OP THE MICROSCOPE. 73 



This quality of objectives of large angles, whereby they 

 are capable of showing distinctly delicate lines or dots placed 

 very closely together, is known as resolving power. In the early 

 days of microscopy, it was called penetrating power, the term 

 penetrating having been applied to that quality of the telescope 

 by which it is enabled to show separately the individual stars 

 of which the nebulae are composed. In the telescope this was 

 supposed to depend upon space-penetrating power as distin- 

 guished from mere magnifying power, and this space-penetrat- 

 ing power was found to depend very largely upon angular aper- 

 ture. In the case of the microscope, however, it is now gene- 

 rally agreed that what was called penetrating power in the 

 telescope, shall be called resolving power, while to the term 

 penetrating power an entirely different meaning has been given. 



Mere resolving power, however, or the power of showing 

 separately lines placed very closely together, is not the only 

 valuable feature of well-corrected object-glasses of high angles. 

 They show delicate lines and fibres, and enable us to make out 

 dilTerencos of structure which are entirely invisible to lenses 

 of low angles. Thus, for example, it has been found during 

 recent researches, that the delicate flagella of certain monads 

 can be seen perfectly with high angle lenses, while with very 

 excellent glasses of low angular aperture they are quite invis- 

 ible. The same fact, probably, holds true in regard to the ulti- 

 mate fibres of nerves and similar objects. 



The researches of Lister and Ross formed, as we have just 

 stated, the first great step in the direction of better correction and 

 increased angular aperture. Whereas, 40 to 65 had previously 

 been regarded as very high angles, even in objectives of the 

 shortest focal distance, Boss, in his objectives, soon attained 

 an aperture of 132 to 135 and, working with the glass at that 

 time available, this was pronounced the highest attainable 

 angle. Attempts had previously been made to obtain a higher 

 angle by the use of the glass which Faraday devised for optical 

 purposes, and which is in fact a borate of lead. But this com- 

 pound is so easily tarnished and disintegrated, that it was 

 found impracticable to use it. It happened, however, that 

 a young American backwoodsman, Charles A. Spencer, of 

 Qanastota, N, Y., a graduate of Hamilton College, had ki3 



