152 Experimental Philosophy. [Lecture 11. 



the sides, the rays are refracted the contrary way, 

 and are made to diverge. 



The reason of this will be sufficiently evident, 

 if it be recollected that all curves or segments of 

 a circle may be conceived as formed of a number 

 of straight lines infinitely short, and inclining to 

 each other like the stones in the arch of a bridge, 

 or the bricks at the top of an arched window- 

 frame. It is evident, therefore, that in fig. 49, 

 where parallel rays are supposed to strike a sur- 

 face of this form, those only which enter the mid- 

 dle part will go in a straight direction, whereas 

 those which strike the sides will strike them ob- 

 liquely, and will consequently be refracted. If 

 the surface, then, be a perfect curve, as in fig. 50, 

 it is plain that only the ray which strikes the 

 centre point of the curve will enter it in a straight 

 direction, and consequently all the rest which 

 strike it obliquely will be more or less refracted, 

 according to the degree of obliquity, and will 

 consequently be made to converge. 



Glasses are usually ground for optical purposes 

 into seven different shapes (see fig. 51). First, 

 the glass may be flat on both sides, as the com- 

 mon pane of a window, No. 1, Or, secondly, 

 it may be flat on one side and convex on the 

 other, plano-convex. No. 2. Or, thirdly, it 

 may be convex on both sides, like our ordinary 

 reading-glasses, No. 3. Or, fourthly, it may be 

 flat on one side and concave on the other, plano- 

 concave, as No. 4. Fifthly, it may be concave 



