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LECTURE XXXV. 



ON THE THEORY OV OPTICS. 



J. HE science of optics is one of the most elegant, and the most important 

 branches of natural and mechanical philosophy. It presents us with experi- 

 ments attractive by their beauty and variety, with investigations affording 

 an ample scope for mathematical refinementSj and with instruments of exten- 

 sive utility both in the pursuit of other sciences, and in the common em- 

 ployments of life; nor is there any department of the study of nature in 

 which an unprejudiced observer is more convincingly impressed with the 

 characteristic marks of the perfect works of a supremely intelligent Artist. 



We shall first consider the essential properties which we discover in light, 

 and which are the basis of our calculations, together with the conclusions 

 immediately deducible from those properties; and next, the application of 

 these laws to practical purposes, in the construction of optical instruments. 

 We shall afterwards proceed to examine the more complicated phenomena, 

 which are derived from the same laws, and which are observed as well in 

 natural as in artificial circumstances, constituting the subdivision of physical 

 optics. The description of the eye, and the explanation of the sense of 

 vision, by means of which all these effects are connected with the human 

 mind, is properly a continuation of the subject of physical optics: the intimate 

 nature of light will be the next subject of investigation, and a historical sketch 

 of the progress of the science pf optics will conclude the second part of this 

 course of lectures. 



In order to avoid all hypothesis in the beginning, it will be necessary to de- 

 fine light from its sensible qualities. The sensation of light is sometimes pro- 

 duced by external pressure on the eye; we mu>>t exclude this sensation from the 

 definitionof light, and must therefore call light an influence capable of entering 



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