VOL. XXXV.] HHlLOSOPHiCAL TRANSACTIONS. 313 



what refraction might do; but here also nothing satisfactory occurred. At last 

 he conjectured, that all the phaenomena hitherto mentioned, proceeded from 

 the progressive motion of light and the earth's annual motion in its orbit. For 

 he perceived that, if light was propagated in time, the apparent place of a fixed 

 object would not be the same when the eye is at rest, as when it is moving in 

 any other direction, than that of the line passing through the eye and object; 

 and that, when the eye is moving in difi^erent directions, the apparent place of 

 the object would be different. 



Mr. B. considered this matter in the following manner. He imagined ca to 

 be a ray of light, falling perpendicularly on the line bd: fig. 1, pi. 7= then if 

 the eye be at rest at a, the object must appear in the direction ac, whether 

 light be propagated in time or in an instant. But if the eye be moving from b 

 towards a, and light be propagated in time, with a velocity that is to the velo- 

 city of the eye, as ca to ba; then light moving from c to A, while 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, he supposed the line cb, to be a tube, inclined to the line bd 

 in the angle dbg, 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 ob- 

 ject must be seen when the eye, as it moves along, arrives at a, would pass 

 through the tube bc, if it is inclined to bd in the angle dbg, 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 incHnation to the line bd. If instead of 

 supposing cb so small a tube, we imagine it to be the axis of a larger; then for 

 the same reason, the particle of light at c, could not pass through that axis, 

 unless it is inclined to bd, in the angle cbd. In like manner, if the eye moved 

 the contrary way, from D towards a, with the same velocity; then the tube 

 must be inclined in the angle bdc. 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 must be in the direction of the tube; 

 but the difference between the true and apparent place will be, casteris paribus, 

 greater or less, according to the different proportion between the velocity of 

 light and that 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, though the eye were in motion; for in that case, ac 

 being infinite with respect to ab, the angle acb, the difference between the 

 true and visible place, vanishes. But if light be propagated in time, which 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 



VOL. VII. S s 



