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And hence, 3 . Since the Point of Difperfion F, is further from the Centre C, if the Refra&io» be in Water than in Air; the Rays will be lefs difpers'd in the latter Cafe than in the former.
8. If the Ray HE (Fig. 62.) fall parallel to the Axis AF, from a denier, upon the Surface of a fpherically Concave rarer Me- dium ,• the refracled Ray will concur with the Axis AF, in the Point F; Co as the Difiance of the Point of Concourfe from the Centre CF, may be to the refraiied Ray FE in the Ratio of the Sine of the refracled Angle, to the Sine of the Angle of In- clination.
Doclrine of Refraction in Glajfes, Lens% &c.
i°. Rfraclion in a Glafs Prifm.— IC a Ray or Light DE {Fig. 62.) fall obliquely out of Air on a Prifm ABC ; being refracted towards the Perpendicular, inftead of proceeding to Fit will de- cline to G, /. e. towards a Line HI, drawn Perpendicular to the Surface AB in the Point of Refraclion E.— Again, fince the Ray EG parting out of the Glafs into Air falls obliquely on CB; it wlllbe ref railed to M.: Co as to recede from the Perpendicular NGO. And hence the various Phenomena of the Prifm. See Prism.
2°. Refraction in a Convex Lens. — If Parallel Rays AB, CD, and EF, (Fig, 63.) fall on the Surface of a Lens 2B 3K; the perpendicular Ray AB will pafs unrefradred to K, where emer- ging into Air perpendicular, as before, it will proceed ftraight to G. But the Rays CD and EF falling obliquely out of Air into Glafs, in D and F, will be refracled towards the Axis of Refracli- on, (/. e. towards Lines HI and LM drawn perpendicular to the refracling Surface in the Points of Refraclion F and D,-) and de- cline to R and Q^— Again, emerging obliquely out of the Glafs into the Surface of the Air, they will be refracted from the Per- pendicular j and therefore DQwill not proceed to X but to G; and FP, not to V but to G : Thus likewife might all the other Rays falling on the Surface of the Glafs, be fhewn to be refracled Co as to meet the others about the Point G. See Focus.
Hence the great Property of Convex Glades; viz. That tloey collect Parallel Rajs, or make them converge into a Point. See Convexity.
3 . Refratlhn in a Concave Lens. — Parallel Rays AB, CD, EF, (Fig. 64.J falling on a Concave Lens GBHIMK; the Ray AB falling perpendicular on the Glafs at B, will pafs unrefraffed to M j where being flail perpendicular, it will pafs into the Air, without Refraction to L. But the Ray CD falling obliquely on the Surface of the Glafs, will be refraiied towards the Perpendi- cular NDO, and proceed to Q; and the RayDQ^ again, falling obliquely out of the Glafs upon the Surface of Air, will be re- fracled from the Perpendicular RQS, and proceed to V. After the fame manner might the Ray EF be fhewn to be refracled to Y, and thence to G-
Hence the great Property of Concave Glaffes, viz. Ihat they dijperfe Parallel Rays, or make them diverge. See Concavity.
4 . Refraclion in a plain Glafs. —If Parallel Rays EF, GH, IL, (Fig. 6y) fall obliquely on a plain Glafs ABCD; the Obli- quity being the fame in all, by reafon of their Parallelifm, they will be all equally refracled towards the Perpendicular ; and ac- cordingly being ftill parallel at M, O, (and Q, will pafs out into the Air equally refracled, again, from the Perpendicular, and ffill Parallel. See Parallel, and Looking-G/«/?.
Thus will the Rays EF, GH, and IL, at their entering the Glafs, be inflected towards the right ; and in their going out as much inflected to the left j fo that the firft Refraclion is here un- done by the fecond : Indeed, not fo as that the Object is feen in its true Place. — For the Ray BQ being produced back again, will not coincide with the Ray LI ; but will fall to the right thereof; and this the more as the Glafs is thicker: However, as to the Point of Colour, the fecond Refraclion does really undo the firft- See Colour.
5 . Refraclion in If and Cry fiat. — The Laws of Refraclion in Ifland Cryftal differ very much from thofe laid down in other Subftances; for here is a double Refraclion* contrary ways, Jwhere- by not only oblique Rays are fplit or divided into two, and re- fraStedto oppofite Parts, but even perpendicular Rays are alfo fplit, and one half of them refracled.
For the Theory hereof, fee Ifland Crystal.
The particular Latvs of Refraclion in the fever al Kinds of Lents, fee under the Article Lens.
Refraction, in Aftronomy, or Refraction of the Stars, is anlnfie&ion of the Rays of thofe Luminaries, in pairing through our Atmofphere; whereby the Altitudes of the heavenly Bodies are apparently in jreasU See STARand Altitude, &c.
This Refraclion arifes hence, that the Atmofphere is unequally denfe in different Stages or Regions.; rareft of all a-top, and denfeft at bottom ; which Inequality in the fame Medium, makes it equivalent to feveral unequal Mediums. See Air and Atmos- phere.
Sir Ifaac Newton has fhewn that a Ray of Light in parting from the higheft and rareft Part of the Atmofphere, down to the loweft and denfeft, undergoes the fame Refraclion it would do in parting immediately, at the fame Obliquity, out of a Vacuum into Air ©f equal Denfity with that in the loweft Part of the Atmofphere.
