250 



HISTORY OF SCIENCE. 



vation to y Draconis and other stars, and after several conjectures were 

 found inadequate to explain the appearances observed, the happy idea 

 occurred to him that all the phenomena proceeded from the motion of 

 light combined with the orbital motion of the earth. A familiar illus- 

 tration may convey to the reader's mind a just idea of the nature of 

 the fact discovered by Bradley. Refer to Fig. 121, and let us imagine 

 a shower of rain descending vertically; but out of the multitude of 

 drops we will consider only the course of one, whose line of fall shall 

 be represented by the vertical line c 1 2 3. Now, suppose we have an 

 open tube, it would be quite easy to make the rain-drop fall down the 

 axis of the tube by simply holding it stationary in an upright position. 

 But now imagine the tube to be moving from left to right at the same 

 time that the drop is falling within it. It is plain that the drops could 

 not possibly pass along the axis of the tube if the latter be maintained 



in a vertical position while it is 

 moved. But it remains for the reader 

 to realize the fact that by giving the 

 tube a certain inclination forward, 

 and it would be possible for the rain- 

 drop to be always vertically falling, 

 yet never quit the axis of our moving 

 tube. In the figure the tube and 

 the rain-drop are represented at three 

 instants of time. A very familiar ex- 

 perience will suggest the necessity for 

 the inclination of the tube in the case 

 supposed. When a person is stand- 

 ing still in a shower of rain descend- 

 ing vertically, an umbrella held up- 

 right effectually screens him ; but if 

 he walks forward he finds it neces- 

 sary to incline his umbrella in the 



direction he is moving, and the faster he walks the greater is the 

 inclination that must be given to the umbrella to prevent the rain 

 from reaching his person. Again referring to Fig. 121, it will be seen 

 that in the time during which the rain-drop is falling from i to 3 the 

 tube moves forwards (we suppose both motions to be uniform) by the 

 distance AI, AS ; hence the velocity of the tube is to the velocity of the 

 drop as the line AI, AS is to line i, 3. Now, instead of rain, imagine that 

 we have light falling vertically, that the tube is the tube of a telescope, 

 and that it is carried forwards by the movement of the earth in its 

 orbit. It will now be obvious to the reader that the telescope which 

 receives light really streaming vertically downwards must be directed, 

 not to the zenith, but to a point at a certain angular distance in ad- 

 vance; and that a star which was truly in zenith c would appear to 

 an observer to be in the direction D. A little reflection will show that 



