538 THE PRINCIPLES OF SCIENCE. [CHAP. 



science. A full account of this discovery may be found in 

 several works, as for instance Herschel's Outlines of 

 Astronomy, and Grant's History of Physical Astronomy, 

 Chapters XII and XIII. 



Predictions in the Science of Light. 



Next after astronomy the science of physical optics has 

 furnished the most beautiful instances of the prophetic 

 power of correct theory. These cases are the more striking 

 because they proceed from the profound application of 

 mathematical analysis and show an insight into the myste- 

 rious workings of matter which is surprising to all, but 

 especially to those who are unable to comprehend the 

 methods of research employed. By its power of prevision 

 the truth of the undulatory theory of light has been con- 

 spicuously proved, and the contrast in this respect between 

 the undulatory and Corpuscular theories is remarkable. 

 Even Newton could get no aid from his corpuscular theory 

 in the invention of new experiments, and to his followers 

 who embraced that theory we owe little or nothing in the 

 science of light. Laplace did not derive from the theory a 

 single discovery. As Fresnel remarks : * 



" The assistance to be derived from a good theory is not 

 to be confined to the calculation of the forces when the 

 laws of the phenomena are known. There are certain 

 laws so complicated and so singular, that observation alone, 

 aided by analogy, could never lead to their discovery. To 

 divine these enigmas we must be guided by theoretical 

 ideas founded on a true hypothesis. The theory of lu- 

 minous vibrations presents this character, and these precious 

 advantages ; for to it we owe the discovery of optical laws 

 the most complicated and most difficult to divine." 



Physicists who embraced the corpuscular theory had 

 nothing but their own quickness of observation to rely 

 upon. Fresnel having once seized the conditions of the 

 true undulatory theory, as previously stated by Young, was 

 enabled by the mere manipulation of his mathematical 

 symbols to foresee many of the complicated phenomena of 

 light. Who could possibly suppose, that by stopping a 



1 Taylor's Scientific Memoirs, vol. v. p. 241. 



