9. There can no Homogeneal Colours be reduced out of Light by Refraction, which were not commixt in it before. Because by Prop. 7. and 8. Refraction changeth not the Qualities of the Rays, but only separates those which have divers Qualities, by means of their different Refrangibility.
10. The Sun's Light is an Aggregate of an indefinite variety of Homogeneal Colours, by Prop. 1, 3, and 9. And hence it is, that I call Homogeneal Colours also Primitive or Original. And thus much concerning Colours.
For a further Illustration of this Doctrine, Mr. Newton, in his Book of Opticks lately published, has by undeniable Experiments explained most of the Principal Phænomena of Light and Colours: To which we refer the Reader.
A Demonstration concerning the Motion of Light, communicated from Paris.
PHilosophers have been labouring for many Years to decide by some Experiment, whether the Action of Light be conveyed in an instant to distant Places, or whether it requireth time. M. Romer, of the Royal Academy of Sciences, hath devised a way taken from the Observations of the first Satellit of Jupiter, by which he demonstrates, that for the distance of about 3000 Leagues, such as is very near the bigness of the Diameter of the Earth, Light needs not one Second of Time.
Let (in Fig. 3. Plate 3.) A be the Sun, B Jupiter, C the first Satellit of Jupiter, which enters into the shadow of Jupiter, to come out at D, and let EFGHKL be the Earth, placed at divers distances from Jupiter.
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.