ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 31 



without moderately high powers and some form of substage 

 condenser. It is therefore a safe rule to always employ a sub- 

 stage condenser unless exceptionally low powers are to be used; 

 this of course does not apply to problems involving examinations 

 with polarized light. 



b. Transmitted Oblique Light is essential for the proper 

 interpretation of appearances under the microscope of objects 

 whose upper and lower surfaces are so placed as to lead to serious 

 confusion if axial light is alone employed. Oblique light also 

 aids in establishing whether the liquid medium or the object 

 immersed in it has the higher refractive index. The value of 

 oblique illumination may be better understood by referring to 

 the diagram shown in Fig. 8. A transparent object O whose 

 upper and lower surfaces are identical 

 and perfectly symmetrical, is shown 

 in section, lying upon an object slide 

 upon the stage, with perfectly axial 

 light as shown by the arrows. It will 

 be obvious that even very careful focus- 

 ing will fail to disclose the probable 

 structure of the lower surface and that 

 even the upper 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 sym- 

 metry 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 objective 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, 



