MICROSCOPES. 



MICROSCOPES may be divided into two classes, Simple and 



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Compound 



SIMPLE MICROSCOPES. 



Simple microscopes consist of one or more lenses, so arranged 

 that the object is viewed directly, and no actual image is formed 

 by one set of lenses, and examined by others, as in the case of 

 compound microscopes. 



The utility and so-called magnifying power of a single lens, 

 or of a simple microscope consisting of a combination of 

 lenses, depends solely on the fact that they bend rays proceeding 

 from an object so near to the eyes, that they would not other- 

 wise form a distinct image on the retina, and cause them to 

 enter the pupil parallel or so slightly divergent as to make distinct 

 vision possible. 



In the construction and use of lenses two great difficulties 

 present themselves. It is practically almost impossible to make 

 small lenses with any other than spherical curves, and unfor- 

 tunately simple spherical lenses do not bring the rays to a perfect 

 and exact focus. If it were possible to construct lenses with 

 elliptical or hyperbolic curves, this so-called spherical aberration 

 would be avoided ; but, even then, since the different rays of the 

 spectrum are refracted differently, the focal length for red light 

 would be greater than for blue, and it would be impossible to 



