RING
, in Astronomy and Navigation, an instrument used for taking the sun's altitude &c. It is usually of brass, about 9 inches diameter, suspended by a little swivel, at the distance of 45° from the point of which is a perforation, which is the centre of a quadrant of 90° divided in the inner concave surface.
To use it, let it be held up by the swivel, and turned round to the sun, till his rays, falling through the hole, make a spot among the degrees, which marks the altitude required.
This instrument is preferred before the astrolabe, because the divisions are here larger than on that instrument.
Ring, of Saturn, is a thin, broad, opaque circular arch, encompassing the body of that planet, like the wooden horizon of an artificial globe, without touching it, and appearing double, when seen through a good telescope.
This Ring was sirst discovered by Huygens, who, after frequent observation of the planet, perceived two lucid points, like ansæ or handles, arising out from the body in a right line. Hence as in subsequent observations he always found the fame appearance, he concluded that Saturn was encompassed with a permanent Ring; and accordingly produced his New System of Saturn, in 1659. However, Galileo sirst discovered that the figure of Saturn was not round.
Huygens makes the space between the globe of Saturn and the Ring equal to the breadth of the Ring, or rather more, being about 22000 miles broad; and the greatest diameter of the Ring, in proportion to that of the globe, as 9 to 4 But Mr. Pound, by an excellent micrometer applied to the Huygenian glass of 123 feet, determined this proportion, more exactly, to be as 7 to 3.
Observations have also determined, that the plane of the Ring is inclined to the plane of the ecliptic in an angle of 30 degrees; that the Ring probably turns, in the direction of its plane, round its axis, because when it is almost edgewise to us, it appears rather thicker on one side of the planet than on the other; and the thickest edge has been seen on different sides at different times: the sun shines almost 15 of our years together on one side of Saturn's Ring without setting, and as long on the other in its turn; so that the Ring is visible to the inhabitants of that planet for almost 15 | of our years, and as long invisible, by turns, if its axis has no inclination to its Ring; but if the axis of the planet be inclined to the Ring, ex. gr. about 30 degrees, the Ring will appear and disappear once every natural day to all the inhabitants within 30 degrees of the equator, on both sides, frequently eclipsing the sun in a Saturnian day. Moreover, if Saturn's axis be so inclined to his Ring, it is perpendicular to his orbit; by which the inconvenience of different seasons to that planet is avoided.
This Ring, seen from Saturn, appears like a large luminous arch in the heavens, as if it did not belong to the planet.
When we see the Ring most open, its shadow upon the planet is broadest; and from that time the shadow grows narrower, as the Ring appears to do to us; until, by Saturn's annual motion, the sun comes to the plane of the Ring, or even with its edge; which, being then directed towards us, becomes invisible, on account of its thinness.
The phenomena of Saturn's Ring are illustrated by a view of this figure. Let S be the sun, ABCDEFGH Saturn's orbit, and IKLMNO the earth's orbit. Both Saturn and the earth move according to the order of the letters; and when Saturn is at A, his Ring is turned edgewise to the sun S, and he is then seen from the earth as if he had lost his Ring, let the earth be in any part of its orbit whatever, except between N and O; for whilst it describes that space, Saturn is apparently so near the sun as to be hid in his beams. As Saturn goes from A to C, his Ring appears more and more open to the earth; at C the Ring appears most open of all; and seems to grow narrower and narrower as Saturn goes from C to E; and when he comes to E, the Ring is again turned edgewise both to the sun and earth; and as neither of its sides is illuminated, it is invisible to us, because its edge is too thin to be perceptible; and Saturn appears again as if he had lost his Ring. But as he goes from E to G, his Ring opens more and more to our view on the under side; and seems just as open at G as it was at C, and may be seen in the night time from the earth in any part of its orbit, except about M, when the sun hides the planet from our view.
As Saturn goes from G to A, his Ring turns more and more edgewise to us, and, therefore, it seems to grow narrower and narrower; and at A it disappears as before.
Hence, while Saturn goes from A to E, the sun shines on the upper side of his Ring, and the under side is dark; and whilst he goes from E to A, the sun shines on the under side of his Ring, and the upper side is dark. The Ring disappears twice in every annual revolution of Saturn, viz, when he is in the 19th degree of Pisces and of Virgo, and when Saturn is in the middle between these points, or in the 19th degree either of Gemini or of Sagittarius, his Ring appears most open to us; and then its longest diameter is to its shortest, as 9 to 4. Ferguson's Astr. sect. 204.
