CHAPTER II. 



THE MICROSCOPE. 



THOSE who have examined a common magnifying glass (or 

 lens) know that it is necessary to hold it exactly at a certain 

 distance from the object viewed through it, in order that such 

 object may be seen with distinctness. The point at which the 

 object must be placed is called the focus of the lens, and the 

 distance from the middle of the lens to the focus is the focal 

 length, or focal distance of the lens. 



The cut represents sections of the different forms of lenses. 

 A, is a plano-convex lens. B, double convex. C, plano-con- 

 cave. D, double concave. E ; a meniscus. 



Fig. 1. 

 C 



The effect of the convex lens or of the meniscus is to cause 

 the rays of light which pass from any object through them, to 

 converge towards a point or focus ; and the eye receiving those 



