224 ox THE THEORY OF THE MICROSCOPE. 



eyepiece, and is inversely proportional to this magnitude. It 

 takes, in every particular, the same form as when the microscope 

 image, supposing it to be still free from this diffractive effect, 

 were observed through a small hole pierced in a diaphragm of the 

 same size as the diameter of that aperture image. This diameter, 

 however, depends on the angle of aperture of the microscope and 

 its collective focal length ; hence, also, on its collective amplifica- 

 tion, and may be calculated on the formula given in (^ iv.) Assum- 

 ing the magnitude of angle of aperture 180^ in air, which cannot 

 be exceeded beyond a few degrees, even by immersion systems, 

 we find, e.g., for an amplification of 1000, the diameter =^L inch, 

 and for amplification of 5000 ^^4-^. inch, without reference to the 

 mode in which the amplification is obtained (through objective 

 and ocular). And if we would know what conditions are involved 

 in such amplifications — as, for instance, 5000 fold, we have only 

 to make a puncture of -^l-j inch diameter with a needle in a card 

 or piece of tinfoil, and through this opening to look at some 

 brightly illuminated object, which has well defined edges {e.g.., a 

 candle flame), and we shall have before our eye of what must be 

 the appearance of the outlines of a microscopic 'object magnified 

 5000 times, even if the microscope itself were absolutely perfect, 

 the diffractive effect excepted. * 



Taking all these circumstances into consideration, it must be 

 concluded that no material exaltation of the absolute optical 

 power (capacity) of the microscope, beyond what is attainable at 

 present with oljectives of -^^ inch focal length, is to be expected 

 in the future, either by shortening of focus or by further improve- 

 ment of construction. And as there exists at this moment no 

 microscope whose serviceable magnifying power reaches even to 

 4000j(if any serious sense of the term ^^ strvicealle " be intended), 

 so will there be none in the future. On the contrary, the facts 

 just stated shew that amplifications of less than half 4000 — such 



* Due to smallness of aperture of a minute lens, and to be carefuly 

 distinguished from the diffractions which is caused by the structure of 

 objects. 



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