io6 GREAT MEN OF SCIENCE 



number of matters which he regards as worthy of considera- 

 tion. 



In spite of these facts, we nevertheless find all too often 

 even to-day, and even in text books of optics, the description 

 as an historical event of importance, of a controversy between 

 the 'emission or emanation theory or hypothesis of Newton' 

 and the 'undulatory or vibration theory or hypothesis of 

 Huygens'; this must be referred to the feeble mental grasp 

 of Newton's and Huygens' contemporaries, and also to the 

 carelessness of their successors, who for the most part became 

 acquainted with these great men at secondhand, along with 

 polemical statements introduced by others. For Huygens 

 likewise attempted simply to bring facts together, as many as 

 possible, which appeared likely to lead to an insight into the 

 nature of light, and this is also quite generally the manner 

 in which our knowledge of nature gradually comes into being. 

 It has never been furthered by mere opposition of different 

 suppositions, but always by discovery of new facts by means 

 of suitable experiments and observations. It is true, how- 

 ever, that narrow minds are not able to keep the living 

 facts in front of them, and are also unable to regard science 

 as something which is continually progressing; hence for 

 their own part they feel the need for schematising what is 

 already known, with the result that it too easily becomes 

 fixed in a rigid and onesided manner. They should not 

 ascribe this mode of thought to the great scientists, who so 

 obviously did not possess it, and they should not obscure the 

 essential in the progress of knowledge by emphasising 

 controversial questions; these have always been fruitless, 

 since they amount to an 'either - or' {tertium non datur), 

 which has nothing to do with nature. 



The case of light, as the result of new and in part fairly 

 recent discoveries, has proved to be an excellent illustration 

 of this fact. Light is certainly a wave motion in the ether, 

 as Huygens already imagined - this however also includes the 



