132 TERRESTRIAL ADAPTATIONS. 



ferent sounds which compose a concert, without 

 attributing them to different parts of space. 



The peculiar property thus belonging to vision, 

 of perceiving position, is so essential to us, that 

 we may readily believe that some particular 

 provision has been made for its existence. The 

 remarkable mechanism of the eye (precisely re- 

 sembling that of a camera obscura,*) by which it 

 produces an image on the nervous web forming 

 its hinder part, seems to have this effect for its 

 main object. And this mechanism necessarily 

 supposes certain corresponding properties in light 

 itself, by means of which such an effect becomes 

 possible. 



The main properties of light which are con- 

 cerned in this arrangement, are reflexion and 

 refraction : reflexion, by which light is reflected 

 and scattered by all objects, and thus comes to 

 the eye from all : and refraction, by which its 

 course is bent, when it passes obliquely out of 

 one transparent medium into another ; and by 

 which, consequently, convex transparent sub- 

 stances, such as the cornea and humours of the 

 eye, possess the power of making the light con- 

 verge to a focus or point ; an assemblage of such 

 points forming the images on the retina, which 

 we have mentioned. 



Reflexion and refraction are therefore the es- 

 sential and indispensable properties of light ; and 

 so far as we can understand, it appears that it 

 was necessary that light should possess such 



