300 WELLS'S NATURAL PHILOSOPHT. 



Several other plans have been devised for determining the velocity of lights 

 the results of whicli agree very nearly with those obtained by the observationt, 

 on the sateUites of Jupiter.* 



■vThen is light 651. When a ray of light strikes against a 

 reflected? surfacc, and is caused to turn back or rebound 

 in a direction different from whence it proceeded, it is said 

 to be reflected. 



■What is ab- ^52. "When rays of light are retained upon 

 fr-ht'?'"^ °^ "^^^ surface upon which they fall, they are said 

 to be absorbed ; in consequence of which their 

 presence is not made sensible by reflection. 



The question as to what becomes of the light which is absorbed by a body, 

 can not be satisfactorily answered. In all probability it is permanently re- 

 tained within the substance of the absorbing body, since a body which absorbs 

 light by continued exposure, does not radiate or distribute it again in any 

 way, as it might do if it had absorbed heat. 



shadow of the planet. If the transmission of light were instantaneous, it is obvious that 

 an observer at T', the most remote part of the eartli's orbit, would see the eclipse begia 

 and end at the same moment as an observer at T, the part of the e.arth's orbit nearest to 

 Jupiter. This, however, is not the case, but the observer at T' sees the eclipse 960 sec- 

 onds later than the observer at T ; and as the distance between these two stations is 198 

 millions of miles, we have, as the velocity of light in one second, 192,000,O00-r-96O = 

 200,000. 



Fig. 232. 



♦ A very ingenious plnn was devised a few years since by M. Fizeau of Paris, by which 

 the velocity of artificial light was determined and found to agree with that of solar light. 

 A disc, or wheel, carrying a certain number of teeth upon its circumference, was made to 

 revolve at a known rate : placing a tube behind these, and looking at the open spaces be- 

 tween the teeth, they become less evident to sight, the greater the velocity of the moving 

 wheel, until, at a certain speed, the whole edge appears transparent. The rate at which 

 the wheel moves being known, it is easy to determine the time occupied while one tooth 

 passes to take the place of the one next to it. A ray of light is made to traverse many 

 miles throngh space, and then passes through the teeth of the revolving disc. It moves 

 the whole distance in just the time occupied iu the movement of a single tooth to theplao» 

 of another at a certain speed. 



