20 CROONIAN LECTURES 



by their discoveries regarding light. Dr. Whe- 

 well says, "By their ideas on the inter- 

 ference of undulations, of double refraction (or 

 the passage of undulations through substances 

 in which the resistance to the undulations is 

 different in different directions). By their ideas 

 of transverse vibration or polarisation and di- 

 polarisation, they did away with the most 

 material part of Newton's ideas on the nature 

 of light, and now the only vestige that remains 

 is the luminiferous aether, the imponderable 

 material substance which is still assumed to 

 carry the motions which constitute light." 



As the question of the existence of this lumi- 

 niferous aether is of great interest in connection 

 with our ideas of matter and force, I must 

 bring before you as shortly as possible the 

 opposite views that now exist regarding it, 

 whether it be regarded simply as the carrier, 

 or as essential to the origination of that form 

 of motion called light. 



The Eev. T. E. Birks, in his work on matter 

 and aether, published in 1862, most powerfully 

 defends its existence and defines its properties. 

 He assumes that it exercises a strong repulsive 

 power among its own particles, and a strong 



