262 



A Short History of Astronomy 



[Cn. X. 



in different directions, the apparent place of the object would be 

 different. 



" I considered this matter in the following manner. I imagined 

 c A to be a ray of light, falling perpendicularly upon the line 

 B D ; then if the eye is at rest at A, the object must appear in 

 the direction A c, whether light be propagated in time or in an 

 instant. But if the eye is moving from B towards A, and light 

 is propagated in time, with a velocity that is to the velocity of 

 the eye, as c A to B A ; then light moving from c to A, whilst 

 the eye moves from B to A, that particle of it by which the object 

 will be discerned when the eye in its 

 motion comes to A, is at c when the eye 

 is at B. Joining the points B, c, I sup- 

 posed the line c B to be a tube (inclined 

 to the line B D in the angle D B c) of such 

 a diameter as to admit of but one particle 

 of light ; then it was easy to conceive that 

 the particle of light at c (by which the 

 object must be seen when the eye, as it 

 moves along, arrives at A) would pass 

 through the tube B c, if it is inclined to 

 B D in the angle DEC, and accompanies 

 the eye in its motion from B to A ; and 

 that it could not come to the eye, placed 

 behind such a tube, if it had any other 

 inclination to the line B D. . . . 



" Although therefore the true or real 

 place of an object is perpendicular to the 

 line in which the eye is moving, yet the 

 visible place will not be so, since that, 

 no doubt, must be in the direction of the 

 tube ; but the difference between the true 

 and apparent place will be (caeteris pari- 



D A B 



FIG. 74. The aberra 



tion of light. From bus) greater or less, according to the 

 Bradley's paper in different proportion between the velocity 

 the Phil Trans. o f light and tnat of the eye. So that if 

 we could suppose that light was propa- 

 gated in an instant, then there would be no difference between 

 the real and visible place of an object, although the eye were 

 in motion ; for in that case, A c being infinite with respect 

 to A B, the angle A c B (the difference between the true and 

 visible place) vanishes. But if light be propagated in time, 

 (which I presume will readily be allowed by most of the 

 philosophers of this age,) then it is evident from the foregoing 

 considerations, that there will be always a difference between 

 the real and visible place of an object, unless the eye is moving 

 either directly towards or from the object." 



