The expression “ colours of thick plates, ” has been appropriated to a class of phenomena discovered by Newton, and described by him in the fourth part of the second book of his Optics. In Newton's experiment, the sun's light was admitted into a darkened room through a hole in the window-shutter, and allowed to fall perpendicularly upon a concave mirror, formed of glass quicksilvered at the back. A white opaque card pierced with a small hole being then interposed, at the distance of the centre of curvature of the mirror, so that the regularly reflected light returned by the same small hole by which it entered, a set of coloured rings was seen depicted on the card encompassing the hole. The existence of these rings was attributed by Newton to the light scattered on entering the glass, and then regularly reflected and refracted; and he succeeded in deducing from his theory of fits the laws of the rings, both as regards the relation between the diameters of successive rings, the order of the colours, the variation of the diameter of a given ring corresponding to a variation either in the radius of curvature of the surfaces or in the thickness of the glass, and even the absolute magnitude of the system formed under given circumstances.