322 M. G. Quincke on the prime Angle of Incidence 



The prime angle of incidence and azimuth are thus deter- 

 mined : — The difference of phase of the two components polarized 

 parallel and perpendicular to the plane of incidence are, in any 



way, made equal to a high multiple of + -. The two then give 



linearly polarized light, which being extinguished by an analy- 

 zing Nicol prism, its plane of polarization can thus be determined. 

 The light originally polarized linearly, and after reflection ellip- 

 tically polarized, can be converted into linearly polarized (1) by 

 being again reflected once or more times under the same condi- 

 tions, (2) by annihilating the difference of phase by a plate of 

 crystal of variable thickness (a plate of calc spar cut perpendi- 

 cular to the optic axis, a Babinet's compensator) whose principal 

 section coincides with the plane of reflection, (3) by turning the 

 principal section of a crystal plate of invariable thickness (a plate 



of mica of ^) toward the reflection- plane. 



The first method has been used by Brewster* and Jaminf; 

 the second by Brewster*, JaminJ, Van der Willigen §, Haugh- 

 ton ||, and me^[ ; the third by De Senarmont**, Kirchhoff, and 

 Eilhard Wiedemann ff. 



The most perfect process is that adopted by Jamin, who 

 threw a pure (objective) spectrum with Fraunhofer's lines upon 

 a polarizing Nicol in azimuth 45°, and with the analyzing Nicol 

 in azimuth j3 extinguished the rays twice reflected under the 

 prime angle of incidence H by parallel mirrors. The angle of 

 incidence and azimuth /3 were altered until the field appeared as 

 dark as possible. 



Azimuth B is then given by the equation 



B= arc (tan =\ZtMi/3) (2) 



Jamin determined in this way the optical constants of several 

 metals for different Fraunhofer lines, and thence deduced the 

 various colours of the metals. 



This procedure has many inconveniences, especially this, that 

 the angle of incidence cannot be determined with sufficient accu- 

 racy. A slight modification of it, which I have employed for 

 more than ten years, consists essentially in observing the spec- 



* Phil. Trans. 1830, pp. 294 & 312. 



t Ann. de Chim. (3) vol. xxii. p. 311 (1848). 



% Ibid. vol. xxix. p. 281 (1850). 



§ Pogg. Ann. vol. cxvii. p. 464 (1862). 



It Phil. Trans. 1863, p. 122. 



^[ Pogg. Ann. vol. cxxviii. pp. 355 & 541 (1866). 

 ** Ann. de Chim. (2) vol. lxxiii. p. 337 (1840). 

 tt E. Wiedemann, Elliptische Polarization des Lichtes. Leipzig: 1872. 



