RING. 



instrument used for taking the sun's alti- 

 tude, &c. It is usually of brass, about nine 

 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 its rays, falling 

 through the hole, make a spot among the 

 degrees, which marks the altitude requir- 

 cd. This instrument is preferred before 

 the astrolabe, because the divisions are 

 here larger than on that instrument. See 



ASTROLABK. 



RISC, of Saturn, is a thin, broad, 

 opaque circular arch, encompassing the 

 body of that planet, like the wooden ho- 

 rizon of an artificial globe, without touch- 

 ing it, and appearing double when seen 

 through a good telescope. See SATUIIX. 



RINGS of colours, in optics, a phenome- 

 non first observed in thin plates of vari- 

 ous substances, by Boyle, and Uooke, but 

 afterwards more fully explained by Sir 

 Isaac Newton. Mr. Boyle having exhi- 

 bited a variety of colours in colourless li- 

 quors, 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 n proper shape and size, and also a 

 black ribband, held at a proper distance 

 between his eye and the sun, showed a 

 variety of little rainbows, ns he calls them, 

 with very vivid colours. Dr. Hook, about 

 nine years after the publication of Mr. 

 Boyle's Treatise on Colours, exhibited 

 the 'Coloured bubbles of soup and water, 

 and observed, that though at first it ap- 

 peared 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 mi- 

 croscope, to be ranged in rings surround- 

 ing the white specks or flaws in them, 

 and with the same order of colours as 

 those of the rainbow, and which were of- 

 ten repeated ten times. He likewise 

 took two thin pieces of glass, ground 

 plane and polished, and putting them one 

 upon another, pressed them till there be- 

 gan to appear a red coloured spot in the 

 middle ; and pressing them closer, he ob- 

 served several rings of colours encom- 

 passing the first place, till at last all the 

 colours disappeared out of the middle of 

 tiie circles, and the central spot appear- 

 ed white. The first colour that appear- 



ed was red, then yellow, then green, then 

 blue, then purple; then again red, yel- 

 low, green, blue, and purple ; and again 

 in the same order ; so that he sometimes 

 counted nine or ten of these circ'.fs, 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 

 says, 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, 

 Sec. 



Sir Isaac 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, ac- 

 cording to their disposition to rehVctthis 

 or that species of rays, pursued the hint 

 suggested by the experiments of Dr. 

 Hook, 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 co- 

 lours, which increased, as the motion of 

 the prisms was continued, and bended 

 more and more about the transparent 

 spot, till they were completed into cir- 

 cles, or rings, surrounding it; and after- 

 wards they became continually more and 

 more contracted. He then took two ob- 

 ject-glasses of a telescope, the one plano- 

 convex, the other a little convex on both 

 sides, he placed one of the faces of this 

 upon the plane face of the former, and 

 pressed the two glasses at first gently, 

 and then, by degrees, more closely 

 against one another. The effect of this 

 gradual pressure was an appearance in 

 the plate of air between the glasses of 

 different coloured circles, which had the 

 point of contact for the common centre, 

 rind which increased in number accord- 

 ing to the greater degree of pressure, in 

 such a manner, that the circle which ap- 

 peared last always surrounded the point 

 of contact, and on a still further pressure 

 i-xt ended its circumference, while it con- 



