22 ELEMENTARY CHEMICAL MICROSCOPY 



surface may be in doubt; but if oblique illumination be employed, 

 usually a very faint shadowy image of the lower surface will 



be observed, slightly out of symmetry 

 with the upper surface. Swinging the 

 mirror to one side or decentering the 

 iris diaphragm of the condenser when 

 this is possible, and noting at the same 

 time any change produced in the image, 

 will show that the image of the upper 

 surface has the appearance of sliding 

 over the lower, providing the object- 

 ive has sufficient penetrating power. 

 Under these conditions the trained 



observer is able to form a fairly accurate conjecture as to the 

 morphology of the object under observation. 



Cleavage planes, infinitely narrow fissures or structures, the 

 arrangement of whose elements is so fine and delicate as to be 

 practically indistinguishable by axial light, may become -easily 

 discernible by oblique illumination; but as intimated above, 

 the character of the information thus gained is necessarily closely 

 associated with the resolving power, penetration and, to a 

 certain extent, the size of field of the optical combination above 

 the stage. 



Transmitted oblique light is desirable and often necessary 

 in the examination of tiny crystals or crystal fragments, in the 

 differentiation of textile and paper fibers, in the study of furs 

 and hairs, in the microscopic examination of foods and drugs 

 for their identification or for the detection of adulteration, etc. 

 In fact in the study of all transparent or translucent objects 

 which have not been cut in thin sections with parallel faces, 

 oblique illumination should always supplement the exami- 

 nation made with axial light. The microanalyst must become 

 thoroughly familiar with the advantages to be derived from 

 oblique transmitted light. 



If necessity requires the study of a preparation with a micro- 

 scope having no substage condenser, illuminate the object with 

 axial light, first using the plane, next the concave mirror. Next 



