4 THE OPTICAL DEFECTS OF THE EYE. 



This will prove not only that the rays diverge, but also that the 

 rays proceed in straight lines.* 



Convex lenses : — We shall now proceed to the consideration of con- 

 vex lenses, which, for our purpose, is the most important part of the 

 subject. Lenses are made of various transparent substances as 

 amber, alum, quartz, glass, diamond, and even of ice. Those in 

 ordinary use are made of glass. "When the two surfaces of a convex 

 lens have the same degree of curvature, the lens is said to be equi- 

 convex. When one of the surfaces is fiat or plane, the lens is called 

 a plano-convex lens. Glass spectacles used by old persons for read- 

 ing, &c., are commonly made double convex. 



In order to simplify the subject as much as possible, let us confine 

 our attention to lenses that are equi -convex. 



In fig. 2 let A be the centre of the circle B, C, D, of which A, B, is 

 the radius, and let E be the centre of the circle F, Gr, H, of which 

 the radius E, E, is equal to the radius A, B. The circle E, Gr, H, will 

 be equal to the circle B, C, D. The part D,H, common to both cir- 

 cles, represent a section of an equi-convex lens. The line A,E, is 

 called the axis of the lens, and the line D, H is called the diameter. 

 The centre of the diameter (where it is intersected by the axis) is 

 the optical centre of the lens. 



Eeading glasses, and burning glasses, are examples of a double 

 convex lens. Many of you have, doubtless, seen the experiment of 



(* Convergent pencils of light do not exist in nature. Parallel pencils or diver- 

 gent pencils of rays can be rendered convergent by means of a convex lens. Thus 

 Id fig. 1, the rays diverging from F, are made to converge to P by the convex lenses, 

 A. and B.) 



