<>j> THE ailCfcOstf'Oi'tf. 75 



ci'oss, we liave drawn three lines, which may be supposed to 

 represent respectively three layers of cells in a section mounted 

 for microscopical examination. If the reader will examine the 

 relation of these lines to the crossing lines in Fig. 18, he will 

 find that the lines a c and b c coincide completely between the 

 upper and under lines, and even outside of them, forming, in 

 fact, a single line. In Figure 19, on the other hand, it is only 

 the middle line that passes through the point at which the lines 

 coincide ; at the point where the upper and lower lines pass, 



Fig. 18. Fig. 19. 



the lines, a' c' and b' c', have already separated to such an extent 

 as to form distinct lines. If we regard the lines, a b, b c and 

 a' b', b' c', as rays of light, it is evident that while those in Fig. 



18 will form images of the upper and lower lines, those in Fig. 



19 cannot do so.* 



Until within a few years it has been accepted, as a thoroughly 

 demonstrated fact, that penetrating and resolving power always, 

 of necessity, exist in inverse ratio to each other, for it is always 

 found that, other things being equal, resolving power increases 

 with the angle of aperture, while penetrating power decreases. 

 Of late, however, it has been claimed that certain lenses 

 of great resolving power possess considerable penetrating 

 power. 



* Those who wish to examine this subject more closely, would do well 

 to examine an article on ''Penetration in Objectives," by Dr. Geo. E. 

 Blackham, published in the American Journal of Microscopy, for July, 

 1880, and the discussions which follow in succeeding numbers. 



