INTRODUCTORY REMARKS. 33 



Microscopes may be classed under two denomina- 

 tions, simple and compound. 



The simple microscope presents an object to our view 

 under a greater angle than that which it is seen under 

 when viewed by the naked eye ; whence it is that the 

 impression is derived of it being amplified or magnified. 

 The amount of this amplification will be in precisely the 

 same ratio as that when a person is seen at different 

 distances from us : the nearer he is, the greater is the 

 angle he will be viewed under ; and so vice versa. 

 Hence it is, that a lens which will admit of an object 

 being placed within one inch of it, will amplify twice* 

 as much as another the focal length of which is two 

 inches. This quality, viz. the focal length of a lens, or 

 its power of admitting an object within a definite distance, 

 so that it may be distinctly visible, is obtained by giving 

 to the surface of any transparent medium (say glass) a 

 spherical or other curved figure. Now it will be evident 

 that, on account of the curvature which is thus given, 

 the rays of light which proceed from any given point in 

 an object to the middle of the lens will pass in a more 

 direct course through it (assuming the lens to be convex) 

 than those which proceed from the same point to the 

 outside or edges ; and consequently the former will be 

 less refracted or bent in their passage than the latter. 

 All the rays, therefore, after permeation, will not proceed 

 towards the same point in the axis of the lens, but 

 crossing it at different distances they will enter the eye 



* See Micrographia, page 63. 

 D 



