THE OPTICAL EQUIPMENT 23 



in aperture is ensured, and one of these stops has to be obtained 

 for each aperture required. With an oil-immersion lens there- 

 fore of, say, an aperture of 1'40, a stop that would reduce it 

 to 1-20 or 110 would probably meet all ordinary requirements. 

 A simple diagram (Fig. 7) may explain more exactly the actual 

 effect in increasing penetration that this stopping down of the 

 objective really has. 



Suppose the line FF to represent one plane of a thick 

 object, and that the object may extend in depth in successive 

 layers in the direction of the point A. The point C may be, 

 therefore, only a little below the focal plane of the objective ; 



FIG. 7. Increased Penetration due to stopping-down of Objective. 



and if the illumination is by a narrow cone of light BB, the 

 appearance of the point, viewed in relation to the line FF, will 

 be less spread, and approximately in better focus than if it were 

 illuminated by a wide cone included within the angle DD. 

 The consequence is that if there were a succession of points in 

 the object lying just below the line FF, these would appear, 

 with a narrow angle, less diffused than if they were viewed with 

 a wide cone of light. It is clear also that the apparent differ- 

 ence between the size of the image-points in depth will be less 

 as the angle of illumination becomes narrower. It is evident, 

 therefore, that with a thick specimen a cone of narrow angle will 

 give images assuming them to be a series of points that 

 vary less in size than they would if the cone were wide ; they 

 will not, however, be such well-defined images, even when 



