R E F 



, 



III. Refraction, images produced by. 



1. The image of a straight line, formed br a plane refracting surface, 

 is a straight line : and if a be the perpendicular distance of any point of 

 the object from the surface, and m the ratio of refraction, the distance 

 of its image m a. 



Cor. In the case of water, m %. Thus the image of the bed of a 

 river is nearer to the surface than the bed itself by 14 of the whole depth. 



2. If the object placed before a lens or a sphere be a circular arc con- 

 centric with it, the image will also be a circular arc concentric with and 

 eimilar to the object. 



3. The image of a straight line formed by a lens or sphere is the arc of 

 a conic section. 



4. The sun's image formed by a lens is a circle, and nearly in the prin. 

 cipal focus : and the density of his rays when viewed with a reflector or 



area of aperture X power 



refractor varies as TH-^ IT-; a 



(Focal length) 2 



TABLE, 



Of the refractive and dispersive power of different substances, with their 

 densities compared with that of water, ivhich is taken as the unit, 



The substances marked (*) are combustible, 



The refraction is supposed to take place between the given substance 

 and a vacuum. 



233 



