CHAPTER III 



THE MICROSCOPE 



The microscope is an optical instrument which 

 produces an enlarged image of a near and usually 

 small object. It differs from the telescope in that 

 the latter instrument produces enlarged images of 

 objects so far distant from the observer that they 

 appear small although they may be very large. The 

 human eye cannot distinguish objects below a certain 

 size, because the extreme light rays proceeding from 

 such an object are so close that the image cast upon 

 the terminations of the optic nerve or retina is too 

 small to cause the stimulation necessary for vision. 

 The function of the microscope lenses is to increase 

 the distance between the extreme rays cast by a small 

 object, and thus render the image received by the 

 retina large enough to cause sufficient stimulation for 

 vision. Magnification by a lens depends upon its 

 ability to refract light rays. This property is in turn 

 dependent upon the principle that light rays passing 

 through objects of differing densities are changed in 

 direction. Light rays passing from a medium of lesser 

 to one of greater density will be bent toward a line at 

 right angles, to the surface of the denser medium 

 (Fig. 8). Light rays passing from a medium of greater 

 to one of lesser density will be bent away from a line 



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