586 THE PRINCIPLES OF SCIENCE. [CHAP. 



Huyghens was not satisfied with an unverified theory. 

 He calculated what might be expected to happen when a 

 crystal of calc-spar was cut in various directions, and he 

 says : " I have examined in detail the properties of the 

 extraordinary refraction of this crystal, to see if each 

 phenomenon which is deduced from theory would agree 

 with what is really observed. And this being so, it is 

 no slight proof of the truth of our suppositions and prin- 

 ciples ; but what I am going to add here confirms them 

 still more wonderfully ; that is, the different modes of 

 cutting this crystal, in which the surfaces produced give 

 rise to refraction exactly such as they ought to be, and as 

 I had foreseen them, according to the preceding theory." 



Newton's mistaken corpuscular theory of light caused 

 the theories and experiments of Huyghens to be disregarded 

 for more than a century ; but it is not easy to imagine a 

 more beautiful or successful application of the true method 

 of inductive investigation, theory guiding experiment, and 

 yet wholly relying on experiment for confirmation. 



Candour and Courage of tlie Philosophic Mind. 



Perfect readiness to reject a theory inconsistent with 

 fact is a primary requisite of the philosophic mind. But it 

 would be a mistake to suppose that this candour has any- 

 thing akin to fickleness ; on the contrary, readiness to reject 

 a false theory may be combined witli a peculiar pertinacity 

 and courage in maintaining an hypothesis as long as its 

 falsity is not actually apparent. There must, indeed, be no 

 prejudice or bias distorting the mind, and causing it to pass 

 over the unwelcome results of experiment. There must be 

 that scrupulous honesty and flexibility of mind, which 

 assigns adequate value to all evidence ; indeed, the more a 

 man loves his theory, the more scrupulous should be his 

 attention to its faults. It is common in life to meet 

 with some theorist, who, by long cogitation over a single 

 theory, has allowed it to mould his mind, and render him 

 incapable of receiving anything but as a contribution to the 

 truth of his one theory. A narrow and intense course of 

 thought may sometimes lead to great results, but the adop- 

 tion of a wrong theory at the outset is in such a mind irre- 

 trievable. The man of one idea has but a single chance of 



