250 



THEORY OF MICROSCOPIC OBSERVATION. 



I I I I I 



of the incident cone of light (placed laterally to the edge), then it 

 is evident from the figure that if the Microscope is adjusted to the 

 surface of the cover-glass, the point p will still appear fully illu- 

 minated. Further to the right the rays, produced backwards, are 

 first partly, and then entirely, reflected towards the left from the 

 vertical edge, or refracted so that they do not reach the diaphragm. 

 The edge of the cover-glass is, therefore, in the shadow. Simi- 

 larly, it may be shown that if the left edge m n of the cover- 

 glass is observed with the same illumination, it will appear bright, 

 and the adjoining portion m t of the plane of adjustment dark. 



If, therefore, we take an object with projecting ridges (Fig. 138 

 A), and adjust the focus to the surface of the elevations, an image 



will be formed like that 

 represented in Fig. 138 B. 

 The right-hand sides of 

 I the ridges, as well as of 

 the depressions between 

 them, will fall in the 

 shadow, and the latter 

 becomes the broader the 

 more oblique the illumin- 

 ation. The same effect 



must necessarily be produced by alternate solid and aqueous layers 

 with vertical bounding surfaces, if the Microscope is focused on 

 the upper surface. 



The importance of oblique illumination for the dioptric image 

 lies therefore principally in the fact that it increases the contrasts 

 between light and shadow, and in addition makes the shadow 

 lines appear broader, and hence more palpable. In particular 

 cases, moreover, if the illumination is sufficiently oblique, their 

 number may be only half as great as by axial illumination, by 

 which, of course, the power of perceiving them is raised in a still 

 higher degree. 



The angle of the incident cone of light, or, more strictly, of its 

 axis, must be specially determined for each particular case by 

 experiment. Its value is dependent not only upon the nature of 

 the object and the aperture o> of the objectives, but also upon the 

 difference between the extreme values of 8. Since the power of 

 discrimination of alternately positive and negative impressions 

 upon the sight is greatest when either the one or the other has 



FIG. 138. 



