e

R

that is, in the progrefs from E to F, or from the infide of the bow to the outfide, in this order, violet, indigo, blue, green, yellow, orange, red. But the violet, by mixture of the white light of the clouds, will appear faint, and inclined to purple. Again, the angle s Go being equal to the angle POG, or 50° 57', will be the leaft angle in which the leaft refrangible rays can, after two reflections, emerge out of the drops, and therefore the leaft refrangible rays must come moft copiously to the eye from the drops in the line o G, and ftrike the fenfe with the deepest red in that region. And the angle SHO being equal to the angle POH, or 54° 7', will be the leaft angle in which the most refrangible rays, after two reflections, can emerge out of the drops, and therefore those rays must come most copiously to the eye from the drops in the line o H, and ftrike the fenfe with the deepest violet in that region. And, by the fame argument, the drops in the regions between G and H will ftrike the fenfe with the intermediate colours in the order which their degrees of refrangibility require; that is, in the progrefs from G to H, or from the infide of the bow to the outfide in this order, red, orange, yellow, green, blue, indigo, and violet. And fince the four lines OE, OF, OG, OH, may be fituated any where in the above-mentioned conical fuperficies, what is faid of the drops and colours in thefe lines is to be underftood of the drops and colours every where in those fuperficies.

Thus there will be made two bows of colours,

an

an interior and ftronger, by one reflection in the drops, and an exterior and fainter by two; for the light becomes fainter by every reflection; and their colours will lie in a contrary order to each other, the red of both bows bordering upon the space GF, which is between the bows. The breadth of the interior bow, EOF, measured crófs the colours, will be 1° 45', and the breadth of the exterior, GOH, will be 3°10', and the distance between them GOF, will be 8° 55', the greatest femidiameter of the innermoft, that is, the angle POF, being 42° 2′, and the leaft femidiameter of the outermoft POG being 50° 57'. These are the meafures of the bows, as they would be, were the Sun but a point; for, by the breadth of its body, the breadth of the bows will be increased, and their distance diminished by half a degree, and fo the breadth of the interior iris will be 2° 15', that of the exterior 3° 40', their distance 8° 25'; the greateft femidiameter of the interior bow 42° 17', and the leaft of the exterior 50° 42'. And fuch are the dimenfions of the bows in the heavens found to be very nearly, when their colours appear strong and perfect.

The light which comes through drops of rain by s two refractions without any reflection ought to appear strongest at the diftance of about 26 degrees from the Sun, and to decay gradually both ways as the distance from the Sun increases and decreases. And the fame is to be understood of light tranf mitted through spherical hail ftones. And if the

T

hail be a little flatted, as it often is, the light tranfmitted may grow fo ftrong at a little lefs diftance than that of 26 degrees, as to form a halo about the Sun and Moon; which halo, as often as the ftones are duly figured, may be coloured, and then it must be red within, by the leaft refrangible rays, and blue without, by the most refrangible ones.

The light, which paffes through a drop of rain after two refractions, and three or more reflections, is scarcely strong enough to cause a fenfible bow.

CHAP. VI.

OF THE SEPARATION OF THE ORIGINAL RAYS OF LIGHT BY REFLECTION OR TRANSMISSION, THAT DEPENDS ON THE THICKNESS OF THE MEDIUM UPON WHICH THEY ARE INCIDENT.

U THE original or component rays of light are separable from each other, not only by refraction, or by varying the angle of incidence on a reflecting furface, but are likewife at like incidences more.or lefs reflexible, according to the thickness or distance between the two furfaces of the medium on which v they fall. They are alfo liable to be turned out of their direct course by approaching within a certain distance from a body, by which means a feparation enfues, the rays being more or lefs deflected as they

differ in colour. Of thefe circumftances it will be proper to give fome account.

If a glafs or lens, whofe furface is convex, or a w portion of a sphere, be laid upon another plain glafs, it is evident that it will rest or touch at one particular point only; and therefore, that at all other places between the adjacent furfaces will be interposed a thin plate of air, the thickness of which will increase in a certain ratio, according to the distance from the point of contact; that is to fay, in arcs whofe verfed fines are very small, as the diameter of the fphere is to the fine of the arc, fo is that fine to the verfed fine or thickness of the air at the distance measured by the fine.

Light incident upon fuch a plate of air is dif- x pofed to be tranfmitted or reflected according to its thickness: thus, at the center of contact, the light is tranfmitted, and a black circular spot appears; this fpot is environed by a circle, the colours of which, reckoning from the internal part, are blue, white, yellow, red; then follows another circular feries, viz. violet, blue, green yellow, red; then purple, blue, green, yellow, red; green, red; greenish blue, red; greenish blue, pale red; greenith blue, reddish white.

These are the colours which appear by reflec- Y tion: by the tranfmitted light the following feries are feen. At the centre white, then yellowish red, black; violet, blue, white, yellow, red; violet, blue, green, yellow, red, &c. fo that the transmitted light at any thickness, inftead of white,

appears

1

appears of the compounded colour which it ought to have after the fubtraction of fome of the conftituent colours by reflection; after which feries the colours become too faint and dilute to be discerned. z It is obfervable, that the glaffes will not come into contact without a confiderable degree of preffure. A By admeasurement it appears, that the rays of any particular colour are difpofed to be reflected when the thickneffes of the plate of air are as the numbers 1, 3, 5, 9, 11, &c. and that the fame rays are difpofed to be tranfmitted at the intermediate thickneffes which are as the numbers O, 2, 4, 6, 8, 10, &c.

B

The places of reflection or tranfmiffion of the.. feveral colours in a feries are fo near each other, that the colours dilute each other by mixture, whence the number of feries in the open day-light feldom exceeds feven or eight: but if the fyftem be viewed through a prifm, by which means the rings of various colours are feparated according to their refrangibilities, they may be feen on that fide towards which the refraction is made, fo numerous, c that it is impoffible to count them. Or, if in a

dark chamber the Sun's light be feparated into its original rays by a prifm, and a ray of one uncompounded colour be received upon the two glaffes heretofore described, the number of circles will become very numerous, and both the reflected and tranfmitted light will remain of the fame colour as the original incident ray. In this experiment it alfo is feen, that in any feries, the circles formed