THE PLANO-CONVEX LENS. 



101 



when converging rays fall upon a double-convex lens they 

 are brought to a focus at a point between the principal 

 focus and the lens itself, as shown in Fig. 92. 



The action of a plano-convex lens, of which our bull's-eye 

 condenser, Fig. 36, is a type, may be studied in the same 

 diagrammatic man- 

 ner. This may be 

 considered as a 

 double-convex lens 

 split down the centre, 

 and so forming two 

 plano-convex lenses ; 

 it is generally used 

 for purposes of microscopy on account of the great work- 

 ing distance of its focus. As already described, parallel 

 rays falling upon a double-convex lens come to a focus 

 very near the centre (radius) of its curvature, but when 

 the same rays fall upon the curved surface of a bull's-eye 

 condenser they are brought to a focus at a distance equal to 

 the diameter of the curvature, or twice the distance of a 

 double-convex lens, as may be seen in Fig. 93 ; and con- 

 versely if we wish to produce rays of parallel light from 



FIG. 92. 



FIG. 93. 



a lamp, the ordinary bull's-eye condenser must be placed 

 twice as far from the luminous point as would be neces- 

 sary in the case of a double-convex lens, a condition of 



