LIGHT. 207 



Light, then, proceeding from a luminous body, impinges on the sub- 

 stances that are within its sphere ; and these, by reflecting the whole 

 or a part of it to the eye, become visible to us. In its course, direct 

 or reflected, its velocity is almost inconceivable. From observations 

 made on the eclipses of Jupiter's satellites, by Homer, Cassini, and 

 other astronomers, it has been calculated, that the light of the sun is 

 eight minutes and thirteen seconds in its passage from that luminary to 

 the earth. The distance between the earth and the sun is thirty-three 

 millions of leagues, so that the velocity of light is sixty-seven thousand 

 leagues, or two hundred thousand miles per second; in other words, in 

 the lapse of a single second it could pass between Washington and 

 Albany supposing the distance to be three hundred miles seven hun- 

 dred times; and could make the tour of the globe in the time it takes 

 us to wink. In consequence of this extreme velocity, in all calcula- 

 tions, regarding the light from bodies on the surface of the globe, it is 

 presumed to reach the eye instantaneously; for, granting that a lumi- 

 nous body at Albany could be seen at Washington, the light from it 

 would reach the eye in the ^jjth part of a second. Inconceivable as 

 this velocity is, it is far surpassed by that of the attractive force ex- 

 erted between the heavenly bodies. "I have ascertained," says M. La 

 Place, "that between the heavenly bodies all attractions are transmitted 

 with a velocity, which, if it be not infinite, surpasses several thousand 

 times the velocity of light; and we know that the light of the moon 

 reaches the earth in less than two seconds." An annotator on the 

 works of this distinguished mathematician is more definite ; affirming, 

 "that the gravific fluid passes over one million of the earth's semi- 

 diameters in a minute of time." Its velocity is eight millions of times 

 greater than that of light. 



A series of particles, succeeding each other in a straight line, is 

 called a ray of light. Light which proceeds from a radiant point, forms 

 diverging cones, which would be prolonged indefinitely did they not 

 meet with obstacles. In its course, it loses its intensity according to a 

 law, which seems applicable to all influences radiating from a centre. 

 If a taper be placed in the middle of a box, each one of whose sides is 

 a foot square, all the light must impinge upon the sides of the box; if 

 it be placed in a box, whose sides are two feet square, the light will 

 shine upon them from double the distance, but it will be distributed 

 over four times the surface. The intensity of the light, then, in this 

 case, as in every other, diminishes according to the square of the dis- 

 tance from the luminous body. According to this rule, those planets 

 which are nearer the sun than ours must receive the light and also the 

 heat for the same law applies to caloric in much greater intensity; 

 whilst the more distant luminaries can receive but little caloric, or light, 

 in comparison with the earth; hence, perhaps, the necessity for the 

 satellites by which they are accompanied, and by whose agency the light 

 of the sun is reflected to the planet, and the deficiency in some measure 

 compensated. 



In proceeding from a luminous body, rays, cones, or pencils of light 

 must traverse intermediate bodies, in order to reach the eye. These 

 bodies are called media. Air is the common medium; and when, in 



