PENETRATION. 



115 



section. The depth of focus does not diminish at such a 

 rapid rate, but the thickness of an object which can be 

 seen under one focussing decreases accordingly as the 

 amplification increases, and therefore it is most import- 

 ant, where binocular vision is essayed, to use the lowest 

 power sufficient for distinctly recognising the object ; and 

 with transmitted light to employ as narrow a pencil as will 

 sufficiently illuminate it. 



The following table by Professor Abbe will sufficiently 

 illustrate these remarks : 



The higher the numerical aperture of an objective the 

 less will the penetration be, though the defining and re- 

 solving power, with quantity of light admitted, increase 

 with the absolute aperture of the objective, provided the 

 aberrations are well corrected. 



The following table (see next page) has been abstracted 

 from the 'Journal of the Royal Microscopical Society' 

 for August 1 88 1, wherein may also be found Professor 

 Abbe's paper from which we have largely quoted. 



From the foregoing considerations, "tests" for pene- 

 tration would seem to be superfluous, seeing that it results 

 from two almost fixed conditions ; nevertheless, it is well 

 to know how it may be observed. A section of frog's lung 

 or of human liver (Fig. 108) is useful for this purpose, as 

 when mounted, the various parts will be found to have con- 



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