ON MATTER, FORCE, AND MOTION IN SPACE. 55 



seeing that the modes of undulation, as regards both 

 time and space, are different for the different coloured 

 rays, and still more divergent in those which evoke 

 heat and chemical effects. Add to these conditions 

 the actual velocity of light (185,000 miles in a second), 

 the yet more inconceivable frequency of the oscillations 

 transmitting it, and the nature of these vibrations 

 transverse to the direction of impulse and then bring 

 the phenomena thus denoted to the conception of a 

 material medium evolving them through all time with- 

 out change or interruption, and we have this great 

 natural problem, the occupation of space, fairly before 

 us for contemplation. 



Still taking our illustrations from light, we have to 

 encounter the fact that this great agent in the universe 

 is transmitted to us, possessed of the same properties, 

 i.e., is transmitted to us in the same way, from innumer- 

 able stars, single or aggregated into groups or nebulae, 

 the light from each one propagated in all directions 

 through surrounding space. How is this endless inter- 

 mingling of oscillations in the medium of transmission 

 compatible with the individuality and clearness of the 

 images they severally convey to us ? Or, giving the 

 question its greatest generality, how do these undula- 

 tions, issuing, primary or reflected, from every luminous 

 body whatsoever, within or without our sphere, traverse 

 space in all directions without conflicting, so as to in- 

 termix and confuse sensible objects to the eye? The 

 undulations of sound offer some important analogies 

 here. But it is the higher prerogative of mathematics 

 to have faced these difficulties, and through methods 



