88 LIGHT. 



CHAPTER II. 



LIGHT. 



THE mechanical properties of light constitute the science of 

 optics,, and belong, therefore, to physics, and not to chemistry. 

 But it may be useful, by a short recapitulation, to recal them to 

 the memory of the reader. 



1 . The rays of light emanate with so great velocity from the 

 sun, that they occupy only 74 minutes in traversing the immense 

 space which separates the earth from that luminary. They travel at 

 the rate of 192,500 miles in a second, and would, therefore, move 

 through a space equal to the circumference of our globe in I -8th 

 of a second. They are propagated continually in straight lines, 

 and spread or diverge at the same time ; so that their density 

 diminishes in the direct proportion of the square of their dis- 

 tance from the sun. Hence, if the earth were at double its 

 present distance from the sun, it would receive only one-fourth 

 of the light ; at three times its present distance, one- ninth ; at 

 four times its present distance, one-sixteenth, &c. 



2. When the solar rays impinge upon a body, they are re- 

 flected from its surface, and bound off as an elastic ball, striking 

 against the same surface in the same direction, would do ; or 

 they are absorbed by the body upon which they fall, and disap- 

 pear, being extinguished ; or lastly, they pass through the body, 

 which in that case is transparent or diaphanous. In the first 

 case, the body becomes visible, appearing white, or of some 

 particular colour, and we see it in the direction in which the 

 rays reach the eye. In the second case, the body is invisible, 

 no light proceeding from it to the eye ; or it appears black, if 

 the surrounding objects are illuminated. In the third case, if 

 the body be absolutely transparent, it is invisible, and we see 

 through it the object from which the light was last reflected. 

 But light is often greatly affected in passing through transparent 

 bodies. 



3. If light enters such media, of uniform density, perpendi- 

 cularly to their surface, its direction is not altered ; but in pass- 



