M. G. Quincke on the Refractive Index of the Metals, 435 



Plane of 



Incidence 



of the Metal. 



Horizontal. 



Vertical. 



Field. 



Light. 



Dark. 



Dark. 



Light. 



I. 



* 



J_ 



* 



_L 



o 





 20 

 30 

 40 

 50 

 60 

 70 



-0-25 



-035 



-0-3 



-0-4 



-0-3 



-0-2 



-0-25 



-0-3 



-0-3 



-0-27 



-0-2 



-0-1 



-0-25 



-0-3 



-0-4 



-0-2 



-0-2 



-0-1 



-0-1 



-0-25 



-0-3 



-0-3 



-0-2 



-0-1 



-0-1 



-0- 



From this it appears that, as a rule, the displacement for 

 light polarized parallel to the plane of incidence is about O'l of 

 the distance of the fringes greater than for light polarized per- 

 pendicular to the plane of incidence, — or that, in accordance 

 with my experiments with the Babinet's compensator, the rays 

 polarized in the plane of incidence are before the others. These 

 last rays polarized perpendicular to the plane of incidence un- 

 dergo at 1 = 70° no more displacement; hence, according to 

 equation (5), 77 = 70°, cos 77 = 71= 0342. 



It is here indeed supposed that the rays polarized perpendicular to 

 the plane of incidence undergo no alteration in phase by 

 traversing the metal plate ; and this, according to other experi- 

 ments of mine, seems to be not in all strictness true. 



The refractive index n may likewise be calculated from the 

 measured displacement A of the interference-bands, the thickness 

 D of the metallic plate being known. Let a be the distance 

 between interference-bands in the spectrum, \ m the length of 

 the undulations belonging to the colour that we are dealing 

 with in the metal, \ the same in air, then 



(7) 



A --Hi-iy-> 



or, if the refractive index of the metal be used, 



nz=z \ 3 



whence follows 



A = «.5( W -l); 



n=l + 



D a 



(8) 



(9) 



* See Neumann, " Law of the Double Refraction of Light in compressed 

 Bodies," Abhand. der Acad, der Wissenschaft zu Berlin, 1841, vol. ii. 

 p. 52. 



2 F2 



