228 HISTORY OF SCIENCE. 



B', the point A must have advanced to some place in the circle efg. 

 Consider now a point E in the wave half-way between A and B : it will 

 arrive at E' in one-half the time required for E to pass to B', and the 

 other half of the time it will be moving in the denser medium ; at 

 the end of the period it must be somewhere in the circle h i k, which 

 is described with a radius = three-fourths of E E'. Therefore the front 

 of the wave when B arrives at B' must be the straight line, B if, which 

 passes through B and touches the two circks ; for it will be seen that 

 whatever point in A B is considered, its place at the end of the in- 

 terval will be determined by a circle which will touch E r f. 



The theory which thus admirably explained ordinarv refraction was 

 then applied by Huyghens to the case of the extraordinary ray in 

 double refraction. He found that by assuming that the undulations 

 in Iceland spar corresponding with this ray were propagated in a 

 spheroidal, and not, as in the ordinary case, in a spherical form, a 

 truthful representation of the observed phenomena might be obtained. 

 But the undulatory theory was for the present advanced only as a hy- 

 pothesis, to be received or rejected according to its applicability or 

 otherwise to other facts. To the optical discoveries, however, which 

 were made soon after the publication of Huyghens' treatise, his theory 

 does not seem to have been applied. 



We have already mentioned the attention given by astronomers to 

 the system of satellites revolving about the planet Jupiter. A remark- 

 able deviation from the known regularity of planetary motions was 

 soon noticed in the periods of these satellites. Thus, lo, the satellite 

 nearest to the planet, revolves round its primary in about 42^ hours, 

 and at each revolution it is eclipsed by passing into the shadow of 

 the planet. The period of the revolution of the satellite could be 

 deduced with accuracy by the application of Kepler's laws from the 

 known distance of the satellite from the primary planet. It was, how- 

 ever, observed that when the earth was at that position in which it is 

 either nearest to or farthest from Jupiter, the interval between the 

 eclipses agreed with the theory ; but when the distance of the earth 

 from Jupiter was decreasing, the eclipses occurred at shorter intervals 

 of time ; while, on the other hand, when the earth was retiring from 

 Jupiter, the intervals of the eclipses were greater. The sum of the 

 differences between the calculated intervals, and those observed while 

 the earth was passing from, amount in each case to about 16^ minutes. 

 The true explanation of this circumstance, as depending on a hitherto 

 unrecognized property of light, probably occurred to the mind of 

 more than one astronomer ; but a Danish astronomer, named OLAUS 

 ROEMER (1644 1710)? was the first to announce, in 1676, that these 

 differences were owing to the time required for the light to travel, 

 and that the i6| minutes represented the period it occupied in pass- 

 ing across the earth's orbit. Roemer calculated from this that light 

 travels at the amazing velocity of 185,000 miles in a second. 



