« ZurückWeiter »
the various colours of the
G, but is reflected a second
efracted in the direction HS,
reason of the rays loft by the Rection. It has been found allo,
least angle SGO, or GOP, Thich the least refrangible rass
to the eye at 0, after two reand two refractions, is 50° 57'; east angle HOP, under which
refrangible rays can come to n this case, is 54° 7': whence blours of the exterior raio-bow, i'med in the drops from G to H; the breadth of this bow, viz. hereas the breadth of the former,
bow, viz. EF, is but 3° 45'i istance between the bows, viz.
ss'. And such would be like -f the bows, were the fun but a ut fince his body fubtends an half a degree, it is evident, by will each bow be increased, and nce diminished. chend rightly the different af. f the rain bow, we must attend llowing particulars. 1. That he rain-bow be occasioned by led and reflected light of the g on the drops of rain, get them is produced by any rays 7 any part of the drop indifbut by those only which fall on ce of the drop BLQG (ibid. 3.) in or about the point N, as 9 N; those which fall nearer farther towards L, being un.
in this production. 2. The bow is produced by two se and one reflection. The firit
is of the incident rays ex•
N to ope common point
hould be refracted together to the point to constitute a beam GR of the same F, that being reflected together from density with the incident heam S N, and thence to G they may there go out pa- therefore capable of exhibiting a vivid rallel
, and so come in quantity fufficient appearance of colours, and for this reason to excite the sensation of colours in a are called efficacious rays. trong and lively manner. Now those Phanomena of the RAINBOW. The first rays, and those only, which are incident is, that each is variegated with all the on the globule about the point N, can
This is a necessary this, as will appear from what fol. consequence of the different refrangilows: for, 4. The point I makes the bility of the rays refracted and reflected arch of a maximum, or the distance in drops of falling rain. Let A (ibid, OF from the axis of the drop SQ is fig. 4.) he such a drop, SN a ray engreater than any other distance from tering it at N, which is refracted to F, whence any other rays nearer to the axis, from whence refracted to G, where, as it as SD, SE, or farther from it, as SH, emerges, it is refracted into all the sea $1, are reflected ; because those which veral forts of rays of wliich it is comare nearer afier the hift refraction tend to poled, viz. GR, the least refrangible points in tbe axis produced more remote or red making ray, GO the orange, GY than that to which the ray SN tends ; the yellow, G Gihe green, G B the blue, and therefore as their distance from the GT the indigo, and GV the violet, or axis increases, fo likewise will the disa most refrangible ray. tances of their points of reflection QP, The truth of this may be easily proved Q0, till the ray becomes SN; after by experiment, by fufpending a glass which the rays more remote from the globe filled with water in the fun-fhine, axis, as SH,'S I, are refracted towards and viewing it in such a position, that the points XY, which are nearer and
the rays SN will fall upon it, and nearer to the axis; and this occasions
emerge to the eye at A, under ihe fe. the points of reflećtion on the farthest
veral angles from SFR to SFV; which side of the drop to decrease again from F towards Q. 5. Hence it will ne
may be easily effected by letting the
globe descend from A to G, by a Itring cellarily happen, that,fome rays above
going over a pulley. and below the ray S N will fall upon the
Hence, the second phænomenon, viz. same point, as ó or P, on the farthelt the circular forin, is accounted for, and fide ; and for that reason they will be so reflected from thence as to go out of the
alio the third, which is the breadth of drop by refraction parallel to each other.
the bow; for that will be equal to the Thus let S E below, and SH above the
angle 4 R G=R G V=1° 45, where ray SN, be refracted both to one point
the ray, as here, emerges afier one re
fection. These particulars are repre. 0; from hence they will be reflected to
fented more completely in fig. 5. where tis true, but alone ; being divested of
BGD is the red circumference formed
by the rotation of the ray AG, that can to a different point É will be reflected in
first come to the eye at A; and Cge is
hence, that after an efficacious ray of ineffectual for exciting the sensation of
as to render them light SN, entering a diop of rain, has colours; and they are therefore called
been twice reflected on the fartheft side inefficacious rays, in contradiftinction
ai F and H, it will emerge retracted in-
Mand L, and will there emerge parallel, their intermediate rays SN, which going a different direction to G, and emerge on one side, and not between those rays, as when they were incident on the drop. All which is evident from the figure. 6. As this will be the case of all the rays which are not indefinitely near to SN, it is plain, that being deprived of the intermediate rays, their density will
be so far diminished,
to those which enter the drop near SN, and which, having the same point F of reflection, are not scattered like the others
, but emerge together at G, so as •
but if it be of the red or last refrangible the interior one. This is the consequences fort, then the said angle is but sol of the rays being twice reflected within 58'=SyA.
the drops which form the outer bow, Therefore, all those drops which are so They who make the experiment in a situated around the eye, that their most dark chamber, may wonder when they refrangible rays shall fall upon it, muit obferve how large a part of the beam with those rays make an angle with the (that enters the globule at N) goes out Jine A P passing through the eye pa. at F, that there should be enough in the rallel to the sun's rays, viz. she angle remaining part F G to exhibit the colours GAP, equal to the angle SYA, or so Itrong and vivid in the first bow as GAP=57° 10'. These rays, there- they appear; but then, considering how fore, will every where exbibit a violet much of this residual ray is refracted at colour in the arch PGL. For the same G, it is rather a wonder how the very reason, those drops whose least refran- small part reflected to H, Nould there, gible rays
when refracted, be in quantity fufficient the angle g AP = 50° 58'; and so the to excite any distinct ideas of colours ray Ag, revolving about the axis AQ, at all, will describe the circular arch MgK, The ninth phænomenon is, that somewhich will exhibit the deepest red; and times more than two bows appear ; as in all the drops between G and g will paint a very black cloud we have observed four, the several other coloured peripheries, and a faint appearance of a fifth : but all which together will complete the ex. this happens rarely. Now, these spurious terior bow.
