ON PROJECTION 5 



A convex or focussing lens must act in the same way, 

 because it is a piece (or combination of pieces) of glass 

 formed so as to be thickest in the middle, the faces gradually 

 becoming more and more inclined till they go off to an edge. 

 It is a circular prism, of gradually-increasing inclination 

 towards its edges. If we place such a lens in a beam of 

 parallel rays, as from the sun, it is easy to see what must 

 happen. The centre ray, striking the glass perpendicularly, 

 proceeds straight on unrefracted. The next outer rays, 

 meeting the glass at a small angle, are a little bent in towards 



the centre or thickest 



part. Then rays farther 



out from the centre are 



more, bent in, because 



they strike the surfaces 



at a greater angle ; and FlG<3 



so the whole beam of 



rays meet practically in one point, F (fig. 3), which is the 



' focus ' for parallel rays, or ' principal focus.' This is the 



well-known phenomenon of a burning-glass. v 



Take the converse, however ; if the rays diverge from the 

 luminous point F at this focus of the lens, they are refracted 

 just the same, being converted into parallel rays. Suppose, 

 however, that the rays diverge from a point, /, farther away 



FlQ. 4 



from the lens than its principal focus, F (fig. 4). What must 

 happen then is, obviously, that all the rays diverging from 

 the point / will be bent in so as to meet again in some other 

 point, F a , on the other side of the lens, which point must have 



