ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 
21 
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. 
All modern compound microscopes are provided with two 
mirrors placed back to back in an annular mounting. One 
mirror has a plane surface, the other a concave surface. The 
mounting is so pivoted as to permit the easy swing of one or 
the other of the mirrors into a position to reflect hght through 
the stage opening. The plane mirror is employed with day¬ 
light or light diffused from a ground glass placed before an arti¬ 
ficial light. With the plane mirror parallel light is obtained. 
The concave mirror serves to obtain parallel rays from a source 
of artificial light placed only a short distance from the mirror or 
may be employed as a collector of rays (converging light) when 
powerful illumination of the object is desired. Microscopes in 
which the linear distance between mirror and stage is fixed 
and unalterable should be provided with diaphragms to fit 
immediately below the object. If no such device is provided, 
it will be found desirable to have at hand several pieces of dull 
black paper or thin card through which have been cut circular 
orifices of different diameters. Unless diaphragms are used 
below the object details of fine structure can rarely be discerned. 
h. Transmitted Oblique Light is essential for the proper inter¬ 
pretation 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 
obhque illumination may be better understood by referring to 
the diagram shown in Fig. 3. 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 focusing will fail to disclose the 
probable structure of the lower surface and that even the upper 