There are various hypotheses concerning this Ring. Kepler, in his Epitom. Astron. Copern. and after him Dr. Halley, in his Enquiry into the Causes of the Variation of the Needle, Phil. Trans. No 195, suppose our earth may be composed of several crusts or shells, one within another, and concentric to each other. If this be the case, it is possible the Ring of Saturn may be the fragment or remaining ruin of his formerly exterior shell, the rest of which is broken or fallen down upon the body of the planet. And some have supposed that the Ring may be a congeries or series of moons revolving about the planet.
Later observations have thrown much more light upon this curious phenomenon, especially respecting its dimensions, and rotation, and division into two or more parts. De la Lande and De la Place say, that Cassini saw the breadth of the Ring divided into two separate parts that are equal, or nearly so. Mr. Short assured M. De la Lande, that he had seen many divisions upon the Ring, with his 12 feet telescope. And Mr. Hadley, with an excellent 5 1/2 feet reflector, saw the Ring divided into two parts. Several excellent theories have been given in the French Memoirs, particularly by De la Place, contending for the division of the Ring into many parts. But finally the observations of Dr. Herschel, in several volumes of the Philos. Trans. seem to confirm the division into two concentric parts only. The dimensions of these two Rings, and the space between them, he states in the following proportion to each other.
| Miles. | |
| Inner diam. of smaller Ring | 146345 |
| Outside diam. of ditto | 184393 |
| Inner diam. of larger Ring | 190248 |
| Outside diam. of ditto | 204883 |
| Breadth of the inner Ring | 20000 |
| Breadth of the outer Ring | 7200 |
| Breadth of the vacant space | 2839 |
| Ring revolves in its own plane, in 10h 32′ 15″.4. |
Dr. Herschel adds, Some theories and observations, | of other persons, “lead us to consider the question, whether the construction of this Ring is of a nature so as permanently to remain in its present state? or whether it be liable to continual and frequent changes, in such a manner as in the course of not many years, to be seen subdivided into narrow slips, and then again as united into one or two circular planes only. Now, without entering into a discussion, the mind seems to revolt, even at first sight, against an idea of the chaotic state in which so large a mass as the Ring of Saturn must needs be, if phenomena like these can be admitted. Nor ought we to indulge a suspicion of this being a reality, unless repeated and well-confirmed observations had proved, beyond a doubt, that this Ring was actually in so fluctuating a condition.” But from his own observations he concludes, “It does not appear to me that there is a sufficient ground for admitting the Ring of Saturn to be of a very changeable nature, and I guess that its phenomena will hereafter be so fully explained, as to reconcile all observations. In the mean while, we must withhold a final judgment of its construction, till we can have more observations. Its division however into two very unequal parts, can admit of no doubt.” See Philos. Trans. vol. 80 pa. 4, 481 &c, and the vol. for 1792, pa. 1 &c. also Hist. de l'Acad. des Scienc. de Paris, 1787, pa. 249 &c.
Rings of Colours, in Optics, a phenomenon first observed in thin plates of various substances, by Boyle, and Hook, but afterwards more fully explained by Newton.
Mr. Boyle having exhibited a variety of colours in colourless liquors, by shaking them till they rose in bubbles, as well as in bubbles of soap and water, and also in turpentine, procured glass blown so thin as to exhibit similar colours; and he observes, that a feather of a proper shape and size, and also a black ribband, held at a proper distance between his eye and the sun, shewed a variety of little rainbows, as he calls them, with very vivid colours. Boyle's Works by Shaw, vol. 2, p. 70. Dr. Hook, about nine years after the publication of Mr. Boyle's Treatise on Colours, exhibited the coloured bubbles of soap and water, and observed, that though at first it appeared white and clear, yet as the film of water became thinner, there appeared upon it all the colours of the rainbow. He also described the beautiful colours that appear in thin plates of Muscovy glass; which appeared, through the microscope, to be ranged in Rings surrounding the white specks or flaws in them, and with the same order of colours as those of the rainbow, and which were often repeated ten times. He also took two thin pieces of glass, ground plane and polished, and putting them one upon another, pressed them till there began to appear a red coloured spot in the middle; and pressing them closer, he observed several Rings of colours encompassing the first place, till, at last, all the colours disappeared out of the middle of the circles, and the central spot appeared white. The first colour that appeared was red, then yellow, then green, then blue, then purple; then again red, yellow, green, blue, and purple; and again in the same order; so that he sometimes counted nine or ten of these circles, the red immediately next to the purple; and the last colour that appeared before the white was blue; so that it began with red, and ended with purple. These Rings, he fays, would change their places, by changing the position of the eye, so that, the glasses remaining the same, that part which was red in one position of the eye, was blue in a second, green in the third, &c. Birch's Hist. of the Royal Society, vol. 3, pa. 54.
