INTRODUCTION TO OPTICS. 



Ixxxix 



at the focus F*. The focal distance, or distance of the focus from the 

 surface of the lens, depends both upon the form of the lens and of the 

 refractive power of the substance of which it is made ; in a glass "lens, 

 both sides of which are equally convex, the focus is- situated nearly at the 

 centre of the sphere of which the surface of the lens forms a portion : it 

 is at the distance, therefore, of the radius of the sphere. 



There are lenses of various forms, which are represented in fig. 24. 

 The property of those which have a convex surface is to collect the rays 



Fig. 24. 



of light to a focus ; and of those which have a concave surface, to dis- 

 perse them : for the rays A, C, falling on the concave lens X Y (Jig. 25), 

 instead of converging towards the ray B, which falls on the axis of the 

 lens, will each be attracted towards the lens, both on entering and quitting 

 it, and will, therefore, by the first refraction, be made to diverge to a, c, 

 and by the second to d, e. Lenses which have one side flat, and the 

 other convex or concave, as~A and B,Ji$. 24, are called plano-convex and 

 plano-concave lenses. The focus of the former is at the distance of the 



Fig. 25. Fig. 26. 



diameter of a sphere, of which the convex surface of the lens forms a 

 portion, as represented in Jig. 26. The three parallel rays, A, 13, C, are 

 brought to a focus by the plano-convex lens X Y at F. 



Thus far we have only spoken of the refraction of parallel rays; if the 



* The refractions will at once appear to be in the same direction if perpendiculars to 

 the surface of the lens be drawn at (he points where the ray enters and quits it: bearing 

 in mind that the lens being denser than the air, the ray is drawn towards the perpen- 

 dicular ou entering, and/m/i it on quitting the lens. 



