274

Of Light and Colours.

Again: Since Light is propagated h*

Right Lines,

its Power or Intensity wl

st

decrea ;

se

tion of Light was instantaneous,, or that it wasgated through immense Spaces in an instant. ' But

pot

d

Plate

XXXV.Fig. I.

Reaumur and other Philosophers about this time, , , sing frequent observations on the Eclipses of Jup^ eirMoons, found that the Time of those Eclipses did ^correspond to the Calculations founded upon the Annomical Tables ; where the Times are all calculifor the Distance of the Centre of the Sun, and conquently, where the Eye of the Spectator must beposed to be in viewing the laid Eclipses, Occultati 0&c. of yfi^rrer's Moons. of

2. To illustrate this Matter ; let S be the Centtf ,the Sun, A B the Orbit of Mercury, C I) the Or^, gVenus, E F that of the Earth, and G H a Part of ^Orbit of Jujiter. Let I be the Body of Jupiter^JK L its Shadow, O M N the Orbit of one of J u ^!d 0 <ftMoons M just entering the Shadow of 'Jupiter .

a Spectator at S would observe the Moon M to ent?t ^Shadow just at the Time which is calculated f rotJl 0 yTables ; but a Spectator at the Earth at T always ^serves it to happen sooner, and when the Earths[ ethe opposite Part of its Orbit R, he will always oh ^ ^it to happen later, by the Space of about 7 Min u Y^.both Cases. This Observation gave the first ur gsLight was progressive, and took up about 14 M 1 sto pals over the Diameter of the Earth's Orbit n° ^to R, or 7 Minutes to pass from the Sun S i0Earth T. p r oof

3. But this, though a sufficient Discovery ° r p*-of the progressive Motion of Light, was yet but an ^periment in the Gross, and not accurate enoughtermine or define the true Rate of Velocity %v f- treally belong to Light. The Metnod by which ‘been more nicely determined, was his upon in * ’ 0 ^lowing Manner : Though Sir Isaac Newton had ^

ftrated the Motion of the Eartls from the Lawsvity, yet as his Book was understood by

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