238 FRESNEL. 
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 found to adapt itself to new 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 thus 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 alternations in the colours of thin plates; 
another totally unconnected theory 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 wnconnected with, and 
independent 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. Every 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 any 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 
in ogee a a a ae 
ts 
