417 



CHAPTER VIII. 



ATTEMPTS TO DISCOVER THE LAWS OP OTHER 

 PHENOMENA. 



THE phenomena which result from optical com- 

 binations, even of a comparatively simple na- 

 ture, are extremely complex. The theory accounts 

 for these results with the most curious exactness, 

 and points out the laws which pervade the apparent 

 confusion ; but without this key to the appearances, 

 it was scarcely possible that any rule or order 

 should be detected. The undertaking was of the 

 same kind as it would have been, to discover all the 

 inequalities of the moon's motion without the aid of 

 the doctrine of gravity. We will enumerate some of 

 the phenomena which thus employed and perplexed 

 the cultivators of optics. 



The fringes of shadows were one of the most 

 curious and noted of such classes of facts. These 

 were first remarked by Grimaldi 1 , (1665,) and re- 

 ferred by him to a property of light which he called 

 Diffraction. When shadows are made in a dark 

 room, by light admitted through a very small hole, 

 these appearances are very conspicuous and beau- 

 tiful. Hooke, in 1672, communicated similar obser- 



1 Physico-Mathesis, de Lumine, Coloribus el Iride. Bologna, 

 1665. 



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