268 THEORY OF LIGHT. 



almost infinite variety of colour, yet still within that range which is 

 agreeable and soothing to the eye, as well as consonant to our feelings ! 

 The human countenance, too, although capable of exciting our warmest 

 sympathies by form and motion alone, has that beauty perfected by colour, 

 varying under the influence of emotion. 



It remains, in order that we may apply these facts to the explanation 

 of the structure of the eye, to show how the rays proceeding from a body 

 and falling upon a convex glass suffer refraction. The ray that strikes 

 upon the centre, being perpendicular to the glass, passes on undeviatingly. 

 But each ray as it strikes a point removed from the centre must 

 impinge with more obliquity, in consequence of the curved surface; and 

 as the refraction of all the rays will be in proportion to the obliquity of 

 their incidence, they will converge towards the central direct ray. 



THE EYE COMPARED WITH OPTICAL INSTRUMENTS. 



It is surprising that the structure of an animal body should so seldom 

 be taken as a model. In the history of inventions, it appears quite 

 extraordinary that the telescope and the microscope should be modern, 

 when, as it should seem, the fine transparent convexity of the eye might 

 have given rise to imitation, as soon as man learned to give shape to 

 natural or artificial glass. It reminds us of the observation of Locke, in 

 speaking of a discovery, that it proved the world to be of no great 

 antiquity. Yet we must estimate the invention of the telescope and 

 microscope as by far the most important in their consequences of either 

 ancient or modern discoveries. The first opens to us an unlimited 

 expanse, not only of new worlds, but systems%f worlds, and new laws 

 evinced in the forces which propel and attract these; since in the 

 heavenly bodies we find no material contact, nor pressure, nor impulse, 

 nor transfer of power nor effect of heat, nor expansion of gases nothing, 

 in short, which can be illustrated by mechanism. By the microscope, 

 we contemplate the minute structure of animals and things but for its 

 aid invisible: the balance of the cohesive and repulsive force as they 

 order the changes in the material of the world, and in that of our own 

 frames. Yet these instruments are not in contrast with the eye; but 

 through the comparison of them we discover the wonderful adapta- 

 tions of that organ; of which it has long ago been said, that it can at one 

 time extend our contemplations to the heavenly bodies and their 

 revolutions, and at another limit its exercise to things at hand, to the 

 sympathies and affections of our nature visible in the countenance. If 



