41 



and extent of its definitions are under a certain mag- 

 nifying power. If a high magnifying power is all that 

 is desired, this may be obtained to an almost unlim- 

 ited extent by means of simple lenses which may be 

 procured at a small pecuniary outlay ; but these do 

 not give a distinct image nor do they make structure 

 visible, which, be it remembered, is the purpose of 

 the microscope to do. 



The normal eye can distinguish from 200 to 250 

 lines to the inch, and in a microscope such magnify- 

 ing power should be used, which will apparently bring 

 the structure which is sought after at least up to this 

 figure. In illustration take a \ inch objective of 98 

 degrees and a \\ inch eye-piece. An objective of this 

 kind properly corrected, resolves pleurosigma angiila- 

 tum, in which the average lines are 00,000 to the 

 inch. With the above eye-piece it is utterly impossi- 

 ble to see them, while if it is replaced by a | inch or 

 | inch, they can easily be distinguished. This is not 

 owing to any peculiar quality of the eye-piece, but 

 merely to the fact that by increasing the magnifying 

 power, the dimensions of the object have been 

 increased to such an extent that the lines have appar- 

 ently been separated and become visible to the eye. 



Beginners as a rule are apt to use too much magni- 

 fying power or amplification, and often attempt to 

 view a large surface with an objective which will show 

 but a small part of it. It must not be forgotten that 

 the apparent field of view is decreased as higher 

 powers are used, and that a low power will give a 



