TJte Velocity of Light. 61 



because while the drop descended the height of the waggon, the 

 waggon itself would have traversed its own length. If the waggon 

 went faster than the drop, it would come out of the waggon before 

 it reached the bottom, but if the drop went faster than the waggon, 

 the drop would strike against the bottom and not come out of the 

 waggon at all. If the waggon were to change its direction, the 

 direction from which the rain came would appear to change, and 

 if finally the waggon were to describe a circle, the direction 

 from which the rain came would also appear to describe a circle. 

 If we thorougly understand these motions of the drops of rain 

 it will be quite easy to understand the phenomena of aberration. 

 Let the rays of light be the rain and the earth, the waggon moving 

 in a circle. If in a waggon we wanted to let a drop pass through 

 a tube without touching the side, we would if stationary point it 

 perpendicular to the waggon, and if moving, point it in that 

 direction from which the drops appear to come ; let then a 

 telescope be our tube ; then we would have to point the telescope 

 in the direction from which the rays appear to come. If light 

 travels instantaneously, the telescope would have to be held in 

 the real direction of the ray, whatever the speed of the earth is, 

 because of course the light would travel through the telescope 

 instantaneously, and the earth could travel no distance in 

 no time. Again, if the earth were not moving the telescope 

 would have to be put in the direction of the real ray, because 

 that would be like the case of our stationary waggon. But 

 if the earth moves and light has a finite velocity the telescope must 

 be inclined, as in the case, when our waggon, is moving, to that 

 direction from which the light appears to come. This deviation 

 has been measured and found to be an angle of 20"445. In this 

 apparent displacement of each star consists the phenomenon of 

 aberration discovered by Bradley. From the angle of aberration 

 20"445, i.e., the angle between the apparent and real position of 

 the star, Bradley calculated that the distance the earth travelled 

 in any time was 10089th what light travelled in the same time, 

 or, in other words, the speed of light was 10089 times the speed of 

 the mean velocity of the earth. The mean velocity of the earth is 

 nineteen miles a second, therefore the velocity of light is, according 

 to Bradley's calculation, 191,691 miles per second. After this 

 discovery of Bradley's the fact that light had a finite velocity could 

 no longer be doubted ; for although the phenomena observed by 

 Roemer might possibly have been due to a real movement in the 

 satellites themselves, yet it would seem to be quite impossible that 

 each star should have a peculiar movement of its own, according to 

 its position in the sky, particularly when those movements coincide 

 so exactly with the phenomena we should observe if light has a 



