160 Experimental Philosophy. [Lecture 11. 



In looking through a glass of this description, 

 that is, a convex or double convex lens, the ob- 

 jects which we look at will appear magnified; 

 for it is a rule in optics, that we see cvi'ry tiling 

 in the direction of that line In which the rays ap- 

 proach us last. When I come to treat of the eye, 

 the reason of this will be explained. Suffice it 

 to say for the present, that the larger the angle 

 under which any object is seen, the larger will 

 any object appear. The convergence of the rays 

 of the convex lens, therefore, enlarges greatly 

 the angle of vision, as must be evident if we 

 continue the lines/D,/E,/T, and/G, fig. 52, 

 in the direction to which they point, and therefore 

 in proportion to the distance the appearance 

 of the objects will be enlarged. The jcommon 

 spectacle-glasses and reading-glasses are of this 

 description. 



The effects of the plano-concave and double 

 concave lenses, No. 4 and 5, are directly op- 

 posite to those of the convex lenses; for the 

 thick parts of these glasses, you see, are towards 

 the edge, and therefore their attractive and re- 

 fractive powers are not towards the centre, but 

 towards the circumference. Parallel rays, there- 

 fore, striking one of these glasses are made to 

 diverge, or are dispersed. Rays already divergent 

 are rendered more so ; and convergent rays are 

 made less convergent. Hence objects seen 

 through these glasses appear considerably smaller 

 than they really are. To prove this, let ab (fig. 53) 



