viii AUTHOR'S PREFACE. 



some amount of repetition has been unavoidable, and the 

 first four may perhaps seem somewhat confusedly thrown 

 together. If I may claim that they have any leading 

 thought, it would be that I have endeavoured to illustrate 

 the essence and the import of Natural laws, and their 

 relation to the mental activity of man. This seems to me 

 the chief interest and the chief need in Lectures before a 

 public whose education has been mainly literary. 



I have but little to remark with reference to individual 

 Lectures. The set of Lectures which treat of the 

 Theory of Vision have been already published in the 

 * Preussische Jahrbiicher,' and have acquired, therefore, 

 more of the character of Eeview articles. As it was 

 possible in this second reprint to render many points 

 clearer by illustrations, I have introduced a number of 

 woodcuts, and inserted in the text the necessary explan- 

 ations. A few other small alterations have originated in 

 my having availed myself of the results of new series of 

 experiments. 



The fifth Lecture, on the Interaction of Natural 

 Forces, originally published sixteen years ago, could not 

 be left entirely unaltered in this reprint Yet the alter- 

 ations have been as slight as possible, and have merely 

 been such as have become necessary by new experimental 

 facts, which partly confirm the statements originally made, 

 and partly modify them. 



The seventh Lecture, on the Conservation of Force, 

 developes still further a portion of the fifth. Its main 

 object is to elucidate the cardinal physical ideas of work, 

 and of its unalterability. The applications and conse- 