bows, as we may call them, cannot be The fifth phænomenon is the_greater
formed in the manoer as the two prinbreadth of the exterior bow. Thus, if cipal bows are, that is, by refraction from 54° 10' we subduct 50° 58', we after a third, fourth, fifth, &c. refraction ; th all have 3° 12'=Gg= the width of for the beam is by much too weak to exthe outer bow; which, therefore, is hibit colours by refraction, even after the almost twice as wide as the interior bow. third reflection only, much less would it The fixth phænomenon is the distance after a fourth or fifth. Besides, though between the two bows, which is thus de- after a third and fourth reflection of the termined: from the angle which the rays they fhould be supposed capable of least refringible ray in the upper bow Thewing their colours, yet the bows made makes with the axis AP, viz. 50° 38', thereby would not appear at the same fubtract the angle. 42° 2', which the time with the other two, nor in the fame most refrangible rays make therewith in part of the heavens, but in the rain bethe lower bow, and the remainder 8. tween us and the sun, and must be viewe 56'=5 AF is the arch of distance be- ed by the spectator's face turned towards tween the bows.
the lun, and not from it, as in the other The seventh phænomenon is the inverse
case. order of the colours in the two bows. The tenth phænomenon, is the appearThis follows from the contrary parts of ance of the bows in that part of the hea. the drop on which the ray is incident, vens opposite to the fun. This necessarily and from whence it emerges and is re. happens from the incident and emergent fracted. Thus, because the rays SN ray being both on one side of the drop; enter the upper part of the drop, and for it is evident, that in order to see the emerge from the lower, it is evident the colours, we must look to that part against rays refracted in this case (viz. in the which the sun fines. inierior bow) will have a fituation quite The eleventh phænomenon is, that they the reverse of those which enter on the never appear but when and where it lower part of the drop, and are re- rains. This is because rain affords a sufa fracted from the upper, as in the exte- ficient plenty of drops, or aqueous rior bow, whose colours are violet, in- spherules, proper to reflect and refract digo, blue, green, yellow, orange, and the light fit for this purpose, which canred; whilst those of the other are red, not be done without a requisite size, orange, yellow, green, blue, indigo, figure, and disposition of the particles, and violet; counting from the upper which the vapour of the cloud does not parts downwards in both.
admit, and therefore clouds alone exThe eighth phanomenon is the faintness bibit no such appearance, of the exterior. bow, in comparison of The twelfth phænomenon is the dimen
fon of the bows. This is determined the rays are reflected from them com.
architecture, are pieces that lie uoder the Lunar RAINBOW. The moon sometimes beams, and over the potts or puncheons.
also exhibits the phænomenon of an iris, RAISINS, grapes prepared by suffering by the refraction of her rays in drops
them to remain on the vine till they are of rain in the night time.
per feally ripe, and then drying them in Ariftorle says, he was the first that ever the sun, or by the heat of an oven. The observed it; and adds, that it is never difference between raisins dried in the visible, but at the time of full moon. fun, and those dried in ovens, is very The lunar iris has all the colours of the obvious: the former are sweet and plea.. folar, only fainter. See Phil. Trans. sant, but the latter have a latent acidity
with the sweetness that renders them Marine RAINBOW, the sea-bow, is a phæ- much less agreeable.
nomenon sometimes observed in a much The common way of drying grapes for agitated sea, when the wind, sweeping raisins, is to tie two or three bunches of part of the tops of the waves, carries them together while yet on the vine, and them aloft ; fo that the rays of the sun dip them into a hot lixivium of wood. are refracted, &c, as in a
alhes with a little of the oil of olives in Thower.
it. This disposes them to thrink and F. Bourzes, in Phil. Trans. observes, wrinkle, and after this they are left on that there are scarce above two colours the vine three or four days separated on distinguilhable, a dark yellow on the fide sticks in an horizontal Gtuation, and then next the fun, aud a pale green on the op. dried in the sun at leisure, after being pofite fide. But there are sometimes cut from the tree. The finest and beit twenty or thirty of them seen together. raisins are those called in some places Da
They appear at noon day, and in a po. mascus and Jube railins; which are die sition opposite to that of the common ftinguished from the others by their fize bow, the concave fide being turned up- and figures : these are flat and wrinkled wards.
on the surface, soft and juicy within, and There is a kind of white colourless rain. near an inch long, and when fresh and bow which Mentzelius and others saw growing on the bunch, are of the fize and at noon-day. M. Marriote, in his fourth shape of a large olive. Essaie de Physique, says, they are form- The raisins of the fun, and jar-raisins, ed in milts, as the others are in showers ; are all diied by the heat of the sun, and having observed several of them both af. there are the forts used in medicine. ter sun-rifing and in the night.
However, all the kinds have much the The want of colours in there, is owing fame virtues ; they are all nutritive and to the exceeding tenuity of the vehicles balsamic; they are allowed to be attenu. of the vapour, which being only little ant, are said to be good in nephritic coin. watery pellicles bloated with air, the plaints, and are an ingredient in pectoral rays of light ondergo but little refraction decoclions, in which cales, as allo in ail is palling out of air into them. Hence others where altringency is not required