246 PROGRESS OF SCIENCE. 



MOTION OF THE EARTH ; invented the GYROSCOPE for 

 detecting the astronomical situation without looking at the 

 heavens. 



b. 1819. Fizeau (1849) a ^ so measured directly the velocity 

 of light by a system comprising a toothed-wheel, a mirror, 

 tubes, and lenses, and found as a result 190,000 MILES PER 

 SECOND a result which practically is the same as that 

 obtained by Roemer and Bessel. The latter gives 192,000 

 miles, by dividing the mean distance of the sun, assumed to be 

 95,274,000 miles, by 8 minutes 13 seconds 3 the time taken by 

 a luminous ray to reach the earth.* The last result found 

 (Cornu's) gives 186,660 miles. Roemer calculated the time 

 taken by a ray of light to reach us from the sun to be 

 8 minutes 16 or 18 seconds, which exceeds the average now 

 accepted by from 3 to 5 seconds. Be that as it may, the 

 velocity of light has become the foot-rule, so to speak, for 

 the measurement of the distances which separate us from 

 the fixed stars whose parallax has been approximately 

 ascertained. The nearest star, a Centauri, is at least 400,000 

 times our distance from the sun, its light taking 3^ years 

 to reach us a fact which gives us a notion of the incom- 

 mensurable infinity of space. A beam takes 9^ years to 

 reach us from 61 Cygni, 12 years from a Lyrse. The light 

 of some stars takes 100 years, 1,000 years, 2,000 years to 

 reach us, so that when we see them, we see them not as 

 they are now, but as they were 100, 1,000, 2,000 years ago 

 (as they were when the light-ray left them). Likewise, an 

 inhabitant of a star thus remote would see the earth, granting 

 that he could perceive such a speck, not as it is now, but 

 as it was 2,000 years ago ; that is, he might at this moment 

 see Julius Caesar landing in Great Britain. 



1820 1 8 . Becquerel (Alex. Edward) discovered OXY- 

 GEN to be A MAGNETIC BODY, and to be to other gases 

 what iron is to other metals ; found chlorine of silver 

 capable of receiving and preserving coloured impressions 

 from light ; invented the PHOSPHOROSCOPE. 



1821 94. Helmholtz invented the OPHTHALMOSCOPE 

 for viewing the interior of the eye ; analysed sound by 

 * Sound would take fourteen years. 



