NOTES TO BOOK IX. 509 



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

 that this phenomena 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, were examined 

 by Sir D. Brewster, and laws of the phenomena made 

 out by him with his usual skill and sagacity. For a 

 time these remained unexplained by the theory. But 

 recently Mr. MacCullagh has traced the consequences of 

 the theory in this case, and obtained a law which repre- 

 sents with much exactness, Sir D. Brewster's observation. 

 (Prof. Lloyd's Report, Brit. Assoc. 1834, p. 374.) 



The phenomenon which Sir D. Brewster, in 1837, 

 called a new property of light, (certain appearances of 

 the spectrum when the pupil of the eye is half covered 

 with a thin glass or crystal,) have been explained by 

 Mr. Airy in the Phil Trans, for 1840. 



I may remark also that there is no force in the objec- 

 tion which has been urged against the admirers of the 

 undulatory theory, that by the fulness of their assent to 

 it, they discourage further researches which may contra- 

 dict or confirm it. We must, in this point of view also, 

 look at the course of the theory of gravitation and its 

 results. The acceptance of that theory did not prevent 

 mathematicians and observers from attending to the ap- 

 parent exceptions, but on the contrary, stimulated them 

 to calculate and to observe with additional zeal, and still 

 does so. The acceleration of the Moon, the mutual dis- 

 turbances of Jupiter and Saturn, the motions of Jupiter's 

 Satellites, the effect of the Earth's oblateness on the 

 Moon's motion, the motions of the Moon about her own 



