EPOCH OF YOUXG AXD FRESXEL. 100 



intervals of the particles (see chap. x. and hereafter, chap, xiii.), are 

 only limitations of what was indefinite in the earlier form of the hypo- 

 thesis. And so far as the properties of visible radiant light are con- 

 cerned, I do not think it at all too much to say, as M. Schwerd has 

 said, that " the undulation theory accounts for the phenomena as com- 

 pletely as the theory of gravitation does for the facts of the solar system." 



This we might say, even if some facts were not yet fully explained ; 

 for there were till very lately, if there are not still, such unexplained 

 facts with regard to the theory of gravitation, presented to us by the 

 solar system. With regard to the undulatory theory, these exceptions 

 are, I think, disappearing quite as rapidly and as completely as in the 

 case of gravitation. It is to be observed that no presumption against 

 the theory can with any show of reason be collected from the cases 

 in which classes of phenomena remain unexplained, the theory having 

 never been applied to them by any mathematician capable of tracing- 

 its results correctly. The history of the theory of gravitation may 

 show us abundantly how necessary it is to bear in mind this caution ; 

 and the results of the undulatory theory cannot be traced without great 

 mathematical skill and great labor, any more than those of gravitation. 



This remark applies to such cases as that of the transverse fringes 

 of grooved surfaces. The general phenomena of these cases are per- 

 fectly explained by the theory. But there is an interruption in the 

 light in an oblique direction, which has not yet been explained ; but 

 looking at what has been done in other cases, it is impossible to doubt 

 that this phenomenon depends upon the results of certain integrations, 

 and would be explained if these were rightly performed. 



The phenomena of crystallized surfaces, and especially their effects 

 upon the plane of polarization, w r ere examined by Sir D. Brewster, 

 and laws of the phenomena made out by him with his usual skill 

 and sagacity. For a time these were imexplained by the theory. 

 But recently Mr. Mac Cullagh has traced the consequences of the 

 theory in this case, 21 and obtained a law which represents with much 

 exactness, Sir D. Brewster' s observation. 



The phenomena which Sir D. Brewster, in 1837, called a new pro- 

 perty of light, (certain appearances of the spectrum when the pupil 

 of the eye is half covered with a thin glass or crystal,) have been ex- 

 plained by Mr. Airy in the Phil. Trans, for 1840. 



Mr. Airy's explanation of the phenomena termed by Sir D. Brew 



' J1 Prof. Lloyd's Report, Brit. Assoc. 183i, p. 374. 



