358 LEADING AMERICAN MEN OF SCIENCE 



tion of any point, however simple, if he found that one of his pupils 

 failed to understand it, even at a time when he refused an offer 

 of five dollars an hour for doing outside work. His customary 

 remark at the end of every demonstration was: "Is it proved?" 

 He was once asked what he meant by this, and explained that 

 different minds required different degrees and kinds of proof. 

 That this is true to a much greater extent than he realized is shown 

 by the fact that some students, not particularly apt in mathematics, 

 found his lectures difficult or impossible to follow. This was due 

 to his complete absorption in his subject and his failure to com- 

 prehend that others could not take as long steps as he could. The 

 tendency of his mind toward generalization, in which lay his unique 

 power, was shown by a remark frequently upon his lips, "The 

 whole is simpler than its parts." 



His lectures were conversational, though usually well prepared 

 and straightforward, never twice alike, so students often took what 

 was nominally the same course a second year. Occasionally if a 

 new idea occurred to him in the course of a lecture, he would for- 

 get his students and work it out on the blackboard. He carried 

 few notes to the class room, and those merely the chief formulas. 

 If he forgot a step in writing out a demonstration, he would stand 

 and softly whistle, occasionally darting nervously across the room 

 to fix the radiator. On leaving the class room he was apt to come 

 back two or three times to see if he had not forgotten something. 



His students were never neglected because they were few or 

 because they interrupted his research work. Except in the very 

 first years his courses occupied on an average of seven hours a 

 week, and they were always freshly prepared and original. Some- 

 times with a single student before him, in such a subject as the 

 electromagnetic theory, he would sit for two hours at a time in 

 his characteristic posture, his hands folded and forefingers 

 touching, developing his own system and extending it into new 

 fields. 



To those who were prepared for them, his lectures were most 

 inspiring on account of their clear and logical demonstrations, 

 their comprehensiveness and their pertinent and graphic illustra- 



