OBLIQUE ILLUMINATION. 



attains a distance of 2 /, with high powers, if, for instance, &> = 80 

 and S' is somewhat greater than 45. The same reasoning holds 

 good also for the penumbra ; the formulae remain the same, only 8 

 is substituted for S'. Hence it follows that oblique illumination 

 with surface-elements, whose breadth amounts to less than 2 f f 

 entirely obliterates the shadow on one side, while it intensifies it 

 on the other. Where, therefore, many such minute convex lenses 

 are joined to one another, the microscopic image will show the 

 same number of bright points which alternate with dark ones ;. 

 this is also the case with axial illumination. But while the 

 latter may render only the penumbra visible, the oblique light 

 will produce the much more distinct umbra, and will increase 

 thereby the contrasts between brightness and darkness. 



If, on the same suppositions, the minute convex lenses are 

 replaced by concave ones, or by surface-elements which cause no 

 deviation, the latter will remain bright, while the concave lenses 

 will exhibit light and shade in the opposite order. The illumin- 

 ated parts of adjacent surface-elements will therefore unite ; each 

 two elements will form together a bright and a dark part. The 

 number of light and dark points in the microscopic image is 

 therefore reduced to half, and it may happen that stride become 

 visible in consequence of this, which are not perceptible by axial 

 illumination. 



Although it is true these considerations do not apply to fine 

 structural details of objects, 

 since, as we have before 

 shown, they are reproduced 

 by interference images, yet 

 it is important to under- 

 stand the influence of ob- 

 lique illumination upon the 

 microscopic image, more 

 especially with reference to 

 the refracted rays. We 

 will therefore explain, as a 

 typical example, the effect 



which the vertical edge of a F 137 



membrane, or of a cover- 

 glass, produces with oblique illumination. If q r (Fig. 137) is the 

 vertical edge, and B the diaphragm which regulates the inclination. 



