ILLUMINATION OF OBJECTS. 
unable to perceive tbe' elevations or depressions, all being pro¬ 
jected upon the same ground-plan, and all being similarly illu¬ 
minated. But if the light fall upon it with a certain and regular 
obliquity, lights and shadows will be produced which will enable 
him to infer the accidents of the surface and the real form of the 
object. 
The due consideration and application of this general optical 
fact will enable the microscopic observer to submit the object of 
his inquiry to such a visual analysis as will unfold at least a close 
approximation to its real form. 
48. If the object be viewed by a front light proceeding from 
the concave mirror m M, fig. 13, or reflected by the Lieberkuhn, 
this effect will not be produced; for although the light reflected 
from the Lieberkuhn is not perpendicular to the object, it is scat¬ 
tered in all possible directions, so as utterly to remove all possi¬ 
bility of lights and shadows. An expedient is sometimes adopted 
in which light projected by a concave mirror or lens, properly 
placed, is directed only on one side of the Lieberkuhn, which is 
necessarily productive of lights and shadows. 
But the purpose is much more simply and effectually attained 
by removing the Lieberkuhn altogether, and directing the illu¬ 
mination with the necessary obliquity upon the object by means 
of a reflector or lens placed as shown at m'm' or l l. 
Those methods are always practicable except when a magnifying 
power is used so high as to render it necessary to bring the object 
almost into contact with the object-glass, in which case the 
mounting of the latter would intercept the light, whether pro¬ 
ceeding from the Lieberkuhn, the lens, or mirror. In such cases 
the object can only be illuminated by a back light. 
If the object be illuminated by a back light thrown obliquely 
upon it, the lights and shadows, strictly speaking, can only be 
produced upon the posterior surface. Nevertheless, the light 
passing obliquely through the anterior surface will produce dark 
and light tints, according to the angle at which it strikes the 
several superficial inequalities and accidents of that side of the 
object. It will be evident, therefore, that very complicated effects, 
in which the disentanglement of the forms which produce them 
is extremely difficult, must ensue. 
Nevertheless, the attentive and practised observer, by presenting 
the illumination successively in various directions, by properly 
varying its intensity, and examining the object as well by front 
as by back illumination, when both are practicable, can generally 
arrive at a pretty clear knowledge of its form and parts. 
49. When the object is illuminated by a back light, optical 
phenomena, called diffraction and interference, are produced, 
4.5 
