Radiant Forces. 141 



total darkness or distinct colours, according to the exact nature 

 of the interference which takes place. 



Optical experiments show that the light we receive from 

 distant bodies, as from the sun, behaves as if it were composed 

 of a number of rays exactly parallel to each other, while the 

 light received from near objects behaves as if it were com- 

 posed of rays not parallel, but diverging from a common 

 centre. These facts seem to throw us back upon the concep- 

 tion of really divergent rays, as contradistinguished from 

 parallel rays, and thus would lead us to set us to reject the 

 supposition of strict analogy between the spherical water waves, 

 already spoken of, and the waves of light. We shall, however, 

 find that the difficulty arises from accepting the term " diverg- 

 ing rays," as expressing a literal fact, and not merely affording 

 an approximate illustration. 



If we pass from light to gravitation we find another 

 instance of a force moving in straight lines, and varying* 

 inversely as the square of the distance at which it operates. 

 Bodies attract each other in proportion to their masses, and 

 the force of that attraction diminishes as the squares of the 

 distances increase. This law is found to hold good for Nep- 

 tune, the remotest known member of our system, and which, 

 as we have remarked, is more than thirty times as far off the 

 sun as the earth is. 



We have as yet no means of showing or establishing a 

 connection between gravitation and other properties of matter. 

 Light, heat, electricity, chemical attraction, magnetism, and 

 mechanical force all stand in a certain relation to each other, 

 and are susceptible of conversion into each other. No one, 

 however, has succeeded in depriving any body of its gravity, 

 and thereby causing the gravitation force to assume a new 

 form. It may be an ultimate and permanent property of 

 matter, or what seems more probable, a property of matter 

 under certain conditions ; but we have no idea how it is 

 correlated to other forces which matter exhibits. We have 

 reason to believe that it is propagated in all directions like 

 light, and Newton showed that bodies gravitate towards each 

 other as if their matter were concentrated in one central point, 

 corresponding with what is known as their centre of gravity. 



A luminous body emits light, whether or not there is any- 

 thing which it can illuminate ; but we only know of gravitation 

 as a reciprocal action of two bodies upon each other. When a 

 man jumps he kicks the earthball from him with a force propor- 

 tioned to his own rebound, and if the earth were a little thing, 

 its position would be displaced. As it is, the displacement, 

 though existing theoretically, is too small for appreciation or 

 for computation in intelligible figures. In like manner, when 



