( 6 47 ) 
itj by which the Obje& will be dilcerned, when the 
Eye in its Motion comes to A, is at C when the 
Eye is at B. Joining the Points B,C, I fuppofed the 
Line CB, to be a Tube (inclined to the Line BD in 
the Angle D B C ) of fuch a Diameter, as to admit 
of but one Particle of Light ; then it was cafy 
to conceive, that the Particle of Light at C (by 
which the Objed muft be feen when the Eye, as it 
moves along, arrives at A) would pafs through the 
Tube B C, if it is inclined to B D in the Angle D B C, 
and accompanies the Eye in its Motion from B to A 3 
and that it could not come to the Eye, placed behind 
fuch a Tube, if it had any other Inclination to the 
Line BD. If inftead of fuppofing CB fo Email a 
- Tube > vve imagine it to be the Axis of a larger ; then 
for the fame Reafon, the Particle of Light at C, could 
not pafs through that Axis, unlels it is inclined toBD, 
in the Angle CBD. In like manner, if the Eye 
moved the contrary way, from D towards A, with 
the fame Velocity ; then the Tube muft be inclined 
in the Angle BDC. Although therefore the true or 
real Place of an Objed is perpendicular to the Line 
in which the Eye is moving, yet the vifible Place 
will not be fo, fince that, no doubt, muft be in the 
Diredion of the Tube ; but the Difference between 
the true and apparent Place will be (ceteris paribus) 
greater or lefs, according to the different Proportion 
between the Velocity of Light and that of the Eye. 
So that if we could fuppofe that Light was propagat- 
ed in an Inftant, then there would be no Difference be- 
tween the real and vifible Place of an Objed, altho* 
the Eye were in Motion, for in that cafe, AC be- 
ing infinite with Refped to AB, the Angle A C B (the 
2 Dif- 
