238 FKESNEL. 



However, the system of emission has few partisans ; 

 but it is not under the blows dealt by Euler that it has 



on single points ; to uphold a solitary experimental fact as decisive one 

 way or the other. A single favourable fact will not prove the theory ; 

 and, on the other hand, the only real conclusion in cases where a 

 single fact appears to stand out as an objection is, that (granting the 

 fact incapable of being otherwise interpreted) the theory requires re- 

 modelling; and that some undue assumption has crept into it. Such 

 reconstruction has always been the process by which it has been suc- 

 cessively fouud to adapt itself to nev/ phenomena, even when at first 

 sight they appeared most opposed to it. But even were it otherwise, 

 the theory is one which is not to be staked on single facts; it rests its 

 claim (in the first instance) in being that which connects by a common 

 principle, and tlms explains the greatest number of facts. Many of the 

 old theories, as of inflexion, attractions, &c., each explained a certain 

 small number of facts; but the real argument against them was, that 

 they did not explain each other. Every new partial explanation of 

 the wave theory, on the contrary, not only explains a certain class of 

 facts, but connects these with some other class similarly explained. 

 Newton had proposed one idea (that of fits of easy reflexion and trans- 

 mission) to account for the altei'nations in the colours of thin plates; 

 another totally unconnected theorj"- of inflexion, or bending in and 

 out in passing the edge of a body, to explain the phenomena of dif- 

 fraction: a third idea of polarity, for double refraction; besides other 

 occasional references to waves, or even a combination of vibrations 

 with molecular emission in some cases; but all unconnected with, and 

 indejjendeni of, each other, and each confessedly a mere arbitrary as- 

 sumption, not pretending to stand on any other ground than that it 

 explained in a certain way the particular phenomenon in relation to 

 which it was adduced. 



On the undulatory view, on the contrary, every subordinate law 

 successively established, and every class of phenomena explained, 

 has become directly connected with all the others. Everj' part is in 

 intimate relation with every other part, and the progressive improve- 

 ment and enlargement of the theory has regularly kept pace with the 

 advance of experimental discovery; every new modification, as it 

 were, has grown out of the simple principles at first laid down by a 

 natural sequence, without anj' new hypothesis, or forced and arbi- 

 trary changes. It is a theory of which an eminent philosopher, by no 

 means unduly biased in its favour, and at a time when it had by no 

 means reached its present point of perfection, emphatically said, " It 



