PHYSICAL FORCES 109 



came to be spoken of, because the hypothesis now popu- 

 larly employed to explain the different colours which 

 objects present to us and seem to possess, explains 

 them by reflexion colours being represented as due to 

 different conditions of the reflexion of light. Light 

 being normally colourless, or white, a white object is one 

 which is supposed to reflect all the different kinds of 

 rays which it receives, and so the object looks white. 

 Any object which is black, is, on the other hand, 

 supposed to absorb all the rays of light and to reflect 

 none. A red object is supposed to absorb all the rays 

 which are not red, while the circumstances of its reflect- 

 ing the red ones is supposed to be the cause of its red 

 appearance. So again blue and all other colours are 

 supposed to be similarly caused by the absorption of 

 certain rays, and the reflexion of those which seem to 

 belong to the several objects seen by us as possessing 

 corresponding tints. In any coloured transparent object, 

 such as red-coloured glass, its colour is deemed to be due 

 to the absorption by it of all the colours save the red 

 rays, which, being unabsorbed, it transmits to the eye of 

 the spectator. 



Those peculiar conditions, activities, or what not, which 

 exist in ordinary light and are called " colours," possess 

 different degrees of refrangibility. This is easily shown 

 by a simple experiment which was first tried by Sir Isaac 

 Newton. He allowed a sunbeam to enter a dark room 

 through a small aperture and throw a bright spot of light 

 on a screen, placed opposite the aperture on purpose that 

 the beam might fall upon it. He then placed a prism of 

 glass the opposite sides of which approached each other 

 from above downwards at a considerable angle in the 

 path of the sun-beam. Thereby the beam became much 

 refracted, so that it produced an elongated bright spot 



