T 
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 
