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LECTURE XXXV. 



ON THE THEORY OF OPTICS. 



THE science of optics is one of the most elegant, and the most important 

 branches of natural and mechanical philosophy. It presents us with 

 experiments attractive by their beauty and variety, with investigations 

 affording an ample scope for mathematical refinements, and with instru- 

 ments of extensive utility both in the pursuit of other sciences, and in 

 the common employments 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 con- 

 clusions immediately deducible from those- properties ; and next, the ap- 

 plication of these laws to practical purposes, in the construction of optical 

 instruments. We shall afterwards proceed to examine the more compli- 

 cated 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 ex- 

 planation 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 of 

 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 

 define light from its sensible qualities. The sensation of light is sometimes 

 produced by external pressure on the eye ; we must exclude this sensation 

 from the definition of light, and must therefore call light an influence 

 capable of entering the eye, and of affecting it with a sense of vision. A 

 body, from which this influence appears to originate, is called a luminous 

 body. We do not include in this definition of the term light the invisible 

 influences which occasion heat only, or blacken the salts of silver, although 

 they both appear to differ from light in no other respects than as one kind 

 of light differs from another ; and they might probably have served the 

 purpose of light, if our organs had been differently constituted. 



A ray of light is considered as an infinitely narrow portion of a stream 

 of light, and a pencil as a small detached stream, composed of a collection 

 of such rays accompanying each other. As a mathematical line is some- 

 times conceived to be described by the motion of a mathematical point, so 

 a ray of light may be imagined to be described by the motion of a point 

 of light. We cannot exhibit to the senses a single mathematical lino, 

 except as the boundary of two surfaces ; in the same manner, we cannot 



