STUDY OF ORES AND METALS. 



437 



-^-B 



Fig. 10. New 

 method for the de- 

 tection and meas- 

 urement of the 

 degree of aniso- 

 tropism of a crys- 

 tal plate. In this 

 method the cleav- 

 age plate of cal- 

 cite serves to pro- 

 duce two adjacent 

 fields whose planes 

 of polarization are 

 normal to each 

 other. The inten- 

 sity of the illumi- 

 nation of the two 

 fields is made 

 equal by rotation 

 of the nicol A''. 



polarized light be 

 fields is different; 



In Fig. lo the non-polar- 

 ized incident light is reflected 

 by the prism R to the plate C 

 whence it passes on reflection 

 through the calcite plate aper- 

 ture^ to the nicol A^, the weak 

 lens D and the microscope eye- 

 piece E. In the writer's mi- 

 croscope the upper nicol can 

 be rotated and the angle of 

 rotation read off to 6' on the 

 stage of the microscope. The 

 function of the low-power 

 lens D is to image the aper- 

 ture A in the focal plane of 

 the weak eye lens E. 



In Fig. 1 1 the calcite plate 

 is replaced by the Koenig- 

 Martens arrangement of aper- 

 ture ^, field lens D (to render 

 rays from aperture A to Wol- 

 laston prism W parallel), and 

 the twin prism F. This gives 

 a better photometric field than 

 the calcite cleavage plate. In 

 both methods artificial light, 

 non-polarized, and either mon- 

 ochromatic or white is used 

 and rendered diffuse either 

 by the interposition of a 

 ground glass or opal glass 

 plate. The fields should be 

 equally illuminated and no 

 junction line between them 

 should be visible. In case 

 present the intensity of illumination of the. two 

 but it can be made equal by rotating the nicol A'' 



O 



Fig. II. In this 

 method the Koe- 

 nig-Martens ar- 

 rangement of wol- 

 laston prism and 

 twin glass prism 

 is substituted for 

 the calcite cleav- 

 age plate of Fig. 

 lo. The intensity 

 of the illumina- 

 tion of the two 

 adjacent fields is 

 rendered equal by 

 rotation of the 

 nicol A^. 



