296 NATURAL THEOLOGY. 



sudden angle, as if broken — and this is refraction. 

 Two circumstances, therefore, influence the ray 

 of Hght ; — the angle at which it falls, and the den- 

 sity of the body into which it passes. When the 

 ray b passes from the denser medium into the 

 rarer, it is again refracted, but away from the per- 

 pendicular, and takes its original course, provided 

 the surface at which it goes out is parallel to the 

 surface at which it entered. 



When a ray strikes upon a body that is not 

 transparent, or only imperfectly so, it is in part 

 reflected, that is, struck off' again, bent back, or 

 reflected, and enters the eye, conveying to us the 

 impression of the form and colour of that object. 



But the expression which we have used requires 

 explanation ; for how is it that the reflected rays 

 should convey the idea of colour ? 



The prism is a piece of glass so formed that the 

 rays must fall obliquely on one or both of the sur- 

 faces, and suffer refraction. Thus the ray a strik- 

 ing into the prism b is refracted ; but all its parts 

 are not equally refracted, and as the light consists 

 of parts differing in colours, and which are differ- 

 ently refracted, it is divided or dissected into se- 



