Wright: vibration plane of the polarizer 643 



In the early morning and late afternoon the sun is near the 

 horizon on the east or west and the plane of maximum sky polari- 

 zation is approximately a north-south vertical plane passing 

 through the zenith. The vibration plane of rays contained in 

 this vertical plane and incident on the microscope facing north 

 is parallel to the vertical cross-hair of the microscope. Measure- 

 ments by H. H. Kimball^ and others prove that between 50 and 

 80 per cent of this light is plane-polarized, the actual percentage 

 varying with a number of factors which need not be discussed 

 here. Under these conditions the intensity of the transmitted 

 light, when the plane of the polarizer is in the plane of symmetry 

 of the microscope, is more than twice that when the polarizer 

 is parallel with the horizontal cross-hair. 



At noon, on the other hand, the sun is in the south and its 

 polar plane is an inclined plane intersecting the horizon on the 

 east and west. The plane of vibration of the rays from the north 

 incident on the microscope is essentially parallel to the hori- 

 zontal cross-hair. Under these conditions the most favorable 

 position of the polarizer plane is from right to left or at right 

 angles with the early morning position. 



On cloudy and misty days the amount of sky polarization 

 decreases rapidly and is practically negligible. On such days 

 the position of the polarizer plane has little effect on the inten- 

 sity of the field illumination in the microscope. 



If on a clear day the observer wishes to obtain the maximal 

 illumination it is necessary to set the plane of the polarizer paral- 

 lel to the vertical cross-hair in the early morning or late after- 

 noon and then toward noon to turn it to the horizontal position. 

 But such a procedure is not always convenient as the polarizer, 

 is commonly fixed in position; in view of the fact that at noon 

 time there is always an abundance of light from a clear sky 

 while in the early morning and late afternoon the sky illumina- 

 tion is less than half that at noon, the position of the polarizer 

 which favors greater intensity of illumination at early and late 



J Proc. Third Convention of Weather Bureau Officials, Peoria, III. 1904, 

 p. 69. Bull. Mt. Weather Observatory, 1, pt. 2: 38-48. 1908. Ibidem, 3, pt.2: 

 113-114. 1910. Jour. Franklin Institute, 171:333-344. 1911. 



