434 



LECTURE XXXVIL 



ON PHYSICAL OPTICS. 



Having examined the general theory of optics, and the construction 

 of optical instruments, we are now to consider those properties and affectiots 

 of Hght, which rather belong to its natural history, than to its mechanical 

 effects; to trace its relations to the particular phenomena of nature; to in- 

 vestigate the manner in which it is connected with our sensations, and to 

 inquire on what intimate mode of action the various effects of light depend. 

 All these subjects may be properly comprehended under the denomination 

 of physical optics, but we shall find it convenient to reserve each of the 

 two last for a separate examination. The sources of light, the velocity of 

 its motion, its interception and extinction, its dispersion into different 

 colours ; the manner in which it is affected by the variable density of the at- 

 mosphere, the meteorological appearances in which it is concerned, and the 

 singular properties of particular substances with regard to it, will be the first 

 subjects of our investigation. 



The sources, from which light is commonly derived, are either the sun or 

 stars, or such terrestrial bodies as are undergoing those changes which consti- 

 tute combustion. The process of combustion implies a change in which a 

 considerable emission of light and heat is produced; but it is not capable of 

 a very correct definition: in general it requires an absorption, or at least a 

 transfer, of a portion of oxygen; but there appear to be some exceptions 

 to the universality of this distinction; and it has been observed that both 

 heat and light are often produced where no transfer of oxygen takes place, 

 and sometimes by the effect of a mixture which cannot be called combus- 

 tion. 



Light is also afforded, without any sensible heat, by a number of vegetable 



