( 6^6 ) 
Caufe of them. I was already convinced, that the 
apparent Motion of the Stars was not owing to a 
Nutation of the Earth’s Axis. The next Thing that 
offered itfelf, was an Alteration in the Direction of 
the Plumb-line, with which the Inftrument was con- 
stantly rectified ; but this upon Trial proved infuffi- 
cient. Then I confidered what Refraction might do, 
but here alfo nothing latisfaCtory occurred. At laft 
I conjectured, that all the Thanomena hitherto men- 
tioned, proceeded from the progreflive Motion of 
Light and the Earth’s annual Motion in its Orbit. 
For I perceived, that, if Light was propagated in 
Time, the apparent Place of afixt ObjeCt would not 
be the lame when the Eye is at Reft, as when it is 
moving in any other Direction, than that of the Line 
paffing through the Eye and ObjeCt; and that, when 
the Eye is moving in different Directions, the appa- 
rent Place of the ObjeCt would be different. 
I confidered this Matter in the fol- 
lowing Manner. I imagined C A to 
be a Ray of Light, falling perpendi- 
cularly upon the Line B D ; then if 
the Eye is at reft at A, the ObjeCt 
muft appear in the Direction A C, 
whether Light be propagated in Time 
or in an Inftant. But if the Eye is 
moving from B towards A, and Light 
is propagated in Time, with a Velo-* 
city that is to the Velocity of the 
Eye, as C A to B A ; then Light mov- 
n ing from C ro A, whilft the Eye 
moves from B to A, that Particle of 
C 
