260 



SCIENCE OF COMMON THINGS. 



Lenses and their varieties. 



What is a focus of light? 



towards the earth, and produce a little light called twi- 

 light. 



1691 What is a lens f 



A piece of glass or other transparent substance, 

 bounded on both sides by polished spherical surfaces, 

 or on the one side by a spherical, and on the other by a 

 plane surface. Rays of light passing through it are 

 made to change their direction, and to magnify or 

 diminish the appearance of objects at a certain distance. 



1603 Sow many varieties of lenses are generally recognised f 



Two : convex and concave. 



Fij?. 74. 



Among convex lenses are the double convex A (Fig. 74) to which the 

 appellation lens was originally applied from its resemblance to a lentil- 

 seed (kns in Latin) being bounded by two convex spherical surfaces 

 whose centres are on opposite sides of the lens ; the plano-convex B, 

 having one side bounded by a plane surface, and the other by a convex 

 surface ; and the meniscus or concavo-convex C, bounded on one side by 

 a concave, and on the other by a convex surface. 



There are also three principal varieties of concave glasses; as the 

 double concave D, bounded by two concave surfaces, forming portions of 

 spheres whose centres are on opposite sides of the lens; the plano- 

 concave E, bounded on one side by a plane, and on the other by a 

 concave surface ; and convexo-concave F, bounded by a convex surface 

 on one side, and by a concave one on the other. 



1693 What is a focus of light f 



"When rays of light continually approach each other, 

 as in moving to a point, they are said to converge, and 

 the point at which the converging rays meet is called 

 tliejocus. 



1694 What sort of a lens is a common burn- 

 ing-glass ? 



A double convex lens. 



Fig. 75 represents the action of a double con- 

 vex lens in causing the rays of light to converge 

 and meet at a focus. 

 Fi gi 75. 1695 What are transparent bodies f 



