162 Experimental Philosophy. [Lecture 11. 



whether concave or convex, are segments of dif- 

 ferent circles, the radii and diameters of which 

 may vary almost to an infinite extent. The 

 distance of the principal focus, or focus of parallel 

 rays, that is, the point where all the parallel rays 

 meet, as the point/; fig. 52, will vary in different 

 lenses, according to their respective degrees of 

 convexity. Hence, when opticians speak of the 

 radius of a lens, when they say it is three or six 

 inches, they mean that the convex surface of the 

 glass is that part of a circle, the radius (that is, 

 half the diameter) of which is three or six inches. 

 The axis of a lens is a straight line drawn through 

 the centre of its spherical surface. 



The principal focus, or focus of parallel rays, 

 in convex lenses, is ascertained (as was before 

 intimated) upon mathematical principles. It may 

 however be found with sufficient accuracy for 

 common purposes, by holding a sheet of paper 

 behind the glass, when exposed to the rays of the 

 sun, and observing when the luminous spot is 

 smallest, and when the paper begins to burn. 

 Or when the focal length does not exceed three 

 feet, it may be found by holding the glass at 

 such a distance from the wall opposite a window 

 sash, as that the sash may appear distinct upon 

 the wall. 



You will observe, that in a double convex lens 

 the rays of light are twice refracted ; first, on 

 entering the convex surface of the dense medium, 

 the glass; and, secondly, on going out of the 



