viii AUTHORS PREFACE. 



Hence 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 treats 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 explanations. 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 alterations 

 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 con- 

 sequences of the law of the Conservation of Force are 

 comparatively more easy to grasp. They have in recent 



