Of the celestial Appearances. 173 Light in a Lanthorn. 6. Ignis Pyramidalis, the pyramidical Fire, when it resembles a Pillar of Fire standing upright. 7. Draco Volans, a flying Dragon, when the middle Parts be thicker and broader than the Ends. 8. Capra Saltans, a skipping Goat, when it appears to have a skipping Motion, and to be fometimes kindled, and fometimes not. 9. Stella cadentes, falling Stars; when the more fubtle Parts are burnt away, they fall by the Weight of their viscous and earthy Matter remaining: And these are all the remarkable fiery Meteors. CHA P. IV. Phantafmatography, or a philofophical Account of the celeftial Apparitions, viz. the Rain-bow, Halos, Parkeliums, Parafelenes, &c. PR A. RAY, Sir, why do you chufe to ufe fuch a long hard Word as Phantafmatography, I proteft I can hardly fpeak it ? B. Because that beft expreffes my prefent Defign, which is to let you understand what the Opinions and Difcoveries of the best Philofophers are concerning the celeftial Appearances, as the Rain-bow, &c. this Word being compofed of Φαντασματα, Fan tomes, 1 tomes, or Appearances, and I expǹ, a Defcrip tion. A. But, Sir, by your leave, I would ask whether it be not an Innovation to call thofe Things by the Names of Phantoms, Appearances, or Apparitions, which (as your felf fays) are by the greatest Philofophers ranked with, and deemed Meteors? not B. Be it an Innovation, or what it will, A. What, I pray you, is there nothing B. Nothing more indeed; they all owe their Existence to one common Cause, viz. the Reflection and Refraction of Light. A. Please to explain a little more particularly the Manner how thefe Phanomena are produced; and firft, how the Rainbow acquires his beauteous and wonderful Form? B. The Rain-bow is one of the most furprifing of all the Works of God (which the Hebrews call Dnb nep, the Bow of God, and the Greeks Oaupalis, i. e. the Daughter of Wonder) This Phanomenon is feen in the falling Rain, or Dew, and not in མ་ Of the Rain-bow, bow caufed; its Colours. 175 in the Cloud whence that Rain, or Dew, proceeds; it is caused by a Reflection and Refraction of the Sun's Rays from the globular Particles of Rain: There are often two Bows to be feen at the fame Time, the interior, as AFB, which is more ftrong and vivid; the exterior Bow, as QHD, which is more faint and weak; the interior Bow is formed by two Refractions of the Rays of Light, and one Reflection of them in the Drops of Water. See Fig. XXVII. A. Pray, Sir, exemplify this Matter. B. I will: In the interior Bow (Fig. XXVII.) let E F be two Drops of the falling Rain, and let Sa be a Ray of Light, falling on the Drop E in a, from whence it is refracted first to e, thence it is reflected to E, whence it is a fecond Time refracted to the Eye, fuppofe at O: In like Manner, the fame Thing happens in the upper Part of this Bow, in the Drop F. A. Well, and what are we to learn from all this? B. Hence you easily understand the Reafon of the Colours of the Bow, if you have not forgot what I delivered to you when we were difcourfing of Light and Colours: For here you fee the Angle COE=40° 2' fhall be the greatest Angle, in which the moft refrangible Rays can after one Reflection be refracted to the Eye; and therefore all the Drops in the Line OE, shall send the most moft refrangible Rays most copiously to the Eye, and thereby ftrike the Senfes with the deepest violet Colour in that Region: In like Manner, the Angle COF=42° 17′ fhall be the greatest, in which the leaft refrangible Rays can after one Reflection be refracted to the Eye; and therefore all those leaft refrangible Rays fhall come most copiously to the Eye in the Line OF, and ftrike the Senses with the deepest Red in that Region: Do you understand me pray? A. Yes, Sir, very well; and I alfo underftand, that by Reafon of the intermediate Degrees of Refrangibility of Rays coming from the Drops between E and F, the Space between E and F fhall be painted with proper intermediate Colours; and therefore the whole Face of the Bow will be tinged with all the primogenial Colours in their natural Order, viz. Violet, Indigo, Blue, Green, Yellow, Orange, and Red, from E proceeding to F. B. I am heartily glad to fee you so very happily understand the Matter; you will with the greatest Ease and Pleasure underftand the Phænomena of the upper, or exterior Bow QHD, which in short are thus produced: Let G and H be two Drops in the extream Parts of the upper Bow; now let Sg be a Ray falling on the Drop in G, whence it is first refracted to e, from thence it is first reflected to F, from F, it is a fecond Of various Affections of the Rain-bow. 177 cond Time reflected to g, and from g it is a fecond Time refracted to the Eye at 0: Now the fame is to be understood in the upper Drop H; hence the Angle COg= 50° 42' is the leaft Angle, in which the leaft refrangible Rays can after two Reflections be refracted to the Eye; and therefore the Drops in the Line Og fhall strike the Eye with the deepeft Red, and the Angle COH=54° 22' fhall be the leaft Angle, in which the most refrangible Rays after two Reflections can emerge out of Drops; and therefore those Rays shall come most copiously from the Drops in the Line OH, and fhall ftrike the Senfe with the deepest Violet in that Region: And by the fame Reasoning, the Drops between GH shall strike the Senfes with the intermediate Colours; and fo the Colours in the whole Width of the upper Bow, fhall lie in this Order from G to H, viz. Red, Orange, Yellow, Green, Blue, Indigo, Violet; contrary to the Order of those in the lower Bow. A. Sir, I readily perceive the Reason of thofe Colours in both the Bows, according to the Laws of Refraction you heretofore mentioned; but, pray, why are the Colours of the exterior Bow fo much fainter than those of the interior one? B. Because the Light is twice reflected in the Drops of the exterior, and but once in thofe N |