WRIGHT: INTERFERENCE FIGURES IN OBJECTIVES 305 



are far more distinct under such conditions than one might expect 

 and the question arises as to the refractive index of the surface film 

 of the glass bead. Such a film is under the influence of surface tension 

 forces and hence is exposed to a different set of mechanical forces 

 than that which obtains inside the bead. It is, therefore, natural 

 that its refractive index (expression of influence of original system of 

 forces on light wave system of forces) should be different. To test 

 this conclusion still further a mixture of alcohol and ether on the one 

 hand was prepared and a second mixture of water and glycerine. Now 

 these mixtures have the same refractive index for sodium light, the 

 same density and approximately the same dispersion. On shaking 

 the two together the alcohol and ether form small drops suspended in 

 the water-glycerine mixture and furnish, therefore, an excellent system 

 for testing the idea that the refractivity index of the surface film may be 

 different from that inside the drops. Although experiments along this 

 line are still in progress it may be stated that the evidence so far ob- 

 tained is not sufficiently definite to warrant definite statements. 



The above experiments prove conclusively that the apparent op- 

 tically positive character for transmitted plane polarized light rays 

 and optically negative character for reflected rays when tested by 

 means of the sensitive tint plate, is due to rotation of the plane of 

 vibration at the steeply inclined surfaces of the isotropic substance. 

 In passing from one isotropic substance to a second the direction 

 of vibration remains, of course, in the same plane but the azimuth 

 of the plane of vibration changes (suffers rotation) because of the 

 change of direction of the ray (wave normal) on refraction.^ 



The phenomena exhibited by the objective on insertion of 



the sensitive tint plate can also be deduced by computation 



from the standard Fresnel formulas. The angle of rotation 



of the plane of vibration for any plane-polarized wave entering 



a plane surface of an isotropic substance of refractive index n 



at an angle of incidence i and an azimuth angle E is given by the 



equations . . 



sin ^ = n sin r, 



cot B = cos (i - r) . cot A, 



For a ray transmitted through a plate, equation 1 is valid. But 

 ordinary white light consists of light of all wave lengths through- 

 out the visible spectrum and the angle of rotation varies slightly 

 with the change in wave length but it is so shght that for the pres- 



' For a more complete discussion of these phenomena, see Carnegie Institution 

 of Washington, Pub. 158: 7.5-79. 1911. 



