258 optics. 



light are directed from the circumference towards 

 the centre, in an angle proportional to the con- 

 vexity of the glass, and meet in a point, at a 

 greater or lesser distance from the glass, as it is 

 more or less convex. This point, where the rays 

 meet, is called the focus; and this focus is nearer 

 or farther off, according to the convexity of the 

 glass ; for as a little convexity throws it to a con- 

 siderable distance, so when the convexity is much, 

 the focus is very near. Its magnifying power is 

 also in the same proportion to the convexity; for 

 as a flat glass scarcely magnifies at all, the less a 

 glass, departs from flatness, the less of course it 

 magnifies ; and the more it approaches towards a 

 globular figure, the nearer its focus is, and the 

 more its magnifying power. 



People's different length of sight depends on the 

 same principle, and arises from more or less con- 

 vexity of the cornea and crystalline humour of the 

 eye : the rounder these are, the nearer will the 

 focus or point of meeting rays be, and the nearer 

 an object must be brought to see it well. The case 

 of short-sighted people is only an over-roundness 

 of the eye, which makes a very near focus; and 

 that of old people is a sinking or flattening of the 

 eye, whereby the focus is thrown to a great dis- 

 tance; so that the former may properly be called 

 eyes of too short, and the latter eyes of too long a 

 focus. Hence, too, the remedy for the last is a 

 convex glass, to supply the want of convexity in 

 the eye itself, and brings the rays to a shorter 

 focus; whereas a concave glass is necessary for the 

 first, to prevent the rays from coming to a point 

 too soon. 



Nothing is more common, than to observe old 

 people holding objects they would examine at a 



