MICROSCOPY AND BACTERIOLOGY. 



INTRODUCTION. 

 THE REFRACTION OF LIGHT AND THE MICROSCOPE. 



THE REFRACTION OF LIGHT. 



Definition. Refraction is the property possessed by trans- 

 parent media of altering the rays of light which pass through 

 them. It is to this property possessed by lenses, the trans- 

 parent media of microscopes, that these instruments owe 

 their magnifying power. 



The Two Fundamental Laws of Refraction. 



I. When a ray of light passes from a denser to a rarer 

 medium, it is refracted away from a line drawn perpendicu- 

 larly to the plane which divides the media ; and vice versa, 

 when the light passes from a rarer to a denser medium, it is 

 refracted toward that perpendicular. 



II. The sines of the angles of incidence and refraction that 

 is, of the angles which the ray makes with the perpendicular 

 before and after its refraction bear to one another a constant 

 ratio for each substance, which is known as its index of 

 refraction. 



The Principles of Refraction by Lenses. 



Microscope lenses are chiefly convex ; those of other forms 

 are used to make certain modifications in the rays passing 

 through the convex lens, and so render their performance 

 more exact. 



2 M. B. 17 



