NEWTON'S RINGS. The colored rings seen when a thin film of air or other substance inter venes between the surfaces of two plates of glass. This phenomenon is named from its dis coverer, Sir tsaac Newton, who in his work on Optics describes how he took a piano-convex lens designed for a fourteen-foot telescope and placed it with its plane side downward on top of a double convex lens constructed for a telescope of about fifty feet in length. On slowly pressing the upper lens against the lower, a number of concentric rings having the point of contact of the lenses as their centre appeared• increasing in size as the pressure was increased. This arrangement of a lens and plane surface is often employed in performing the experiment, and the thickness of the film and the wave lengths of the different kinds of lights can be ascertained. The effect is due to the interference (q.v.) of the waves of light reflected from the upper and lower surfaces of the thin film of air, which from nothing at the point of contact gradually increases in thickness with the distance from the centre. If light of one color, or, speaking more scientifically, of a single wave length, is used• the rings will be alternately bright and dark, the bright waves being produced by the eombination of the various waves caused by interference and overlapping. Therefore the colors of Newton's rings are not pure spectral colors, though they are extremely brilliant and varied. In the centre, where the glass surfaces are in contact, there is a dark spot. and as the air film begins to have an appreciable thickness the rings are formed. They were divided by
Newton into a series of orders, seven in number, though usually nut more than four or five are seen. The colors of the first order, from the central black circle outward, are gray, whitish, straw color, orange, brick-red, and dark purple, the last color corresponding to a thickness of the film of .000011 of an inch or .00028 of a millimeter. In the second order we have the purest colors, wwhic•h run through a succession of violet, blue, peacock, yellow, orange, red, and violet, the thickness of the air film in the last instance amounting to .000022 of an inch or .00055 of a millimeter; in the third and fourth orders we also have a succession of colors: in the fifth we have pale green. pale rose, and rose: in the sixth, pale peacock. and pale rose and rose; and in the seventh, pale green and rose. When the thickness of the film reaches .001 mil limeter the color appears as pale green, and is included in the fourth order, but from this point the colors begin to fade and disappear. By means of these rings we are enabled to measure the distance between two transparent surfaces which are in seeming contact. In case the light passes through the plates instead of being re flected, the colors are reversed, and we have the rings formed with the complementary colors. See LIGHT, sections Interference and Diffraction. Con sult: Thompion, Light risible and Invisible; Preston, Theory of Light (New York, 1894).