's line F. The negative sign of the dis- 

 placement indicates that w<l. The first column contains 

 the angle of incidence; the following contain the observed 

 displacement in fractions of the distance of the fringes according 

 as the light was polarized parallel ( ^=), or perpendicular (_[_) to 

 the plane of incidence of the transparent metallic plate. For 

 the second and third columns this plane of incidence was hori- 

 zontal ; for the fourth and fifth vertical. It is at the same time 

 recorded whether the field of vision was light or dark. 

 * Le Verrier, Ann, de I'Observatoire Imperiale, vol. v. p. 179. 



