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hould be refracted together to the point F, that being reflected together from thence to G they may there go out parallel, and fo come in quantity fufficient to excite the fenfation of colours in a

to conflitute a beam GR of the fame density with the incident beam S N, and therefore capable of exhibiting a vivid appearance of colours, and for this reafon are called efficacious rays. Phænomena of the RAINBOW. The first is, that each is variegated with all the prifnatic colours. This is a neceffary confequence of the different refrangibility of the rays refracted and reflected in drops of falling rain. Let A (ibid. fig. 4.) he fuch a drop, SN a ray entering it at N, which is refracted to F, from whence refracted to G, where, as it emerges, it is refracted into all the feveral forts of rays of which it is compoled, viz. GR, the lealt refrangible or red making ray, GO the orange, GY the yellow, GG the green, GB the blue, GI the indigo, and GV the violet, or moft refrangible ray.

ftrong and lively manner. Now thofe rays, and thofe only, which are incident on the globule about the point N, can do this, as will appear from what follows: for, 4. The point F makes the arch QF a maximum, or the distance OF from the axis of the drop SQ_is greater than any other diftance from whence any other rays nearer to the axis, as SD, SE, or farther from it, as SH, SI, are reflected; because those which are nearer after the fift refraction tend to points in the axis produced more remote than that to which the ray SN tends and therefore as their distance from the axis increases, fo likewife will the diftances of their points of reflection QP, QO, till the ray becomes SN; after which the rays more remote from the axis, as SH, SI, are refracted towards the points XY, which are nearer and nearer to the axis; and this occafions the points of reflection on the farthest fide of the drop to decreafe again from F towards Q5. Hence it will fieceffarily happen, that some rays above and below the ray SN will fall upon the fame point, as O or P, on the farthelt fide; and for that reafon they will be fo reflected from thence as to go out of the drop by refraction parallel to each other. Thus let SE below, and SH above the ray SN, be refracted both to one point 0; from hence they will be reflected to M and L, and will there emerge parallel, 'tis true, but alone; being divefted of their intermediate rays SN, which going to a different point F will be reflected in a different direction to G, and emerge on one fide, 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 cafe 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 fo far diminished, as to render them ineffectual for exciting the fenfation of colours; and they are therefore called inefficacious rays, in contradiftinction to those which enter the drop near S N, and which, having the fame point F of reflection, are not fcattered like the others, but emerge together at G, fo as

VOL. IV.

The truth of this may be easily proved by experiment, by fufpending a glafs globe filled with water in the fun-fhine, and viewing it in fuch a pofition, that the rays SN will fall upon it, and emerge to the eye at A, under the feveral angles from SFR to SFV; which may be easily effected by letting the globe defcend from A to G, by a string going over a pulley.

Hence, the fecond phænomenon, viz. the circular form, is accounted for, and allo the third, which is the breadth of the bow; for that will be equal to the angle ARG= R G V = 1° ‍45′, where the ray, as here, emerges after one reflection. Thefe particulars are reprefented more completely in fig. 5. where BGD is the red circumference formed by the rotation of the ray AG, that can first come to the eye at A; and CgE is the violet arch formed by the leaft refrangible ray gA; after which the rays are all refracted below the eye. And thus, by the intermediate rays and colours, the whole interior bow is produced.

The fourth phænomenon is the appearance of two bows. This follows from hence, that after an efficacious ray of light SN, entering a drop of rain, bas

been twice reflected on the fartheft fide at F and H, it will emerge refracted into all its fimple or conitituent rays at G upon the upper fide of the drop, fo as to make with the incident ray the angle GYN, SYA, 54° 10′, if that ray

be the violet fort, or most refrangible;

15 S

but

but if it be of the red or last refrangible fort, then the faid angle is but 50o 58' Sy A.

Therefore, all thofe drops which are fo fituated around the eye, that their most refrangible rays fhall fall upon it, mut with thofe rays make an angle with the Jine AP paffing through the eye parallel to the fun's rays, viz. the angle GAP, equal to the angle SYA, or GAP 54° 10. Thefe rays, therefore, will every where exhibit a violet colour in the arch PG L. For the fame reason, thofe drops whofe leaft refrangible rays fall upon the eye at A, make the angle g AP 50° 58'; and so the ray Ag, revolving about the axis AQ, will defcribe the circular arch MgK, which will exhibit the deepest red; and all the drops between G and g will paint the feveral other coloured peripheries, all which together will complete the exterior bow.

The fifth phænomenon is the greater breadth of the exterior bow. Thus, if from 54° 10′ we fubduct 50° 58', we fhall have 3° 12' Gg the width of the outer bow; which, therefore, is almost twice as wide as the interior bow. The fixth phænomenon is the distance between the two bows, which is thus determined: from the angle which the leaft refringible ray in the upper bow makes with the axis AP, viz. 50° 58′, fubtract the angle 42° 2′, which the moft refrangible rays make therewith in the lower bow, and the remainder 8 56'g AF is the arch of distance between the bows.

The feventh phænomenon is the inverse order of the colours in the two bows. This follows from the contrary parts of the drop on which the ray is incident, and from whence it emerges and is refracted. Thus, because the rays SN enter the upper part of the drop, and emerge from the lower, it is evident the rays refracted in this cafe (viz. in the interior bow) will have a fituation quite the reverse of thofe which enter on the lower part of the drop, and are refracted from the upper, as in the exterior bow, whofe colours are violet, indigo, blue, green, yellow, orange, and red; whilst thofe of the other are red, orange, yellow, green, blue, indigo, and violet; counting from the upper parts downwards in both.

