642 WRIGHT: VIBRATION PLANE OF THE POLARIZER 



The chief factors which produce partial polarization in the 

 incident light are: (a) polarization by reflection at the surface 

 of the substage reflector; (b) sky polarization. 



(a) At first thought it would seem that the first factor is ex- 

 ceedingly important because of the pronounced polarization ef- 

 fect which a plane glass surface produces on reflected rays. The 

 line of vibration for such reflected rays is the normal to the plane 

 of incidence; hence the natural inference is that the preferable 

 position for the plane of vibration of the polarizer is from right 

 to left, parallel to the horizontal cross-hair. But in the case of 

 the silvered mirror we have to do not only with reflection at a 

 glass surface but chiefly with metallic reflection ; and for such a 

 surface the percentage intensity of polarization by reflection is 

 in general much less than at a glass surface. Measurements, 

 with a Koenig-Martens polarization photometer, of the relative 

 intensities of the light rays reflected from the substage mirror 

 (metallic reflection) show that the intensity of the rays vibrating 

 normal to the plane of incidence is less than 10 per cent greater 

 than that of the rays vibrating in the plane of incidence; this 

 difference is for practical purposes not of sufficient importance 

 to exclude either position. Unless, therefore, the source of light 

 is distinctly polarized we must conclude that, practically, the 

 one position is as good as the other. 



(b) It has been known for nearly a century that much of the 

 light from the sky on a clear day is polarized. Arago, Babinet, 

 and Brewster were the first to discover and to study sky polariza- 

 tion, and since their time many investigators have studied the 

 phenomenon. The literature on the subject is voluminous. 

 The general results of this study so far as it pertains to the pres- 

 ent problem are: The light from the different parts of the sky 

 is not uniformly polarized but is at a maximum in the plane 

 polar to the sun ; this plane is also the plane of vibration of these 

 polarized rays. The percentage of sky polarization decreases 

 from the polar plane toward, and away from, the sun until the 

 neutral points of no polarization (called Babinet and Arago points 

 and situated about 20° from the sun and its antipoint respectively) 

 are reached; beyond these the sky polarization again increases. 