Newton, having demonstrated that every different colour consists of rays which have a different and specific degree of refrangibility, and that natural bodies appear of this or that colour, according to their disposition to reflect this or that species of rays (see Colour), pursued the hint suggested by the experiments of Dr. Hook, already recited, and casually noticed by himself, with regard to thin transparent substances. Upon compressing two prisms hard together, in order to make their sides touch one another, he observed, that in the place of contact they were perfectly transparent, which appeared like a dark spot, and when it was looked through, it seemed like a hole in that air, which was formed into a thin plate, by being impressed between the glasses. When this plate of air, by turning the prisms about their common axis, became so little inclined to the incident rays, that some of them began to be transmitted, there arose in it many slender arcs of colours, which increased, as the motion of the prisms was continued, and bended more and more about the transparent spot, till they were completed into circles, or Rings, surrounding it; and afterwards they became continually more and more contracted.
By another experiment, with two object glasses, he was enabled to observe distinctly the order and quality of the colours from the central spot, to a very considerable distance. Next to the pellucid central spot, made by the contact of the glasses, succeeded blue, white, yellow, and red. The next circuit immediately surrounding these, consisted of violet, blue, green, yellow, and red. The third circle of colours was purple, blue, green, yellow, and red. The fourth circle consisted of green and red. All the succeeding colours became more and more imperfect and dilute, till, after three or four revolutions, they ended in perfect whiteness.
When these Rings were examined in a darkened room, by the coloured light of a prism cast on a sheet of white paper, they became more distinct, and visible to a far greater number than in the open air. He sometimes saw more than twenty of them, whereas in the open air he could not discern above eight or nine.
From other curious observations on these Rings, made by different kinds of light thrown upon them, he inferred, that the thicknesses of the air between the glasses, where the Rings are successively made, by the limits of the seven colours, red, orange, yellow, green, blue, indigo, and violet, in order, are one to another as the cube roots of the squares of the eight lengths of a chord, which sound the notes in an octave, sol, la, fa, sol, la, mi, fa, sol; that is, as the cube roots of the squares of the numbers 1, 8/9, 5/6, 3/4, 2/3, 3/5, 9/6, 1/2. These Rings appeared of that prismatic colour, with which they were illuminated, and by projecting the prismatic colours immediately upon the glasses, he found that the light, which fell on the dark spaces between | the coloured Rings, was transmitted through the glasses without any change of colour. From this circumstance he thought that the origin of these Rings is manifest; because the air between the glasses is disposed according to its various thickness, in some places to reflect, and in others to transmit the light of any particular colour, and in the same place to reflect that of one colour, where it transmits that of another.
In examining the phenomena of colours made by a denser medium surrounded by a rarer, such as those which appear in plates of Muscovy glass, bubbles of soap and water, &c, the colours were found to be much more vivid than the others, which were made with a rarer medium surrounded by a denser.
From the preceding phenomena it is an obvious deduction, that the transparent parts of bodies, according to their several series, reflect rays of one colour and transmit those of another; on the same account that thin plates, or bubbles, reflect or transmit those rays, and this Newton supposed to be the reason of all their colours. Hence also he has inferred, that the size of those component parts of natural bodies that affect the light, may be conjectured by their colours. See COLOUR and Reflection.
Newton, pursuing his discoveries concerning the colours of thin substances, found that the same were also produced by plates of a considerable thickness, divisible into lesser thicknesses. The Rings formed in both cases have the same origin, with this difference, that those of the thin plates are made by the alternate reflexions and transmissions of the rays at the second surface of the plate, after one passage through it; but that, in the case of a glass speculum, concave on one side, and convex on the other, and quicksilvered over on the convex side, the rays go through the plate and return before they are alternately reflected and transmitted. Newton's Optics, p. 169, &c. or Newton's Opera, Horsley's edit. vol. 4, p. 121, &c. p. 184, &c.
