Petrographic Microscope Work. 



67 



is the index finger placed below the 

 condenser in such a position that it 

 allows only marginal rajs to enter at 

 one side of the condenser. In this 

 manner a bundle of rajs incident on 

 the section under a high angle and in 

 an j azimuth can be readily obtained ; 

 if, for anj special purpose, rajs of a 

 given angle of emergence are re- 

 quired, the j can be had bj using a 

 small sliding stop (iris diaphragm), but 

 such instances are rare and the index 

 linger is satisfactory for all practical 

 purposes. 



(3) This method is similar to the 

 second in principle but slightly dif- 

 ferent in its details. In this method 

 the index finger or a sliding stop 

 below the polarizer* is used and 

 serves to cast a shadow over part of 

 the microscopic field. The path of 

 the rays under these conditions is 

 illustrated in fig. 7, in which the fin- 

 ger or sliding stop is represented by 

 the arrow AB. AB is imaged by 

 the condenser at A I B I , by the ob- 

 jective at A 11 B", by the field lens of 

 the ocular at A m B m , by the eye lens 

 of the ocular at A IV B IV , and by the 

 eye at A v B v behind the retina. In 

 fig. 7 the microscope is focussed on 

 the object plane and the image A V B V 

 does not appear, therefore, in sharp 

 focus (coincide with the retina), but 

 rather as a dark shadow with wide 

 blurred edges. It is with these semi- 

 dark edges that the method has to do. 



In figs. 8, 9, 10 these relations are 

 shown on a larger scale than in fig. 7, 

 and for the condenser part of the 

 microscope system only. In fig. 8 

 the condenser is in such a position 

 that the object AB is sharply imaged 

 in the object plane of the draw-tube 

 lens system ; under these conditions 

 the edge of the finger or sliding stop 



*F. E. Wright, Min. petr. Mitt., xx, 239, 

 385-887. 1904. 



Fig. 7. 



IMAGE PLANE 



FOCAL-PLANE 



1901 ; this Journal (4), xvii, 



