438 



WRIGHT— POLARIZED LIGHT IN THE 



through an angle a ; in this case the observed intensity from the 

 one field of intensity /, is / cos^ (45° — a) ; that of the second, h, 

 is / cos- (45° -|- a). Since the observed intensities are equal we 

 have 





COS^ (45° + a) 

 COs2 (45° - a) 



= tanM45°-«). 



(47) 



A series of values computed by means of this equation is listed 

 in Table 9. For practical purposes a large-scale curve plotted on 

 millimeter cross section paper is useful for ascertaining intermediate 

 values. 



TABLE 9. 



In this table are listed the relative intensities of the sources of illumina- 

 tion of the two fields of a calcite rhomb or a Koenig-Martens photometer for 

 different settings a of the nicol N at which equal field intensities are obtained. 



Of the two devices the Koenig-Martens photometer is the better 

 because it gives a better photometric field and is an attachment com- 

 plete in itself which may be used on any microscope in place of the 

 eyepiece. The only additional accessory required is the reflecting 

 prism or mirror with which every metallographic microscope is 

 equipped. 



These methods are superior to the Koenigsberger method, not 

 only because of increased sensitiveness, but also because the par- 

 ticular plate under investigation can be viewed directly (at high or 

 low magnifications) and the test made directly on the plate in full 

 view. In the Koenigsberger method the Savart bands are not readily 

 distinguished unless the plate is brought out of sharp focus and even 

 then the irregularities of the image tend to render indistinct and 

 uncertain the faint Savart bands. 