The abbé Mazeas, in his experiments on the Rings of colours that appear in thin plates, has discovered several important circumstances attending them, which were overlooked by the sagacious Newton, and which tend to invalidate his theory for explaining them. In rubbing the flat side of an object-glass against another piece of flat and smooth glass, he found that they adhered very firmly together after this friction, and that the same colours were exhibited between these plane glasses, which Newton had observed between the convex object glass of a telescope, and another that was plane; and that the colours were in proportion to their adhesion. When the surfaces of pieces of glass, that are transparent and well polished, are equally pressed, a resistance will be perceived; and wherever this is felt, two or three very fine curve lines will be discovered, some of a pale red, and others of a faint green. If the friction be continued, the red and green lines increase in number at the place of contact; the colours being sometimes mixed without any order, and sometimes disposed in a regular manner; in which case the coloured lines are generally concentric circles, or ovals, more or less elongated, as the surfaces are more or less united.
When the colours are formed, the glassesadhere with considerable force; but if the glasses be separated suddenly, the colours will appear immediately upon their being put together, without the least friction. Beginning with the slightest touch, and increasing the pressure by insensible degrees, there first appears an oval plate of a faint red, and in the centre of it a spot of light green, which enlarges by the pressure, and becomes a green oval, with a red spot in the centre; and this enlarging in its turn, discovers a green spot in its centre. Thus the red and green succeed one another in turns, assuming different shades, and having other colours mixed with them. The greatest difference between these colours exhibited between plane surfaces, and those by curve ones, is, that, in the former case, pressure alone will not produce them, except in the case above mentioned.
In rubbing together two prisms, with very small refracting angles, which were joined so as to form a parallelopiped, the colours appeared with a surprising lustre at the places of contact, and differently coloured ovals appeared.
In the centre there was a black spot, bordered by a deep purple; next to this appeared violet, blue, orange, red tinged with purple, light green, and faint purple.
The other Rings appeared to the naked eye to consist of nothing but faint reds and greens. When these coloured glasses were suspended over the flame of a candle, the colours disappeared suddenly, though they still adhered; but being suffered to cool, the colours returned to their former places, in the same order as before. At first the abbé Mazeas had no doubt but that these colours were owing to a thin plate of air between the glasses, to which Newton has aferibed them; but the remarkable difference in the circumstances attending those produced by the flat plates and those produced by the object glasses of Newton, convinced him that the air was not the cause of this appearance. The colours of the flat plates vanished at the approach of flame, but those of the object glasses did not. Nor was this difference owing to the plane glasses being less compressed than the convex ones; for though the former were compressed ever so much by a pair of forceps, it did not in the least hinder the effect of the flame. Afterwards he put both the plane glasses and the convex ones into the receiver of an air-pump, suspending the former by a thread, and keeping the latter compressed by two strings; but he observed no change in the colours of either of them, in the most perfect vacuum that he could make. Suspecting still that the air adhered to the surface of the glasses, so as not to be separated from them by the force of the pump, he had recourse to other experiments, which rendered it still more improbable that the air should be the cause of these colours. Having laid the coloured plates, after warming them gradually, on burning coals; and thus, when they were nearly red, rubbing them together, he observed the same coloured circles and ovals as before. When he ceased to press upon them, the colours seemed to vanish; but they returned, as he renewed the friction. In order to determine whether the colours were owing to the thickness of some matter interposed between the glasses, he rubbed them toge- | ther with suet and other soft substances between them; yet his endeavour to produce the colours had no effect. However by continuing the friction with some degree of violence, he observed, that a candle appeared through them encompassed with two or three concentric greens, and with a lively red inclining to yellow, and a green like that of an emerald, and at length the Rings assumed the colours of blue, yellow, and violet. The abbé was confirmed in his opinion that there must be some error in Newton's hypothesis, by considering that, according to his measures, the colours of the plates varied with the difference of a millionth part of an inch; whereas he was satisfied that there must have been much greater differences in the distance between his glasses, when the colours remained unchanged. From other experiments he concluded, that the plate of water introduced between the glasses was not the cause of their colours, as Newton apprehended; and that the coloured Rings could not be owing to the compression of the glasses. After all, he adds, that the theory of light, thus reflected from thin plates, is too delicate a subject to be completely ascertained by a small number of observations. Berlin Mem. for 1752, or Memoires Presentes, vol. 2, pa. 28—43. M. du Tour repeated the experiments of the abbé Mazeas, and added some observations of his own. See Mem. Pres. vol. 4, pa. 288.
Musschenbroeck is also of opinion, that the colours of thin plates do not depend upon the air; but as to the cause of them, he acknowledges that he could not satisfy himself about it. Introd. ad Phil. Nat. vol. 2, p. 738.
See on this subject Priestley's Hist. of Light, &c. per. 6, sect. 5, pa. 498, &c.
For an account of the Rings of colours produced by electrical explosions, see Colours of natural bodies, CIRCULAR spots, and Fairy circles.
