508 NOTES TO BOOK IX. 



this " without making any new physical hypotheses ;" for 

 the transverse direction of vibrations, the different optical 

 elasticities of crystals in different directions, and (if it 

 be adopted) the hypothesis of finite intervals of the par- 

 ticles (see p. 454), are only limitations of what was inde- 

 finite in the earlier form of the hypothesis. And so far 

 as the properties of visible radiant light are concerned, 

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

 has said, (see p. 452,) that "the undulation theory ac- 

 counts for the phenomena as completely 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 as I have shown, in the (Notes to Book 

 vii.), such unexplained facts still remain in the solar system. 

 With regard to the undulatory theory, these exceptions 

 are, I think, disappearing quite as rapidly and as com- 

 pletely as in the case of gravitation. It is to be ob- 

 served 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 shew 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 labour, any more than 

 those of gravitation. 



This remark applies to such cases as that of the trans- 

 verse fringes of grooved surfaces. The general phenomena 

 of these cases are perfectly explained by the theory. There 

 is an interruption in the light in an oblique direction, 

 which has not yet been explained ; but looking at what 



