34 FRAGMENTS OF SCIENCE. 



The aether suffers no rupture of continuity at the 

 surface of the eye, the inter-molecular spaces of the 

 various 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. 1 Thus the sensation of light reduces itself to 

 the acceptance of motion. Up to this point we deal 

 with pure mechanics ; but the subsequent translation 

 of the shock of the aethereal 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 aether 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 aether at rest ; 

 light as aether in motion. But in reality the aether 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 aether, 

 that constitutes what we call the ' temperature of space.' 

 As the air of a room accommodates itself to the require- 



1 The action here described is analogous to the passage of sound- 

 waves through thick felt whose interstices are occupied by air. 



