RAINBOW. 



is different in different places : whence alfo it follows, that 

 every different fpcftator fees a different bow. 



Fourthly, Wliy the bow is fometinies a larger portion of 

 a circle, fomcti-nes a lefs ? its magnitude depending on the 

 greater or lefs part of the furface of the cone, above the 

 hirface of the earth, at the time of its appearance ; and 

 that part being greater or lefs, as the line of afpeft, or axis 

 of vifion, is more inclined or oblique to the furface of the 

 earth ; which inclination, or obliquity, is greater as the fun 

 is higher : whence, alio, the higher the fun, always the lefs 

 the rainbow. 



Fifthly, Why the bow never appears when the fun is 

 above a certain altitude ? the furface of the cone, in which 

 it Ihould be feen, being loll in the ground, at a little diltance 

 from the eye, when the fun is above 4.2^ high- 



Sixthly, Why the bow never appears greater than a femi- 

 circle, on a plane ? fincc be the fun ever fo low, and even 

 in the horizon, the centre of the bow is ftill in the line of 

 afpeft ; which, in this cafe, runs along the earth, and is 

 not at all raifed above the furface. 



Indeed, if the fpcftator be placed on a very confiderable 

 eminence, and the fun in the horizon, the line of afpeft, 

 wherem the centre of the how is, will be notably raifed 

 above the horizon (confidcring the magnitude of the circle 

 whereof the bow ufes to be a part). Nay, if the eminence 

 be very high, and the rain near, it is poflible the bow may 

 be an entire circle. 



Seventhly, How the bow may chance to appear inverted, 

 i. e. the concave fide be turned upwards ? ws. a cloud hap- 

 pening to intercept the rays, and prevent their fhining on 

 the upper part of the arch : in which cafe, only the lower 

 part appearing, the bow will feem as if turned upfide down ; 

 which, probably, has been the cafe in feveral prodigies of 

 this kind, related by authors. 



Indeed the bow may appear inverted from another caufe: 

 for, if when the fun is 41° 46' high, his rays fall upon the 

 fmooth furface of fome fpacious lake, in the middle of which 

 a fpeftator is placed ; and if, at the fame time, there be 

 rain falling, to which the rays may be reflected from the 

 lake, it will be the fame as if the fun fhould fhine below the 

 horizon, and the line of view be extended upwards : tluis 

 the furface (.-f the cone, wherein the coloured drops are to 

 be placed, will be wholly above the furface of the earth. 



But fince the upper part will tall among the unbroken 

 clouds, and only the lower part be found among the drops 

 of rain, the arch will be inverted. 



Eighthly, Why the bow fometimes appears inclined? the 

 accurate roundnefs of the bow depending on its great 

 diftance, which prevents us from judging of it exaftly ; if 

 the rain, which exhibits it, chance to be much nearer, we 

 (hall fee its irregularities ; and if the wind, in that cafe, 

 drive the rain fo that the higher part be farther from the eye 

 than the lower, the bow will appear inclined. 



Ninthly, Why the legs of the rainbow fometimes appear 

 unequally diilant ? if the rain terminate on the fide of the 

 fpeftator, in a plane fo inclined to the line of afpeft as to 

 make an acute angle on the left hand, and an obtufe anglr 

 on the right, the furface of the cone, which determines 

 what drops will appear, will fall upon them in fuch manner 

 as that ihofe on the left hand will appear farther from the 

 eye than thofe on the right. For the line of afpeift being 

 perpendicular to the plane of the bow, if you fuppofe two 

 reftangular triangles, a right and left, the cathetus of each 

 to be the line of view, and the bafe of the femi-diameter of 

 the bow, inclined as above ; it is evident, fince thofe angles 

 of the triangles, next tlie eye, muft always be the fame 

 (viz. 43° in the inner bow), the bafis of the right-hand 



triangle will, in this cafe, appear much longer than that of 

 tlie left. 



