74 THE MICROSCOPE. 



tives is a ^ inch, having an extreme angular aperture of 1S0 

 degrees. 



The reasons of this are evident from what has been said, viz: 

 that amplifying power belongs to eye pieces as well as to objectives, 

 and the further fact that an angular aperture of 180 degrees reaches 

 the limit of construction, and that lines or elements of structure of 

 objects having a little less than yj'ooWo ^ ncn ' s tne nmit 0I the re ' 

 solving power of objectives. 



The power of resolving lines and spaces between them, or 

 other elemental structures of objects having the size of ^r^nns P arts 

 of an inch is the work of that factor of an objective's power which 

 is given by its angular aperture and not by its magnifying 

 power. When these lines are resolved by the objective they must 

 then be amplified by refractive lenses until they are large enough to 

 form a visual angle, and it is found that an amplification of 750 or 

 1,000 diameters is sufficient to do this work. A -Ay objective with a 

 one-inch eye piece will give these lines and spaces an amplification 

 of 1,000 diameters, and therefore a -fa objective, having an angular 

 aperture of 180 degrees, resolving 150,000 lines to the inch (nearly), 

 is the present limit of microscope construction, and represents all 

 that eye pieces. and objectives can do as aids to human vision. 



Now a ^l objective, having an angular aperture of 180 degrees, 

 is no better than a A, T objective having the same resolving power or 

 angular aperture for these reasons: If we have the 150,000 lines 

 resolved by an objecthe, it is necessary to magnify them i,cco 

 diameters to see them. It is immaterial whether they are magnified 

 with an objective or with an eye piece. If the objective is a -A^, a 

 one-inch eye piece is requried to magnify the image of the resolved 

 lines to i,oco diameters. If the objective is a ^ F , an eye piece' of 

 two inch focal distance is required. So there is no possible advan- 

 tage in having a -Aj, or a -Ag-. or a -Aj, or even a y 1 ^ objective, for a £ 

 inch focal distance objective will do all the work that any objective 

 can do with the proper eye pieces. 



To magnify a resolved image to 1,000 diameters we can use 

 either of the following combinations: 



1 inch objective, and \ inch eye piece. 



; u i< it . i, (( u 



1 I) ' 



1 t< i< a „ a if u 



2 



The character of the image which is magnified, so far as the 



