RINGS of colours, in optics, a phenome non first observed in thin plates of vari ous substances, by Boyle, and _Hooke, 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 a 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, as 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 soap 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. De 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 the 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 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 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, &c.
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 reflect this or that species of rays, pursued the hint suggested by the experiments of Dr. Hook, with regard to thin transparent substances. Upon compressingtwo 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 betide(' 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. Ile then took two ob ject-glasses of a telescope, the one piano 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, and which increased in number accord ing to the greater degree of pressure, in such a manlier, that the circle which ap peared last always surrounded the point of contact, and on a still further pressure extended its circumference, while it con.