POLARIZED LIGHT IN THE STUDY OF ORES AND 



METALS.i 



By FRED. E. WRIGHT. 

 (Read April 14, IQ17.) 



The measurement of the optical properties of transparent min- 

 erals, even in minute, irregular grains, is a simple task with modern 

 petrographic microscope methods and is accomplished by petrolo- 

 gists as part of ordinary routine work. But the determination of 

 the optical constants of opaque substances is difficult and is rarely 

 attempted by microscopists ; all observations are necessarily made in 

 reflected light and are restricted commonly to the determination of 

 color, of the character of crystallization, and of the behavior of the 

 mineral or metal plate toward reagents and abrasives. It is gen- 

 erally recognized that if methods were available by means of which 

 the optical constants of opaque substances in fine particles could be 

 readily ascertained these methods would be of great value not only 

 to students of ores and opaque minerals, but especially to metal- 



1 The manuscript of this paper was finished in March, 1917, and is here 

 presented without alteration. A brief resume of the resuUs of the investiga- 

 tion was given at the meeting of the American Philosophical Society in April, 

 1917. With our entrance into the war the writer joined the Army, and the 

 publication of this paper was accordingly postponed. 



In the theoretical section of this paper certain standard equations are 

 derived and expressed in Cartesian coordinates. The expressions would have 

 been much simpler and shorter had the methods of vector anatysis been em- 

 ployed; but this was not done and the equations are developed' in the usual 

 notation in order that they may be easily accessible to the reader interested 

 in this particular subject. Many of the problems of crystal optics are, how- 

 ever, essentially vectorial in character and yield most readily, as do many 

 problems in electricity (alternating currents, wireless telegraphy), to treat- 

 ment by vector analysis. In vector analysis the imaginary quantity f -^ V — i 

 is treated simply as an operator rotating a vector through 90°. This greatly 

 simplifies the interpretation of equations containing complex quantities. An 

 interpretation of this kind of many of the equations in the present paper 

 would undoubtedly render them more intelligible, but this would have greatly 

 increased the length of the paper and was accordingly not attempted. 



PROC. AMER. PHIL. SOC, VOL. LVIII, Z, JAN. 21, I92O 



401 



