PRESENT PROBLEMS 101 



believed. In arriving at our present ideas of the mechanism through 

 which matter reacts on matter, we have not reached them by finding 

 that the old ideas must be renounced, in order to explain some new 

 phenomenon which is apparently out of harmony with the explan- 

 ation previously made. It is rather that each new development has 

 confirmed what had gone before, has made it seem more reasonable, 

 and has filled in some gap in the knowledge of the past. The ether, 

 which only a few years since was assumed to exist because it seemed 

 to be necessary, has become more and more centrally important, 

 and has finally come to monopolize most of the attention of those 

 who would seek to understand matter. It is no reproach to modern 

 ideas concerning the physics of matter, that they are complex. The 

 fact that they are also harmonious and beautiful, and that they 

 furnish an explanation of why a mass of matter has inertia, and pro- 

 mise the explanation of other long-standing puzzles, converts the 

 accusation of complexity into a crowning glory. 