The effect of this Refraction may be thus conceived. Suppofe ZV (Tab.Ajironomy, Fig. 57.) a Quadrant of a vertical Circle, defcribed from the Centre of the Earth T, under which is AB ■ a Quadrant of the Circumference of the Earth; and GH a Qua- drant o\ the Surface of the Atmofphere: And fuppofe SE a Ray of Light emitted by a Star at S, and falling on the Atmofphere at R.— This Ray coming out of the Jitherial Medium, which is much rarer than our Air, or perhaps out of a perfect Vacuum, and falling on the Surface of the Atmofphere, will berefra&ed to- wards the Perpendicular : And fince the upper Air, again, is rarer than that near the Earth, and grows frill denfer as it approaches us ; The Ray, in its Progress will be continually refracted* fo as to arrive at the Eye in the Curve Line EA.
Suppofing, then the right Line AF to be a Tangent to the Arch in A, the Ray will enter the Eye A, according to the Di- rection of AF.— And fince Objects are always Ceen in that Line, according to the Direction whereof the Rays enter the Eye, the Star will appear in AF; that is, in the Heavens at Q\ which is nearer the Zenith than the Star really is.
Hence arife the Phenomena or the Creptfcukm, or Twilight. See Crepusculum.
And hence alio it is that the Moon is fometimes feen eclipfed, when fhe is below the Horizon, and the Sun above it. * See Eclipse.
That there is a real Refraclion of the Stars, <&c. is deduced not only from phyiical Conliderauons, and from Arguments a priori, and a Sivniltudine ; but alio from preciie Agronomical Obler- vations : Thus,
The Diitanceof the two Stars, Spic.i Virgin;:, and the Lyons- Tail, when neer the Meridian, or even near tiie Weft, iscon- ftancly found j}° 2'. But when the Lion's-Tail is rifen in the Faff. 34° 30' high, SptcaPtr'gims is obferv'd to be inalmoft the fame vertical Circle.— Add to this, an Obfervation of the Dutch, who winter'd at Nova Zembla in 1597, from whom the Sun totally dif- appeared en the 14-th of November ; and again began to appear on the 24th of January, which was fix Days iboner than he ihould have return'd, according to Agronomical Calculations ; as is ob- Cervcd mthe Atia Eruditorum, A. 1607.— Ncr mult it be omitted that Charles XL King of Siuedeh, being in 1604, at Tornou, in Wefi-Bothnia, in the Latitude of 65 ° 33', obferv'd that the Sun ne- ver Cet between the 14th and 15th Day of June $ but was vifibie in the middle of the Night : The following Year he appointed two Mathematicians, Bilembergius undSpotha, to obferve the fame more accurately ; who accordingly found that ar Tarnou in the middle of the Night, between the loth and nth of June, the Sun was \ of his Diameter above the Horizon ; and on the 14th of June at Kangis, in the Latitude of 66 9 15', they found the Sun at Miunight, two Diameters above the Horizon.
Hence 'tis argued, That as all Light is propagated in right Lines, no Rays could reach the Eye from a Luminary below the Ho- rizon, unlets they were deflected out of their Courfe at their Entrance into the Atmofphere : 'Tis evident, therefore, the Rays are refrdcJed in parting through the Atmofphere.
Hence, the Stars appear higher by Refraclion than they really are,* fothat to bring the obferv'd or apparent Altitudes to tbetrueones, theQuantity of Refraction rnuft befubftracted. See Altitude.
And hence, as the Antients were uu acquainted with the Re- fraclion; reckoning upon too great Altitudes, 'tis no Wonder they fometimes committed confidtrable Errors.
From the Doctrine of Refractions it appears that we never fee the real Sun rifing or letting, but only a Phantom, or Image thereof; theSLinhimfelf being at that time hid below the Horizon.
And from the Observations juft, mentioned, it follows, that the Refractions are greater near the Pole, than at a lefs Latitude; dbubrtefs from the greater Denfity of the Atmofphere, and the greater Obliquity of the Incidence.
M, de la Hire, allure? us, he could never find any Differences in the Meridian Altitudes of the Stats ; fo that the Rtfraclions remain always the fame.— Though he owns, near the Horizon the diffe- rent Conititution of the Air, &c. may occauon fomc Variations in the Refraclions.
Stars in the Zenith are not fubjeel to any Refraclion.— -Thofe in the Horizon have the greateil— From the Horizon the Re- fraclion continually decrcafes to the Zenith : All which follows hence, that in the firft Cafe the Rays are perpendicular j in the fecond their Obliquity is greateft; in the third, 'tis continually increafing.
At the fame Altitudes the Sun and Stsrs all undergo the fame Refraclion, for at equal Altitudes the incident Rays have the fame Inclinations : but the Sines of ibe refracled Angles, ate as the Sines of the Angles of Inclination, $>c.
Indeed Tycho Brahe, who firft deduced the Refraclions of the Sun, Moon, and fix'd Stars from Obfervation; mak?s the Solac Refraclions greater than thofe of the fix'd Stars; and the Lunar Refraclions fometimes greater than thofe of the Stars, fometimes lefs. — But the Theory of Refraclions (which we have obferved is owing to Snelhus) was not fully underftood in his Age. — De la Hire and Cajji?ii, find the Refraclio?i the fame in all.
M. de la Hire gives us a Table of the Refraclions of the Stars, in their feveral Degrees of Altitude; deduced from the fureft and molt accurate Obfervations; as follows.
TMe