The eighth phanomenon is the faintness of the exterior. bow, in comparison of

the interior one. This is the confequence of the rays being twice reflected within the drops which form the outer bow. They who make the experiment in a dark chamber, may wonder when they obferve how large a part of the beam (that enters the globule at N) goes out at F, that there should be enough in the remaining part F G to exhibit the colours so strong and vivid in the first bow as they appear; but then, confidering how much of this refidual ray is refracted at G, it is rather a wonder how the very fmall part reflected to H, fhould there, when refracted, be in quantity fufficient to excite any distinct ideas of colours at all.

The ninth phænomenon is, that sometimes more than two bows appear; as in a very black cloud we have observed four, and a faint appearance of a fifth: but this happens rarely. Now, these spurious bows, as we may call them, cannot be formed in the manner as the two principal bows are, that is, by refraction after a third, fourth, fifth, &c. refraction; for the beam is by much too weak to exhibit colours by refraction, even after the third reflection only, much less would it after a fourth or fifth. Befides, though after a third and fourth reflection of the rays they should be fuppofed capable of fhewing their colours, yet the bows made thereby would not appear at the fame time with the other two, nor in the fame part of the heavens, but in the rain between us and the fun, and must be viewed by the spectator's face turned towards the fun, and not from it, as in the other cafe.

The tenth phænomenon, is the appearance of the bows in that part of the heavens oppofite to the fun. This neceffarily happens from the incident and emergent ray being both on one fide of the drop; for it is evident, that in order to fee the colours, we must look to that part against which the fun fhines.

The eleventh phænomenon is, that they never appear but when and where it rains. This is because rain affords a fufficient plenty of drops, or aqueous fpherules, proper to reflect and refract the light fit for this purpose, which cannot be done without a requifite fize, figure, and difpofition of the particles which the vapour of the cloud does not admit, and therefore clouds alone exhibit no fuch appearance.

The twelfth phænomenon is the dimen

fion of the bows. This is determined eafily; for continuing the axis AP to Q, the center of the bows, we have the femidiameter of each bow in the angle QAg, or QAG; the double of which gives the angles which the whole diameters of the bows fubtend, and are therefore the measure of their magnitude, The thirteenth phænomenon is the altitude of the bow above the horizon, or furface of the earth. This is equal to the angle GAT, which may be taken by a quadrant, or it may be known for any time by having given the fun's altitude, which is equal to the angle TAQ; which therefore fubducted from the constant angles QAF, or QAY, will always leave the angle of the apparent height of the bow. Lunar RAINBOW.

The moon fometimes alfo exhibits the phænomenon of an iris, by the refraction of her rays in drops of rain in the night-time.

Ariftotle fays, he was the firft that ever obferved it; and adds, that it is never vifible, but at the time of full moon. The lunar iris has all the colours of the folar, only fainter. See Phil. Tranf. n° 331. Marine RAINBOW, the fea-bow, is a phanomenon sometimes obferved in a much agitated fea, when the wind, fweeping part of the tops of the waves, carries them aloft ; fo that the rays of the fun are refracted, &c, as in a common shower.

F. Bourzes, in Phil. Tranf. obferves, that there are fcarce above two colours distinguishable, a dark yellow on the fide next the fun, aud a pale green on the oppofite fide. But there are fometimes twenty or thirty of them feen together. They appear at noon day, and in a pofition oppofite to that of the common bow, the concave fide being turned upwards.

There is a kind of white colourless rainbow which Mentzelius and others faw at noon-day. M. Marriote, in his fourth Effaie de Phyfique, fays, they are formed in mifts, as the others are in fhowers ; having obferved feveral of them both after fun-rifing and in the night.

The want of colours in thefe, is owing to the exceeding tenuity of the vehicles of the vapour, which being only httle watery pellicles bloated with air, the rays of light undergo but little refraction in palling out of air into them. Hence

the rays are reflected from them compounded, as they come.

Rohault mentions coloured rainbows on the grafs formed in the morning dew.

RAISER, in building, a board fet on edge under the fore fide of a step, ftair, &c. See the article STAIR CASE. RAISING, in the manege, one of the three actions of a horfe's legs, the other two being the ftay and the tread. In caprioles, curvets, &c. the railing or lifting up of his leg is elleemed good, if he perform it hardily and with ease; not croffing his legs, nor carrying his feet too much out or in, yet bending his knees as much as is needful.

RAISING-PIECES, or REASON PIECES, in

architecture, are pieces that lie under the beams, and over the pofts or puncheons. RAISINS, grapes prepared by fuffering them to remain on the vine till they are perfectly ripe, and then drying them in the fun, or by the heat of an oven. The difference between raifins dried in the fun, and thofe dried in ovens, is very obvious: the former are sweet and plea.. fant, but the latter have a latent acidity with the fweetnefs that renders them much less agreeable.

The common way of drying grapes for raifins, is to tie two or three bunches of them together while yet on the vine, and dip them into a hot lixivium of woodafhes with a little of the oi! of olives in it. This difpofes them to fhrink and wrinkle, and after this they are left on the vine three or four days feparated on fticks in an horizontal fituation, and then dried in the fun at leifure, after being cut from the tree. The finest and best raifins are thofe called in fome places Damafcus and Jube raifins; which are 'diftinguished from the others by their fize and figures: thefe are flat and wrinkled on the surface, soft and juicy within, and near an inch long, and when fresh and growing on the bunch, are of the fize and fhape of a large olive.

The raifins of the fun, and jar-raisins, are all dried by the heat of the fun, and thefe are the forts used in medicine. However, all the kinds have much the fame virtues; they are all nutritive and balfamic; they are allowed to be attenuant, are faid to be good in nephritic complaints, and are an ingredient in pectoral decoctions, in which cafes, as alfo in all others where altringency is not required

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