Now suppose the _Earth_, being in _L_, towards the second Quadrature of _Jupiter_, hath seen the first Satellit, at the time of its emersion, or issuing out of the shadow at _D_, and that about 42½ Hours after (_viz._ after one Revolution of this _Satellit_) the _Earth_ being in _K_, do see it return'd in _D_: It is manifest, that if the Light require time to traverse the Interval _LK_, the _Satellit_ will be seen return'd later in _D_, than it would have been if the _Earth_ had remained in _L_. So that the Revolution of the _Satellit_ being thus observ'd by the Emersions, will be retarded by so much time, as the Light shall have taken in passing from _L_ to _K_; and that on the contrary, in the other Quadrature _FG_, where the _Earth_ by approaching goes to meet the Light, the Revolutions of the Emersions will appear to be shortned, by so much as those of the Emersions had appear'd to be lengthned. And because 42½ Hours, which this _Satellit_ very near takes to make one Revolution, the distance between the _Earth_ and _Jupiter_, in both the Quadratures, varies at least 210 Diameters of the _Earth_: It follows, that if for the Account of every Diameter of the _Earth_ there were required a Second of Time, the Light would take 3½ Minutes for each of the Intervals _GF_, _KL_; which would cause near half a quarter of an Hour between two Revolutions of the first _Satellit_, one observ'd in _FG_, and the other in _KL_, whereas there is not observed any sensible difference. Yet doth it not follow hence, that Light demands no time. For after M. _Romer_ had examin'd the thing more nearly, he found that what was not sensible in two Revolutions, became very considerable in many being taken together; and that, for Example, forty Revolutions observed on the side _F_, might be sensibly shorter, than forty others observ'd in any place of the _Zodiack_ where _Jupiter_ may be met with; and that in proportion of Twenty two for the whole Interval of _HE_, which is the double of the Interval that is from hence to the Sun. The necessity of this new Equation of the Retardment of Light, is establish'd by all the Observations that have been made in the _Royal Academy_, and in the _Observatory_, for the space of eight Years; and it hath been lately confirmed by the Emersion of the first _Satellit_ observ'd at _Paris_, the _9th_ of _November_ last, at 5 a-clock 35' 45" at Night, 10 Minutes later than it was to be expected, by deducting it from those that had been observ'd in the Month of _August_, when the _Earth_ was much nearer to _Jupiter_; which M. _Romer_ had predicted to the said Academy from the beginning of _September_.