REFLECTION OF LIGHT. 



All ordinary surfaces are more or less rough. The light which falls upon 

 them is irregularly reflected by them ; each point upon them being illuminated, 

 disperses the light which strikes upon it in every direction around it, and it 

 is thus that the point itself becomes visible to an eye placed anywhere within 

 view of it. The surfaces of bodies in general are by this means seen from 

 every quarter around. 



But as the light of the sun is of one uniform color and quality, it will be 

 asked how it happens that the surfaces of different bodies and different parts 

 of the surface of the same body produce different effects upon vision, appear- 

 ing to have a variety of colors and tints of colors. If they reflect to the eye 

 no light except that which falls upon them, and if that which falls upon them 

 be all of a uniform quality, how, it may be asked, does it happen that the 

 light reflected by different surfaces impresses the eye with the perception of 

 different colors ? In answer to this it is necessary to explain that although 

 the light of the sun is, in a certain sense, of a uniform quality and color, it is 

 nevertheless not simple and homogeneous ; it is, in fact, a compound principle, 

 produced by the mixture of lights of different colors in different proportions. 

 It is this mixture which produces the white light of the sun. 



Now, the surfaces of opaque bodies are endowed with various properties of 

 reflecting light. Some possess the virtue of reflecting light, of one color, while 

 they absorb or extinguish light of another. One, for example, will have a 

 strong power of reflecting red light, but will be altogether incapable of reflect- 

 ing blue light ; in short, various surfaces have infinitely various powers of 

 reflecting lights of different colors. 



Why, then, does one opaque object appear to the eye red, while another 

 appears blue ? Because in the compound light of the sun, which equally falls 

 on both of these objects, there is contained both red and blue light ; the sur- 

 face of the object which appears red absorbs or extinguishes all the elements 

 of the solar light except the red rays which it reflects ; and the object, which 

 appears blue, on the other hand, absorbs all the elements of the solar light ex- 

 cept the blue rays, which alone are reflected by it. 



Thus it appears that all objects, Avhether natural or artificial, derive their 

 peculiar tints of color from the property which they possess of decomposing 

 solar light. Such elementary colors as they have the power of reflecting blend- 

 ed together produce the peculiar tints which characterize them, the other con- 

 stituents of the solar light being stopped. 



But besides the colors presented by visible objects, they exhibit various de- 

 grees of illumination, or, what is familiarly called, various degrees of light and 

 shade. Ttois arises from the more or less favorable position which different 

 parts of their surfaces have with respect to the light which falls upon them, and 

 it is by this means that the form and shape of bodies are perceivable by the 

 eye. ' 



Buf if the surface of an opaque body, instead of being more or less rough, 

 so as to render each of its points separately a centre of reflected light, could 

 be rendered perfectly smooth and polished, then the light would not be re- 

 flected from it in the manner now described. The various points upon it would 

 not then become centres from which light would be dispersed in every direc- 

 tion ; on the contrary, the rays of light falling on such a surface would be re- 

 flected by peculiar laws. 



REFLECTION AT PLANE SURFACES. 



Let us suppose that A B,fig. l,is such a surface, and that a ray of light proceed- 

 ing from the sun at S illuminates a point I, placed upon this surface. In the 



