IN THE LAST HALF-CENTURY. 35 



and the undulatory theory, which has 

 superseded the corpuscular theory and 

 has proved one of the most fertile of in- 

 struments of research, is based on the 

 hypothesis of the existence of an 'ether,' 

 the properties of which are defined in 

 propositions, some of which, to ordinary 

 apprehension, seem physical antinomies. 

 It sounds paradoxical to say that the 

 attainment of scientific truth has been 

 effected, to a great extent, by the help of 

 scientific errors. But the subject-matter 

 of physical science is furnished by obser- 

 vation, which cannot extend beyond the 

 limits of onr faculties ; while, even within 

 those limits, we cannot be certain that 

 any observation is absolutely exact and 

 exhaustive. Hence it follows that any 

 given generalisation from observation may 

 be true, within the limits of onr powers 

 of observation at a given time, and yet 

 turn out to be untrue, when those powers 

 of observation are directly or indirectly 



