102 EXPERIMENTS, &c. , 



directed to the same objects, without the inter- 

 vention of such glasses. Hence the nearer the 

 object is, the greater will be the effect of the 

 variation in the inclination of the axes produced 

 by spectacles with convex glasses ; which is 

 the order of things, the best adapted to the 

 wants of those who use them. But with respect 

 to short-sighted persons, since the refracting 

 angles of their glasses, considered as prisms, 

 decrease, in proportion as the objects seen 

 through them become more remote ; they must, 

 consequently, derive the least benefit from an 

 alteration in the mutual inclinations of the 

 optic axes occasioned by their spectacles, at 

 the time they most require it. 



If it were asked, then, what is the real founda- 

 tion of the common reproach against spectacles 

 for long-sighted people ? I should answer, a 

 very different one from that, which is, for the 

 most part, assigned. For the change, in the 

 conformation of the eyes, which renders them 

 useful, seems to be one of those which nature 

 has destined to take place at a particular age, 

 and to which there is no gradual approach 

 through the preceding course of life. A per- 

 son, for instance, at forty, sees an object di- 

 stinctly, at the same distance that he did at 

 twenty. When he draws near to fifty, the 

 change I have spoken of commonly comes on, 



