reconcile this with the fact that numerical aperture only 

 does it ? Why, if this is true, should we use a higher 

 power objective with a prescribed aperture in pre- 

 ference to a lower one of the same aperture ? 



The normal eye can see about 200 lines to the inch, 

 or structures which are 1-200 of an inch apart and it is 

 therefore evident that any structures under the micro- 

 scope must be separated at least to this extent in the 

 virtual image, in order that they may become visible. 

 While we have shown that magnifying power may be 

 increased 



by increasing the power of the objective, 

 by increasing the power of the eyepiece, 

 by increasing the tube length, 



we are limited in the last direction mainly by such 

 length of tube as has been found convenient in the 

 construction of the stand ; in the case of eyepiece on 

 the one hand by convenience in use and on the other 

 by the fact that too great an increase causes an indis- 

 tinctness, so that, although variation in eyepiece within 

 narrow limits is admissible, we are compelled to select 

 an objective which, with a medium power eyepiece, 

 will give sufficient separation to structure and fine 

 detail, that it will be visible to the eye without any 

 undue strain. 



84 



