450 miciielson's recent researches on light. 



vaue altered. He asked the sailors, Why ? All they could say was, 

 that it always did. Ketlectiug' upou the matter, Bradley concluded 

 that the uiotiou of the boat was compounded with the velocity of the 

 wind, and that the vane represented the resultant direction. He was 

 not slow in seeing the application of this homely illustration of the 

 parallelogram of motion lo his astronomical puzzle. The velocity of light 

 was compounded with the velocity of the earth in its orbit, so that its 

 api)arent direction differed by a small angle from its true direction, and 

 the difference was called aberration. In spearing a fish or shooting a 

 bird, the sportsman does not aim at them, but ahead of them. This 

 inclination from the true direction is similiar, in angular measure, to 

 what the astronomer calls aberration. Struve's measurement of aber- 

 ration combined with the velocity of the earth in its orbit gave for the 

 velocity of light 191,513 miles a second. Both of the two methods de- 

 scribed for obtaining the velocity ot light depend for their accuracy 

 upon the assumed distance of the earth from the sun. The distance 

 adopted was the one found by the transits of Venus in 1761 and 1769, 

 viz. 95,360,000 miles. 



During the last forty years, the opinion has been gaining ground 

 among astronomers that the distance of the sun, as deduced from the 

 transits of Venus in 1761 and 1769, was too large by 3 per cent. Ex- 

 peditions have been sent to remote parts of the earth for observing 

 the planet Mars in opposition. The ablest mathematical astronomers, 

 as Laplace, Pontecoulant, Leverrier, Hansen, Lubbock, Airy, and 

 Delaunay, have applied profound mathematical analysis to the numer- 

 ous perturbations in planetary motions, and proved that the sun's dis- 

 tance must be diminished about 2,000,000 miles in order to reconcile ob- 

 servations with the law of gravitation. Airy reduced the distance of 

 the sun by more than 2,000,000 miles, to satisfy the observations on 

 the transit of Venus in 1874. Glasenapp derived from observed eclipses 

 of Jupiter's satellites a distance for the sun of only 92,500,000 miles. 

 From these and similar data, Delaunay concluded that the velocity of 

 light is about 186,420 miles a second. 



These triumphs of astronomical theory recall the witty remark of 

 Fontenelle, that Newton, without getting out of his arm chair, calcu- 

 lated the figure of the earth more accurately than others had done by 

 travelling and measuring to the ends of it. And Laplace, in contem- 

 ])latiou of similar mathematical achievements, says: " It is wonderful 

 that an astronomer, without going out of his observatory, should be 

 able to determine exactly the size and figure of the earth, and its dis- 

 tance from the sun and moon, simply, by comparing his observatiop^ 

 with analysis ; the knowledge of which formerly demanded long anci 

 laborious voyages into both hemispheres." 



The ancients supposed that light came instantaneously from the 

 stars ; a consolation for those who believed that the heavens revolved 

 around the earth in twenty -four hours. Galileo ^od the academioiail§ 

 of Florence obtained eveu negative results. 



