RECAPITULATION. 459 



ABSTRACT OF OPTICS. 



1. The particles of light, which are incon- 

 ceivably small, proceed in straight lines from the 

 luminous body. 



2. Consequently, the density of light is in- 

 versely as the square of the distance from the 

 luminous centre. 



3. Light moves at the rate of 150,000 miles in a 

 second. 



4. When light strikes upon a surface, it is re- 

 flected so that the angle of reflection is equal to 

 the angle of incidence : on this the properties of 

 mirrors depend. 



5. Convex mirrors cause parallel rays to diverge. 



6. Concave mirrors collect parallel rays, or cause 

 them to converge to a point called the focus. 



7. When light passes out of one medium into 

 another, it changes its direction, and either moves 

 farther from, or nearer to, the perpendicular, as 

 the medium into which it enters is rarer or denser 

 than the other medium. The properties of lenses 

 are derived from this law. 



S. Convex lenses collect the rays of light, or 

 make them converge to a focus. 



9. Concave lenses disperse the rays of light. 



10. Light is composed of seven primitive colours, 

 which have not yet been decomposed. But in the 

 practice of painting, all the various tints may be 

 made of three colours, blue, red, and yellow. 



11. The rainbow is owing to the separation 

 of the rays of light into its original primitive 

 colours, by the drops of falling rain. 



12. The human eye is an optical instrument, 

 resembling a camera obscura. 



