LIGHT AND SOUND. 



LIGHT. 



T IGHT (German, licht ; Latin, lux, lumen; 

 I j French, lumtire). That branch of physical 

 science which treats of light and vision is termed 

 Optics, from a Greek word meaning to see. To 

 explain the production of light, let us first state 

 two suppositions, which are now all but universally 

 admitted to be true. First, all matter is built up 

 of small particles, which are in a constant state of 

 vibration. There are many ways of increasing 

 the rapidity of these vibrations, of which it is 

 sufficient to mention friction, chemical action, 

 heat. When the vibrations become sufficiently 

 rapid, the body becomes luminous. Second, per- 

 vading all space, and even the interstices between 

 the component particles of matter, there is a 

 subtle, highly elastic medium called ether, which 

 can be agitated by the vibrations of these particles, 

 thus forming ethereal waves. We see objects by 

 these waves entering our eyes, and acting on the 

 nerves of the retina. This theory has been suc- 

 cessful in explaining all the facts of vision, and 

 some even that at their discovery seemed opposed 

 to it. Besides, consequences have been deduced 

 from the theory that have been afterwards verified 

 by experiments, so that its truth is in the highest 

 degree probable. The only other theory at all 

 deserving of consideration was that devised by 

 the illustrious Newton, who supposed that lumi- 

 nous bodies throw off streams of particles in all 

 directions, some of which entering the eye, produce 

 vision. There are now so many facts that are 

 utterly inconsistent with this hypothesis, that it 

 has been generally abandoned ; though, owing to 

 the influence of Newton's name, it long reigned 

 supreme. This is called the emission theory, as 

 the former is called the wave or undulatory theory 

 of light. In what follows, we shall adhere to the 

 wave theory. 



With regard to their power of exciting the sense 

 of vision, bodies may be divided into luminous 

 and non-luminous, or those whose vibrating par- 

 ticles can, and those whose vibrating particles can- 

 not, produce those ethereal waves fitted to act on 

 the retina. The sun, the fixed stars, a fire, the 

 flame of a candle, a piece of glowing metal, are 

 instances of the first class, or luminous bodies, 

 which can be seen in the absence of other bodies ; 

 while the second class cannot be seen, except in 

 the presence of one of the first, in a way to be 

 presently explained. 



Light proceeds from a luminous body in all 

 directions ; thus the flame of a candle is seen on 

 whichever side of it we stand. Also each small 

 part of a luminous surface gives out its own share 

 of the whole light ; so that, when we wish to have 

 a luminous point, we place a screen, with a small 

 hole in it, in front of the luminous surface. 

 Bodies are said to be transparent when they 

 permit light to pass through them, as a sheet of 

 glass ; and opaque, when they stop it completely, 

 as a thick sheet of metal. No body is so trans- 

 16 



| parent as to allow all the light to pass through it, 

 ' for even the purest glass, if thick enough, will stop 

 some light ; and, on the other hand, if we take 

 a thin enough sheet of any opaque body, some 

 light will be transmitted. Gold-leaf, for example, 

 exhibits a pale green tinge when held between a 

 light and the eye. 



In every uniform medium, such as air, water, 

 glass, a vacuum, light is propagated in straight 

 lines. Thus we can see through a straight tube, 

 but not through a bent one. A shadow is a 

 correct outline of the object which throws it, 

 which could not be the case if light did not travel 

 in straight lines. Any one of these lines, pro- 

 ceeding from a luminous point, is called a ray of 

 light ; and a bundle of rays is called a pencil of 

 rays, which may be a parallel, diverging, or con- 

 verging pencil, according as the distance between 

 the component rays remains the same, increases, 

 or diminishes. 



Light is not transmitted instantaneously, but 

 travels at the rate of 186,000 miles per second. This 

 fact was first established in 1675 by a Danish 

 astronomer, Romer, whose reasoning may be 

 illustrated by the following diagram. 



Fig. i. 



Let S represent the sun ; E, F, the earth in two 

 positions in its orbit, at considerably different 

 distances from J, the planet Jupiter ; and abc, a 

 portion of the orbit of Jupiter's first satellite. At 

 b, the satellite is just plunging into Jupiter's 

 shadow, and a spectator at e sees it suddenly 

 disappear. Six months afterwards, the earth is 

 about F, while Jupiter has not materially altered 

 his place (the small circle below J indicates his 

 real place) ; and as the satellite revolves in a little 

 over forty-two hours, the spectator at F again sees 

 the eclipses, but they are now about a quarter of an 

 hour later than the times calculated from those at 

 E. Romer simply and rightly explained this fact 

 by saying that the eclipses really happened at the 

 calculated times, but that light took the additional 

 time to travel the extra distance to F. By careful 

 calculations he thus deduced the velocity of light 

 to be 195,000 miles per second ; but as the sun's 

 distance from the earth is now known to be less 



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