PHYSICAL FORCES 107 



original direction, although there is a slight change of 

 position. If the opposite sides of the glass are not 

 parallel then that piece of glass is what is called a prism. 

 By it the direction of the rays is permanently changed, 

 and the more so, the more inclined to each other the 

 two surfaces of the glass may be. It has also certain 

 other effects, respecting colour, which will be referred 

 to a little further on. 



The light of day having so very distant a source, may 

 be practically regarded as consisting of parallel rays, 

 just as the force of gravity on the earth's surface may, 

 as we before saw,* be conveniently treated as if acting 

 in parallel lines, although really it acts in lines radiating 

 from the earth's centre. Rays of light, then, which fall 

 upon and pass through bi-convex or bi-concave bodies, 

 follow laws similar to those which we have already seen 

 apply to rays of heat.f Thus it is that by a judicious 

 combination of glasses, those instruments, so valuable for 

 Science, the microscope and the telescope, are constructed. 

 Thus also an image of external objects can be made to fall 

 upon the surface of a table in a camera obscura. Bi- 

 convex glasses, or lenses, bring the rays to a focus at 

 different distances according to the curvature of their 

 surfaces, and the distance in each case is called ihe focal 

 length of such lens. 



When treating of heat, but very little could be said 

 about refraction. But now, after what has been said 

 about light, it will be easy to understand how and why 

 rays should thus be made to converge or diverge accord- 

 ing to the shape of the surface, whereon they impinge. 

 For however many parallel rays fall upon a curved 

 surface, they must all have different inclinations to it, 



* See ante, p. 43, t See ante, p, 101, 



