which is the Medium by which the Rays of thofe luminous Particles are tranfmitted or propagated to our Eyes, and illuminate them with their Light.

A. You then fuppofe, I find, that what we call Rays of Light are real Bodies, or Particles of Matter, endued with the Quality of Light, and not the Quality it self. B. You hit it; I do indeed not only fuppofe it, but am sure of it.

A. Are you indeed! then I am fure not only my felf, but Thousands more, have been long mistaken. Pray, is not the Sun the Fountain of Light originally?

B. No: He is only the greatest Body that emits Light in our planetary Syftem: Light it felt being a Quality innate to. thofe Bodies, which emit it naturally.

A. But why do you fay naturally?

B. Because opake Bodies, when heated beyond a certain Degree, will emit Light; and all Bodies, efpecially fulphureous ones, having their Parts fufficiently agitated do fhine or emit Light: Whether this be by Percuffion, as Quick-filver, when shaken in Vacuo; or by Attrition, as a Cat's Back, or Horfe's Body, &c. rubbed in the Dark; or by Putrefaction, as happens in Wood, Flefb, &c. when putrified; or by any other Way. A. Is the Motion of Light inftantaneous or fucceffive?

t

B. It

Of the Nature of Light.

55

B. It being found (by Obfervation) that Light is fucceffive, or propagated in Time, hath at Length ended this long and difficult Controverfy.

A. By what kind of Obfervations is this difcover'd?

B. Aftronomical Obfervations; the Principal of which is, that of the Eclipfes of the Satellites of Jupiter; for thofe Eclipfes, when the Earth is between the Sun and Jupiter, happen feven or eight Minutes too foon; and when the Earth is beyond the Sun, they happen as much later than they fhould by the Tables; the Reason of which is, that Light hath farther to go in the latter Cafe than in the former, by the Diameter of the Earth's annual Orbit.

A. Why, if this be the Cafe, you may nearly compute the Motion of Light; can you not?

B. Yes, eafily, could we tell precifely the Distance of the Sun: Some make it 10000, and others 20000 Diameters of the Earth; let us then take the Mean 15000, and it is well known the Diameter of the Earth is about 7935-475 English Miles; therefore the Diameter of the Earth's Orb is 119032125 Miles: Hence at the Rate of 7 Minutes, or 450 Seconds in paffing from the Sun, Light will be found to fly 264516 Miles in a Second of Time.

A. Pray make this incredible Swiftness of Light a little more intelligible by Example.

B. I will: 'Tis found that à Bullet, at his first Discharge from the Muzzle of a Cannon, flies one Mile in a little above 8 Seconds, and therefore would be 32/1/ Years in arriving to the Sun; hence the Proportion of Swiftnefs in a Cannon-Ball and Light, is as I to 3112 nearly; that is, Light flies three thoufand one hundred and ' twelve Times faster than a Bullet at his first Discharge.

A. Wonderful are the Works of God! Not only past finding out, but past Belief when but imperfectly known! But, pray, what other Properties of Light are dif

covered?

B. Light is not only found to be a Body, but it is alfo found to confift of Rays tin&tured with all the original Colours in Na

ture.

A. In what Order are the Rays of Light found to be coloured?

B. According to the different Degrees of Refrangibility in the Rays: From Rays lets Refrangible to thofe that are moft fo, the Colours appear in this Order, viz. Red, Orange, Yellow, Green, Blue, Indigo, deep Violet: But more of Colours when we difcourfe directly thereof by and by.

A. Fray

Colours, Reflexion, Refraction of Light. 57 A. Pray what other Affections of Light are obfervable ?

B. Its Reflexibility and Refrangibility. A. What do you call the Reflexibility of Light?

B. The Difpofition of the Rays to be reflected, or turned back from the Surface of any Bodies on which they fall: Thus the Ray aB falling on the Surface AC, in the Point B, is reflected or turned back again in the Direction Bd. See Fig. IX.

A. In what Manner or Order is this?

B. The Law of Reflection of Light is invariable; for the Angle aBc, of the incident Ray aB, is ever equal to the Angle cBd, of the reflected Ray Bd, and the Perpendicular cB. See Fig. IX.

A. Pray what do you mean by the Refrangibility of Light?

B. The Difpofition of the Rays of Light to be refracted, or broken out of their direct Course, in paffing through one tranfparent Body or Medium into another: Thus the Ray aB, paffing through the Medium of Air to B, and there ftriking on the Surface of Water AC, is refracted or broken out of its direct Courfe BE, into another BF, towards the Perpendicular BG, See Fig. X.

A. Doth the Refrangibility of Rays allo obferve any ftated Law or Order?

B. Yes:

B. Yes: For the Line HI, of the Angle of Incidence CBH, is always in the fame Proportion to the Line MF=IK, of the Angle of Refraction GBF; as 4 to 3 in Water; and as 17 to 11 in Glass: That is, HI:MF:: 4:3 in Water; and HI: MF:: 17:11 in Glafs. See Fig. X.

A. What may we learn by this Doctrine of the Reflection of Light?

B. The Reason why Objects appear just as far behind any Plane Speculum, or Looking-glass, as they really are before it, and the fame Way: Thus, fuppofe AC the Section of a Looking-Glas, 'tis plain any Object placed before it at a, as the Arrow, will appear juft fo far behind the Glass AB as E, and directed to the fame Point in the Glafs B: For all Objects appear in that Ray Ed, that meets the Eye in'd, be it any how reflected or refracted. See Fig. IX.

A. And are our Eyes thus deceived by the Refraction of Light?

B. Beyond your Imagination, I believe. A. Aye, indeed! Pray oblige me with an Example how and in what Respects.

B. I will; and that by a very vulgar Experiment: Suppofe AGHB be a Veffel, whofe Length GH is 56 Inches; let any Object, as a half Crown, be placed exactly in the Middle thereof at F; then let the Veffel be filled with Water to the Height CD, 24 Inches; let FP be a Ray of Light paffing