THE ACTION OF LIGHT 



plano-concave. There are other combinations, for 

 example, one face may be inwardly curved and the 

 other outwardly curved, but the four kinds we have 

 described are all that need trouble us. 



It does not require a great amount of imagina- 

 tion to recognise that the double convex lens, that 

 is the lens with two outwardly curved faces is little 

 more than a pair of prisms placed base to base, or 

 more accurately, a number of prisms so arranged 

 as shown in the diagram. Parallel rays of light 



falling upon such an arrangement of prisms would 

 be bent from their course, as shown by the arrows, 

 and this is just what happens with a double convex 

 lens. Now rays of light from an object, passing 

 through a lens of this shape may follow any one of 

 three courses, according to the position of the object 

 with regard to the lens. In one position and one 

 only the rays after passing through the lens will be 

 parallel to one another, as shown in the diagram. 



The only position of the object for the above to 

 take place is when it coicides with a point known 

 as the principal focus of the lens, conversely the 



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