Page:A short history of astronomy(1898).djvu/326

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

"I considered this matter in the following manner. I imagined to be a ray of light, falling perpendicularly upon the line the direction, whether light be propagated in time or in an instant. But if the eye is moving from  towards , and light is propagated in time, with a velocity that is to the velocity of the eye, as  to ; then light moving from  to , whilst the eye moves from  to , that particle of it by which the object
 * then if the eye is at rest at, the object must appear in

will be discerned when the eye in its motion comes to, is at when the eye is at. Joining the points, I supposed the line to be a tube (inclined to the line  in the angle ) 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  (by which the object must be seen when the eye, as it moves along, arrives at ) would pass through the tube , if it is inclined to  in the angle , and accompanies the eye in its motion from  to ; and that it could not come to the eye, placed behind such a tube, if it had any other inclination to the line. . ..

"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 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 propagated 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, being infinite with respect to, the angle  (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."