130 TERRESTRIAL ADAPTATIONS. 



calculations founded upon the theory, which, 

 indicating new and untried facts, are found to 

 agree exactly with experiment. 



We cannot here introduce a sketch of the pro- 

 gress by which the phenomena have thus led to 

 the acceptance of the theory of undulations. But 

 this theory appears to have such claims to our 

 assent, that the views which we have to offer 

 with regard to the design exercised in the adap- 

 tation of light to its purposes, will depend on the 

 undulatory theory, so far as they depend on 

 theory at all.* 



2. The impressions of sight, like those of hear- 

 ing, differ in intensity and in kind. Brightness 

 and Colour are the principal differences among 

 visible things, as loudness and pitch are among 

 sounds. But there is a singular distinction be- 

 tween these senses in one respect : every object 

 and part of an object seen, is necessarily and 

 inevitably referred to some position in the space 

 before us ; and hence visible things have place, 

 magnitude, form, as well as light, shade, and 

 colour. There is nothing analogous to this in the 

 sense of hearing ; for though we can, in some 

 approximate degree, guess the situation of the 

 point from which a sound proceeds, this is a 

 secondary process, distinguishable from the per- 



* The reader who is acquainted with the two theories of light, 

 will perceive that though we have adopted the doctrine of the 

 ether, the greater part of the arguments adduced would be equally 

 forcible, if expressed in the language of the theory of emission. 



