JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VI MAY 4, 1916 No. 9 



PHYSICS. — Polarized skylight and the petrographic microscope. 1 

 W. S. Tangier Smith. (Communicated by F. L. Ransome.) 



When skylight is used with the petrographic microscope, there 

 is often a notable loss of available light owing to its partial 

 polarization. This polarization, like the blue color of the sky, 

 is due to the effect of light waves on particles of matter in the 

 atmosphere, the diameter of which is small compared with the 

 wave-length of light, so that the light which strikes them, instead 

 of being reflected, sets up harmonic vibrations in the particles 

 or the surrounding ether, these vibrations in turn giving rise 

 to light waves and resulting in what is commonly referred to 

 as "scattered light." This scattered light, composed mainly 

 of blue and violet rays, is polarized to a greater or less extent, 

 since at any one point the vibrations which give rise to it are 

 confined to a single plane transverse to the direction of trans- 

 mission of the original beam of light. 



The more numerous the minute particles which scatter light, 

 and the fewer the larger reflecting particles which mask the 

 scattered light, the bluer is the sky and the greater the polariza- 

 tion of its light when viewed in certain directions. 



The blueness of the sky and the polarization of its light being 

 due to the same cause, they vary for the most part together, 

 and the polarization is therefore greatest under those circum- 



1 Read before the Cordilleran Section of the Geological Society of America. 

 April 11, 1913. , 



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