28 LIGHT. 



reflects particles of light, or vibrations, from every point 

 of its surface, which move in straight lines and in all di- 

 rections, and which by their action on the eye cause the 

 body to be seen. The quantity of the rays of light re- 

 flected depends upon the nature of the body, the state 

 and colour of the surface, and the angle of incidence. 

 Under all these circumstances, the angle of reflection is 

 equal to the angle of incidence. Of the various bodies 

 which reflect light, metals possess this property in the 

 greatest degree, and the lightest colours reflect the most. 

 The difference in the two extremes, white and black, are 

 rendered evident by laying a slip of white cloth, and 

 another of black, on the snow in the sunshine ; the black 

 will be found to sink soon into the snow, by absorbing 

 the light, and with it the heat ; while the white piece 

 will retain its place on the surface of the snow. While the 

 science of Dioptrics depends on the refraction of light, and 

 explains the construction of telescopes, microscopes, &c., 

 that of Catoptrics depends on the reflection of light, and 

 explains its laws and. properties as reflected from mirrors 

 or specula of all kinds, whether plain, concave, or convex. 



It has been already observed that bodies are seen by 

 the reflection of the rays of light from their surfaces ; but 

 the contrivance by which the idea of the body seen is 

 conveyed to the mind, so as to form a correct judgment 

 of it, is at once most wonderful and most effective. The 

 Eye is the instrument for this purpose, and it is composed 

 of such parts as best to answer the object for which it is 

 designed. 



Vision, or sight, is performed by rays of light reflected 

 from the body seen passing through the humours of the 

 eye, and painting the object on the retina at the back of 

 the eye, whence it is conveyed by the optic nerve to the 

 brain. The eye must for this purpose necessarily be of 

 a convex figure, and of such a degree of convexity as the 

 refractive powers of the several humours demand for 

 forming the image at the given focal distance. The ex- 

 ternal part of the eye-ball is called the cornea, or horny 

 coat,from its resemblance to a piece of transparent horn. 

 Immediately behind this coat, there is a fine, clear hu- 

 mour, which from its similarity to water, is called the 



