ILLUMINATION OF OBJECTS. 



unable to perceive the 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 31 x, 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, 



45 



