104 ELEMENTS OF SCIENCE 



consideration of bodies very distant from the earth's 

 surface. Light can travel with perfect freedom through 

 any glass vessel, the interior of which is as perfect a 

 vacuum* as we can possibly make, therefore if ether is 

 necessary for the existence of light, ether must fill any 

 such glass vessel, and therefore its interior can be no 

 real vacuum. Like heat, light is eminently capable 

 of reflexion,f wherein it may be said to follow in a 

 general way the laws which determine the reflexion 

 of heat. The quantity of light reflected varies greatly, 

 and rarely amounts to one-half so much being generally 

 absorbed. Any object seen appears the darker, the 

 greater the quantity of light thus absorbed. All surfaces 

 equally reflect light, but those which are "dull" are 

 minutely rough i.e., have minute surfaces turned in 

 many different directions and these reflect light equally 

 in all possible directions. When a body appears 

 " glittering," it is because it bears a quantity of small 

 surfaces, or facets, which are nearly smooth, and therefore 

 each such facet reflects light similarly, and for the same 

 reason a polished surface as that of a mirror appears 

 to reflect light most perfectly. It reflects so perfectly, 

 because its smoothness causes it to reflect with order and 

 regularity according to the law of equal angles of inci- 

 dence and reflexion the rays which reach it with order 

 and regularity from neighbouring objects. There are 

 certain phenomena of reflexion connected with colour 

 which will be best considered together with the next 

 property of light, namely refraction.^ 



Rays of light, like those of heat, are refracted as they 

 pass from one medium into another. Light falling on a 



* See also page 60. t See ante, p. 99. 



+ See post, p. 108. 



