232 THE PRINCIPLES OF SCIENCE. 



ghens 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.' 



The supremacy of Newton's mistaken corpuscular doc- 

 trine 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 the Philosophic Mind. 



Perfect readiness to reject a theory inconsistent with 

 fact is, then, a primary requisite of the philosophical mind. 

 But it would be a mistake to suppose that this candour 

 has anything akin to fickleness ; on the contrary, readiness 

 to reject a false theory may be combined with 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 under-estimate or pass over the unwelcome results of 

 experiment. There must be that scrupulous honesty and 

 flexibility of mind, which assigns an adequate value to all 



