34 FRAGMENTS OF SCIENCE. 



humours are filled with it; hence the waves generated 

 by the glowing platinum can cross these humours and 

 impinge on the optic nerve at the back of the eye.* 

 Thus the sensation of light reduces itself to the ac- 

 ceptance of motion. Up to this point we deal with 

 pure mechanics; but the subsequent translation of the 

 shock of the ethereal waves into consciousness eludes 

 mechanical science. As an oar dipping into the Cam 

 generates systems of waves, which, speeding from the 

 centre of disturbance, finally stir the sedges on the 

 river's bank, so do the vibrating atoms generate in the 

 surrounding ether undulations, which finally stir the 

 filaments of the retina. The motion thus imparted 

 is transmitted with measurable, and not very great 

 velocity to the brain, where, by a process which the 

 science of mechanics does not even tend to unravel, the 

 tremor of the nervous matter is converted into the con- 

 scious impression of light. 



Darkness might then be defined as ether at rest; 

 light as ether in motion. But in reality the ether is 

 never at rest, for in the absence of light-waves we have 

 heat-waves always speeding through it. In the spaces 

 of the universe both classes of undulations incessantly 

 commingle. Here the waves issuing from uncounted 

 centres cross, coincide, oppose, and pass through each 

 other, without confusion or ultimate extinction. Every 

 star is seen across the entanglement of wave-motions 

 produced by all other stars. It is the ceaseless thrill 

 caused by those distant orbs collectively in the ether, 

 tha.t constitutes what we call the * temperature of 

 space.' As the air of a room accommodates'itself to the 

 requirements of an orchestra, transmitting each vibra- 



* The action here described is analogous to the passage of 

 sound-waves through thick felt whose interstices are occupied 

 by air. 



