Surface-Film in Total Reflexion. 



13 



one medium to the value it lias in the other ; or it can he 

 reoarded as a nearly homogeneous layer with a definite index 

 o£ refraction, produced in polishing the surface. The formula 

 for the deviation has been proved experimentally by Jamin * 

 and Quincke t for the case of ordinary reflexion. This paper 

 describes some observations made to prove the formula in the 

 case of total reflexion, which was first given by Drude {, 

 though it follows naturally from the formula for the case of 

 ordinary reflexion, which was found some years earlier. 

 Fresnel's laws for the case of total reflexion were proved by 

 Jamin § and Quincke ||, who worked with fresh surfaces for 

 which the effect of the surface-film would be naturally very 

 small. 



The apparatus used was a spectroscope, the collimator of 

 which was fitted with a polarizing nicol, and the telescope 

 of which was fitted with a Soleil-Babinet compensator and an 

 analvsino- nicol. A sodium-flame was the source of lioht. 



O 1 • 1 



The incident light was polarized at 45 to the vertical ; after 

 total reflexion it was once more made plane-polarized by the 

 compensator, and then cut off by the crossed analyser. 



The Soleil-Babinet compensator consists essentially of a 

 quartz plate and two quartz wedges with equal angles. The 



A 



two wedges have their optical axes in the same direction, 

 parallel or perpendicular to the edge of the wedge ; the 

 wedges are parallel, but in opposite directions. The axis of 

 the plate is perpendicular to the axes of the wedges. The 

 relative phase-difference produced by reflexion &c. is com- 

 pensated by moving the one quartz wedge relatively to the 

 other wedge and the plate, which are fixed together. 



The elementary theory disregards the reflected waves. It 

 assumes also that the axes of the two wedges and the plate are 



* Jamin, Ann. de Phys. et Chim. (3) vol. xxix. p. 263 (1850). 



t Quincke, Poga\ Ann. vol. cxxviii. p, 355 (1866). 



t P. Drude, W'ied. Ann. vol. xliii. p. 146 (1891). 



§ J. Jamin, Ann. de Phys. et Chim. (3) vol. xxx. p. 257 (1850). 



|| G. Quincke, Pogg. Ann. vol. cxxvii. p. 217 (1866). 



