288 SCIENTIFIC AMUSEMENTS. 



8. Radiant Action : Visible Light. 



CHAPTER XXII. 



EMISSION OP VISIBLE LIGHT ILLUMINATION OF OBJECTS NOT SELF- 

 LUMINOUS COLOUR AND ABSORPTION REFLECTION AND RE- 

 FRACTION AT PLANE SURFACES. 



The various sources of visible light known to us may conveni- 

 ently be divided into two chief classes, viz., those of origin outside 

 the world, and those of terrestrial origin. The first class includes 

 the heavenly bodies, such as the sun, moon, planets, fixed stars, 

 comets, and meteors, &c., the study of which in the main belongs 

 to that branch of science known as astronomy rather than to 

 general elementary physics, excepting in so far as the light from 

 these sources obeys the same general laws as the artificial and 

 natural lights producible on the earth. Of late years, by means 

 of an instrument termed the spectroscope, much valuable informa- 

 tion has been gained respecting the various heavenly bodies, 

 especially when photography (Chapter XX Y.) is applied to the 

 indications of the spectroscope, so as to obtain permanent records, 

 instead of. having to trust to the comparatively fleeting results of 

 eye observations ; by the examination of the light reaching us from 

 the sun, for example, and comparing it with artificial lights pro- 

 duced in various ways by employing chemical or electrical action 

 with different kinds of elementary substances (e.g., when elements 

 such as metals are burnt in contact with oxygen, or when electric 

 sparks are made to pass between pieces of metal) a variety of 

 results are deducible, leading amongst other things to the conclu- 

 sion that many forms of matter known to us here on earth as 

 elements are also present in the sun ; analogous remarks also apply 

 to various of the fixed stars. 



The terrestrial sources of light are again divisible into two kinds, 

 viz., those spontaneously produced in nature, and those de- 

 veloped by artificial means. To the former class belong the light 

 emitted in volcanic regions by " burning mountains " and natural 

 highly heated substances, this source of light being simply due 

 to the existence of sources or stores of heat sufficient to cause the 

 heated bodies to become luminous, just as when a poker made 

 "red hot" in a fire becomes visible in a dark room by virtue of 



