8O THE A B C OF PHOTO - MICROGRAPHY 



popular as of old. I cannot, however, pass it 

 by totally unnoticed, though my remarks must 

 be limited in space. For photographic pur- 

 poses especially (though this holds good in 

 all other cases) both prisms polarizer and 

 analyzer should be mounted to revolve freely 

 in their settings, and both should be of largest 

 possible dimensions. The lower prism in my 

 own apparatus measures 16 mm. across its 

 shortest diameter, admitting an immense 

 amount of light. The analyzer must be car- 

 ried between the objective and compound 

 body of the microscope, and not above the 

 ocular, as was formerly a common practice, 

 since it limits the field of view very seriously. 

 If the construction of substage permit, the 

 polarizing prism and condenser should be 

 carried together, the latter in front, of course. 

 The field of view should be completely dark- 

 ened by rotation of both prisms to the proper 

 point before placing the object upon the 

 stage, and a selenite should not be used, ex- 

 cepting in certain necessary cases. The ex- 

 posures must be considerably lengthened, as 

 with dark field and opaque illuminations. Two 

 illustrations of this method in lighting are 

 given. The first (Fig. 17) shows the large 

 starch grains of canna root. These, mounted 



