APPENDIX 155 



light. It is diffused light which makes nonluminous 

 bodies visible. "From the inside of our rooms we well 

 see external objects, for they are powerfully illumi- 

 nated; but from the outside we only see confusedly the 

 objects in the interior for they receive but little light.'' 

 It is the great diffusion on the outside and the slight 

 amount inside which causes the difference. When the 

 room is illuminated at night the opposite is true. 



Refraction. When light passes obliquely from a less 

 dense through a denser medium it is bent away from its 

 original path or refracted (from a Latin word meaning 

 broken). Glass is denser than air. A lens is a piece of 

 glass which may be flat on one side and hollow on the 

 other planoconcave flat on one side and bulging on 

 the other planoconvex, bulging on both sides bi- 

 convex or double convex, or concave on both sides 

 biconcave or double concave. A ray passing through 

 the exact center of any lens is not bent; but a ray which 

 strikes obliquely, as any ray not in the exact center 

 must strike on concave or convex surfaces, is bent toward 

 a common center in convex and away from the center in 

 concave lenses, or the rays are converged by convex and 

 diverged by concave lenses. As animal eyes have double 

 convex lenses which do not always fit the eye in which 

 they are found, oculists make use of this principle of 

 refraction to place glasses in front of the eye to con- 

 verge or diverge the rays and thus correct the refrac- 

 tive error of the natural eye. 



The point at which all of the rays must meet after 

 passing through a lens is called its "focal point." The 

 more convex a lens, the nearer to the lens is the focal 

 point ; i.e., the more abruptly it breaks or bends the rays. 

 The path through the center of the lens is its principal 

 axis and parallel rays emitted from the lens are con- 



