Apeil 8, 1910] 



SCIENCE 



531 



dom, if ever, been surpassed in the history 

 of physics. 



But leaving matter aside, and consider- 

 ing only the ether, what is the net result? 

 Practically this, that electromagnetic dis- 

 turbances, including light waves, are propa- 

 gated through space with a, speed of 300 

 million meters per second. This, I con- 

 ceive to be the criticism which every sound 

 metaphysician, but only some sound physi- 

 cists, would pass upon our present knowl- 

 edge of the ether. This is the one fact 

 concerning the ether which we know in the 

 same sense in which we are said to "know" 

 the ordinary everyday facts of physics. 



In conclusion, and still dealing with 

 limitations, I beg to offer for your consid- 

 eration a definition {i. e., a delimitation) of 

 physics recently given to me by an eminent 

 metaphysician. 



Last summer I had the pleasure of sev- 

 eral times meeting Professor Miinsterberg ; 

 and on one of these occasions I took the 

 liberty of submitting to him, for criticism, 

 a definition of physics, which I myself had 

 formulated. Following is his definition 

 of the physics of to-day which he, in re- 

 turn, submitted to me and which is, I 

 am inclined to think, unsurpassed in point 

 of accuracy, clearness and completeness: 



Physics deals with changes in the world of over- 

 individual objects, in so far as they are not 

 changes of composition. It consists of those 

 judgments which have proved themselves by trial 

 to determine most accurately our justified expecta- 

 tions concerning these changes. In dealing with 

 objects it separates itself from the knowledge of 

 will-acts; in dealing with over-individtuil objects 

 it separates itself from psychology; in abstract- 

 ing from changes of composition, it separates itself 

 from chemistry. The over-individual objects may 

 be matter or ether or electrons. 



CONCLUSION 

 The view of physics here presented is 

 that of a half truth or partial truth. But 

 this is very far from saying it is an un- 



truth. The essential point— the only es- 

 sential point— is for us to recognize the 

 facts; to know ourselves; to admit our 

 limitations. Then the more nearly we re- 

 main inside these limitations, and avoid 

 ' ' the den of the metaphysician, ' ' the better. 



That flexibility of mind which it is de- 

 sirable to secure by not translating every 

 temporary opinion into a hard-and-fast 

 fact of nature is well illustrated by a re- 

 cent remark of Professor Schuster^^ who is 

 himself one of the small group of men who 

 have established the pulse theory of white 

 light. "These two representations of 

 white light (by homogeneous waves and by 

 impulses) are," he says, "not mutually 

 exclusive: They represent two points of 

 view, and we may adopt either one or the 

 other in different problems according to 

 our convenience. ' ' 



Less fixity and more flexibility in our 

 views concerning the ether might, for in- 

 stance, permit a more cordial consideration 

 of Professor Osborne Reynolds's theory of 

 gravitation which, so far as I understand 

 it, has much to recommend it. 



Lest what I said at the outset concerning 

 the experimental side of physics should be 

 forgotten, let me, in justice to myself, re- 

 mind you once more of my attitude toward 

 the experimentalist, towards that group 

 which in Italy includes Galileo, Volta, Mel- 

 loni and Righi ; the skillful group which in- 

 cludes Oersted, Kirchhoff , Hertz, Roentgen ; 

 the French group of laboratory workers, 

 Mersenne, Presnel, Regnault, the Curies; 

 in England, Gilbert, Boyle, Joule, Ray- 

 leigh; and those dextrous men, our own 

 countrymen, Franklin, Henry, Rowland, 

 Michelson. Toward the experimentalist as 

 compared with the friendly critic and re- 

 viewer, my feeling is precisely that of Lin- 

 coln toward the soldiers who fought at 

 Gettysburg. You all remember his sen- 



^Phil. Mag., (6), 18, 767 (1909). 



