JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. IV JUNE 19, 1914 No. 12 



PHYSICS.^ — The optical character of the faint interference figure 

 observed in high power objectives between crossed nicols. Fred. 

 Eugene Wright, Geophysical Laboratory. 



To the petrologist the appearance, under crossed nicols, of a 

 faint, apparently uniaxial interference figure in an objective 

 of short focal length is a matter of common observation. It was 

 at first considered to be the result of strain in the objective lenses 

 but Rinne^ in 1900 gave the correct explanation of the phenomenon 

 and ascribed it to the rotation of the vibration plane of the trans- 

 mitted plane-polarized waves at the steeply inclined lens surfaces. 

 Rotation of this kind was noted by FresneP who deduced, in 1822, 

 for plane-polarized waves transmitted through an isotropic plane 

 parallel plate, the equation 



cot 5 = cos- (i-r) . cot A, (1) 



in which the angle of rotation B is expressed as a function of (a) 

 the angle of incidence i, (b) the azimuth A of the plane of vibra- 

 tion of the incident plane-polarized light wave, and (c) the re^ 

 fractive index of the glass plate (contained in the expression 

 i - r, r being the angle of refraction) . This equation has been 

 frequently verified by experiment and found to represent the 

 observational data satisfactorily. 



From Fresnel's equation it is evident that: (1) in case the 

 vibration direction of the incident wave is parallel or normal to 

 the plane of incidence {A = 0° or 90°) the vibration plane of the 



1 Centralblatt f. Miner. 1900, 88-89; see also C. Cesaro, Bull, de I'Acad. Proy. 

 de Belgique, CI. d. Sci. 1906, 459; and F. E. Wright, Amer. J. Sci. (4), 31: 187. 

 1911; Carnegie Inst, of Washington, Pub. 158: 74-76. 1911. 



^Oeuvres, 1: 441-799. 



301 



