OF NATURAL PHILOSOPHY. 253 



between them. These colours were examined by 

 Newton with a minuteness and care altogether un- 

 exampled in experimental philosophy at that time, 

 and with which few researches undertaken since 

 will bear to stand in competition. Their result was 

 a theory of a very singular nature, which he 

 grounded on an hypothesis of what he termed fits 

 of easy transmission and reflection ; and which sup- 

 posed each ray of light to pass in its progress 

 periodically through a succession of states such as 

 would alternately dispose it to penetrate or be 

 reflected back from the surface of a body on which 

 it might fall. The simplest way in which the reader 

 may conceive this hypothesis, is to regard every 

 particle of light as a sort of little magnet revolving 

 rapidly about its own centre while it advances in its 

 course, and thus alternately presenting its attractive 

 and repulsive pole, so that when it arrives at the 

 surface of a body with its repulsive pole foremost, 

 it is repelled and reflected ; and when the contrary, 

 attracted, so as to enter the surface. Newton, 

 however, very cautiously avoided announcing his 

 theory in this or any similar form, confining himself 

 entirely to general language. In consequence, it 

 has been confidently asserted by all his followers, 

 that the doctrine of fits of easy reflection and trans- 

 mission, as laid down by him, is substantially nothing 

 more than a statement of facts. Were it so, it is 

 clear that any other theory which should offer a 

 just account of the same phenomena must ultimately 

 involve and coincide with that of Newton. But 

 this, as we shall presently see, is not the case ; and 

 this instance ought to serve to make us extremely 



