214 LECTURES TO SCIENCE TEACEEES. 



in the first case the light has to pass over a distance greater 

 than in the second case by a quantity that may amount to 

 the whole diameter of the earth's orbit. This will be easily 

 understood by a glance at the diagram. This is exactly 

 the kind of effect that would be produced on the assumption 

 that light takes a sensible time to pass over space. Roemer 

 found that the maximum difference of time was about 

 eighteen minutes. At that time there was no knowledge 

 about the size of the earth's orbit, except from some inge- 

 nious speculations of the accomplished J eremiah Horrocks. 

 But with our present knowledge of the distance of the 

 sun, this would indicate a velocity of 310,000 miles a 

 second. 



On reading the scientific literature of that day we find 

 that there was a general scepticism prevalent as to the 

 brilliant discovery of Roemer. It was not until Bradley 

 announced his discovery of the aberration of light, in 1728, 

 that men of science generally accepted these views. 

 Bradley was employed at that time in making accurate 

 observations on the star Y Draconis, to attempt to find a 

 change in the place of the star produced by the change of 

 position of the earth in her orbit, which had been 

 announced as observed by Hooke. In technical language, 

 he was trying to measure the parallax of the star. He 

 chose this particular star because it passed so near the 

 zenith, and so the errors of refraction were reduced to a 

 minimum. He certainly found an annual change in the 

 position of the star. But it was produced at a time when 

 parallax could not produce any eftect. 



To understand how the gradual propagation of light 

 could produce such an effect, let us look at the diagram. 

 If the earth remained fixed in the position E, the star s 

 would of course be seen in the direction E a. But if the 

 earth move over the distance B E, while the light from the 

 star passes over the distance A E, then the star will be seen 

 in the direction E s', for that is the direction of the ray of 

 light relatively to the earth. An analogy will make this 

 clearer. In a steady downpour of rain, if you are standing 

 still you hold your umbrella upright ; but if you are walk- 

 ing fast you incline your umbrella forward to catch the 

 rain drops. So when the earth is moving in a direction at 

 right angles to the ray of light coming from a star, you 



