JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. V DECEMBER 4, 1915 No. 20 



PETROLOGY. — The position of the vibration plane of the polarizer 

 in the petrographic microscope. F. E. Weight, Geophysical 

 Laboratory. 



In petrographic microscopes the polarizer is mounted in one 

 of two positions, so that its plane of light transmission is parallel 

 either to the vertical cross-hair or to the horizontal cross-hair 

 of the eyepiece. Certain makers prefer the first position, 

 others the second; but with them the matter seems to be one of 

 long standing habit rather than one of definite reason. It is, 

 therefore, of interest to inquire into the relative merits of the 

 two positions and, if possible, to obtain data which enable us 

 to ascertain definitely which is the better position. In antici- 

 pation of the final result it may be stated that for an observer 

 using light from the northern sky, the correct position depends 

 on, and varies with, the time of day at which the observations are 

 made. The evidence on which this unexpected conclusion is 

 based is presented in the following paragraphs. 



Of the two positions of the plane of vibration of the polarizer 

 that position is preferable which transmits the most light. In 

 case the light entering the polarizer is strictly non-polarized, 

 the same amount of light is transmitted by the polarizer in either 

 position; under these conditions both positions are equally good. 

 If, however, th« incident light be partially polarized, the inten- 

 sity of illumination varies with the azimuth of the polarizer 

 plane; that position which allows the greater percentage of the 

 polarized light to pass is then obviously the better. 



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