LIGHT. 133 



properties, in order that it might form a medium 

 of communication between man and the external 

 world. We may consider its power of passing 

 through transparent media (as air) to be given 

 in order that it may enlighten the earth ; its 

 affection of reflexion, for the purpose of making- 

 colours visible ; and its refraction to be bestowed, 

 that it may enable us to discriminate figure and 

 position, by means of the lenses of the eye. 



In this manner light may be considered as 

 constituted with a peculiar reference to the eyes 

 of animals, and its leading properties may be 

 looked upon as contrivances or adaptations to fit 

 it for its visual office. And in such a point of 

 view the perfection of the contrivance or adapta- 

 tion must be allowed to be very remarkable. 



3. But besides the properties of reflexion and 

 refraction, the most obvious laws of light, an 

 extraordinary variety of phenomena have lately 

 been discovered, regulated by other laws of the 

 most curious kind, uniting great complexity with 

 great symmetry. We refer to the phenomena of 

 diffraction, polarization, and periodical colours, 

 produced by crystals and by thin plates. We 

 have, in these facts, a vast mass of properties and 

 laws, offering a subject of study which has been 

 pursued with eminent skill and intelligence. But 

 these properties and laws, so far as has yet been 

 discovered, exert no agency whatever, and have 

 no purpose, in the general economy of nature. 

 Beams of light polarised in contrary directions 



