64 



FIRST YEAR SCIENCE 



Fig. 37. 



it has been found that the angles between each of these two 

 rays, and the line drawn perpendicularly to the reflecting 



surface are always equal, or in 

 other words the angle of reflec- 

 tion is always equal to the angle 

 of incidence. This explains 

 why, if you are standing in a 

 room beyond one side of a 

 mirror, you can see in the mirror only the opposite side 

 of the room. 



33. The Speed of Light. In the latter part of the seven- 

 teenth century a Danish astronomer by the name of 

 Roemer, after carefully watching the brightest of Jupi- 

 ter's satellites or moons as it re- 

 volved around the planet, noticed 

 that the time of occurrence of 

 its eclipses or passages behind 

 the planet showed a peculiar 

 variation. He accurately deter- 

 mined the interval between two 

 eclipses or the time it took for a 



Fig. 38. 



complete revolution of the satellite around the planet. 



Using this interval he computed the time at which 

 other eclipses should take place and found that as the 

 earth in its revolution around the sun moved away from 

 Jupiter the eclipses appeared to take place more and more 

 behind time. Determining the exact time at which an 

 eclipse took place when the earth was nearest to Jupiter, 

 and computing the time an eclipse should take place six 

 months later when the earth was farthest from Jupiter, 

 he found that the actual time of the eclipse was 22 min- 

 utes behind the computed time. This slowness he said 

 must be due to the time required by the light in crossing 

 the earth's orbit. 