Dr. Langwith, in the Phil. I'ranf. N" 375, defcribes a 

 remarkable rainbow, in which lie obfirved feveral feries of 

 colours, which incrcafed the bow to a breadth far exceeding 

 what had been determined by calculation. The colours of 

 the primary rainbow, he fays, were as ufual ; under tliib 

 was an arch of green, and then alternately two arches of 

 reddifh-purple, and two of green, and under all a faint ap- 

 pearance of another arch of purple. Tlie order of the 

 colours was, i. Red, orange, yellow, green, liglit blue, 

 deep blue, and purple. 2. Light green, dark green, and 

 purple. 3. Green and purple. 4. Green, and faint vanirti- 

 ing purple. 



M. Bouguer frequently faw a phenomenon of this kind, 

 when he was upon the mountains of Peru, where the fky 

 is often extremely ferene. Similar appearances have been 

 alfo obfervcd by others. Dr. Pemberton has attempted to 

 explain them by means of the Newtonian dodlrine of fits of 

 eafy refledlion and tranfmiiiion ; but he goes upo/i the fup« 

 pofition that the differently-coloured rays have their feparate 

 fits, on tlieir arrival at the furface of any medium, without 

 any regard to the thicknefs of it ; whereas it is plain, 

 that, in fuch a cafe, all kinds of rays are reflefted or tranf- 

 mitted promifcuoufly. It is mod probable that lliefe co- 

 lours are formed in very minute drops of water or vapour, 

 intermixed witli the larger drops, that their formation de- 

 pends upon the fame principle with the colours of thin 

 plates, and that they arc fimilar to thole of feveral kinds of 

 halos. Dr. Pemberton himfelf obferves, that it is moll 

 likely that thefe additional rings of colours are formed in 

 the vapour of the cloud, which the air, being put in motion 

 by the fall of the rain, may carry down along with the large 

 drops. This, he fays, may be the reafon why thefe colours 

 appear under the upper part of the bow only, this vapour 

 not defcending very low. As a farther confirmation of this, 

 thefe colours, he obferves, are feen ftrongeit when the rain 

 falls from very black clouds, which caufe the fiercefl rains ; 

 by the fall of which the air will be moll agitated. Phil. 

 Tranf. abr. vol. vi. p. 140. prielUey on Light and Co- 

 lours, p. 593. 



It has been a fubjeft of controverfy among biblical critics, 

 whether the antediluvian atmofphere prefented any fuch 

 phenomenon as a rainbow. The occafion of this debate is 

 a pailage in Genefis, ch. ix. 12 — 17. Mr. Whiteiiurlt, in 

 his " Inquiry into the original State and Formation of the 

 Earth, &c." attempts to eftablifh the probability that the 

 antediluvian atmofphere was fo uniformly temperate, as to 

 be fubjeft to no llorms, tcmpefts, or rain. The opinion is 

 by no means probable, nor does the above-cited paflage 

 warrant any fuch conclufion. The rainbow was then made 

 the fign or token of God's covenant with Noah, that no 

 fuch calamity fiiould again take place. ^Mi denotes a fign 

 or token, and in this fenfe it is ufed in the cafe of Cain, 

 Gen. iv. 15. The mark fet upon Cain was, as Shuckford 

 conceives, a fign or token (nX) given to him, that he 

 (hould be prefervcd from the evil which he deprecated. 



Rainbow, Lunar. The moon fometimes alfo exhibits 

 the phenomenon of an iris, or bow, by the refraftion of her 

 rays in the drops of rain in the night-time. 



Arillotle fays, he was the firft that ever obferved it ; and 

 adds, that it never happens, /'. e. is never vifible, but at the 

 time of the full moon ; her light, at other times, being too 

 faint to affeft the fight after two refractions, and one re- 

 flection. 



The lunar iris has all the colours of the folar, very dif- 

 tindl and pleafant ; only faint, in comparikin of the other ; 



both 



