X PREFACE 



extent, the works of the men themselves. Indeed, it was 

 above all necessary to find out which investigators really be- 

 longed to the series of great men, by whose activities our 

 knowledge of nature has principally been developed and 

 increased. 



This study of the works of great investigators taught me 

 that they had frequently achieved much more than they 

 were usually given credit for. The richer the contents of 

 their work, the more of it appeared to have been forgotten in 

 course of time by the writers of histories and text books; or 

 rather, the credit tended to be given to others who had later 

 turned their attention to the subject and enlarged upon it 

 when it was no longer new. But there can be no doubt who 

 is the originator, when a piece of knowledge is found, quite 

 unexpectedly, already accurately stated, and is not to be 

 found closely anticipated in the work of predecessors. In 

 contrast to the work of investigators to whom the above re- 

 marks apply, there are others whose works disappoint us; 

 they prove to contain less than what is usually ascribed to 

 them. We find, when we are acquainted with their pre- 

 decessors, that their debt to these is very great; in other 

 cases they have done no more than make assertions without 

 having devoted serious attention to the subject; and these 

 assertions have not been forgotten for the simple reason that 

 others, by expending the necessary thought and trouble, 

 have shown that something similar is actually true. But 

 without the labours of these latter workers, the suppositions 

 of the former would have remained nothing more than sup- 

 positions, and not have become a real addition to our know- 

 ledge. It is astonishing how certain estimates in the history 

 of science, estimates which will not bear investigation for a 

 moment, have continued to be held, and have proved im- 

 possible to correct. I have tried, by reading the original 

 literature, to avoid these false estimates entirely. 



If we follow up in this way the sources of our knowledge, 



