THE EEASON WHY. 109 



1 And God saw the light, that it was good : and God divided the light from the 

 darkness." GEN. i. 



A pencil of light is a body of rays which come from, or move 

 towards a point. 



462. What is the radiant point 'f 



The radiant point is that from which diverging rays of light 

 are emitted. 



463. What is the focus? 



The focus is the point to which converging rays are directed. 



Diverging, starting from a point, and separating. Converging, drawing 

 together towards a point. 



464. What is the constitution of a ray of light ? 



A ray of white light, -as we receive it from the sun, is composed 

 of a number of elementary rays, which, with the aid of a triangular 

 piece of glass, called a prism, may be separated, and will produce 

 under refraction the following colours : 



1. An extreme red ray a mixture of red and blue, the red 

 predominating. 



2. Red. 



3. Orange red passing into and combining with yellow. 



4. Yellow the most luminous of all the rays. 



5. Green yellow passing into and combining with the blue. 



6. Slue. 



7. Indigo a, dark and intense blue. 



8. Violet blue mingled with red. 



9. Lavender grey a neutral tint. 



10. E-ays called fluorescent, which are either of a pure silvery 

 blue, or a delicate green. 



465. Why is a ray of light, which contains these elementary 

 rays, white ? 



Because the colour of light is governed by the rapidity of the 

 vibrations of the ether-waves. When a ray of light is refracted 

 by, or transmitted through a body, its vibrations are frequently 

 disturbed and altered, and thus a different impression is made 

 upon the eye. 



Light which gives 37,640 vibrations in an inch, or 

 468,000,000,000,000 in a second of time, produces that sensation 



