THE MICROSCOPE 



25 



perpendicular to the surface of the medium of lesser 

 density (Fig. 8). Glass being of greater density than 

 air, light rays passing through it will be bent toward 

 a line at right angles to its surface. 



Six different types of lenses are formed by com- 

 binations of curved surfaces, either with each other 



Fig. 8. — Refraction of Light Rays. 



1. Path of ray passing from less dense medium (.A) to one of greater density (B). 

 2. Path of ray passing from medium of greater density (4) to one of less density (B) . 



or with flat or plane faces. A lens similar to an ordi- 

 nary magnifying glass is termed convex or double convex. 

 If the lens is thinnest at the center it is termed con- 

 cave or double concave. Two variations of convex 

 lenses are recognized, plano-convex and concavo-convex 

 or convergent meniscus. Similarly there are two 

 variations of concave lenses, plano-concave and convexo- 



