There are many ways by which the retardation necessary to the production of interference is effected. The splendid colours of a soap-bubble are entirely due to interference; the beam falling upon the transparent film is partially reflected at its outer surface, but a portion of it enters the film and is reflected at its _inner_ surface. The latter portion having crossed the film and returned, is retarded, in comparison with the former, and, if the film be of suitable thickness, these two beams will clash and extinguish each other, while another thickness will cause the beams to coincide and illuminate the film with a light of greater intensity. From what has been said it must be manifest that to make two red beams thus coincide a thicker film would be required than would be necessary for two blue or green beams; thus, when the thickness of the bubble is suitable for the development of red, it is not suitable for the development of green, blue, &c.; the consequence is that we have different colours at different parts of the bubble. Owing to its compactness and to its being shaded by a covering of debris from the direct heat of the sun, the ice underneath the moraines of glaciers appears sometimes of a pitchy blackness. While cutting such ice with my axe I have often been surprised and delighted by sudden flashes of coloured light which broke like fire from the mass. These flashes were due to internal rupture, by which fissures were produced as thin as the film of a soap-bubble; the colours being due to the interference of the light reflected from the opposite sides of the fissures. If spirit of turpentine, or olive oil, be thrown upon water, it speedily spreads in a thin film over the surface, and the most gorgeous chromatic phenomena may be thus produced. Oil of lemons is also peculiarly suited to this experiment. If water be placed in a tea-tray, and light of sufficient intensity be suffered to fall upon it, this light will be reflected from the upper and under surfaces of the film of oil, and the colours thus produced may be received upon a screen, and seen at once by many hundred persons. If the oil of cinnamon be used, fine colours are also obtained, and the breaking up of this film exhibits a most interesting case of molecular action. By using a kind of varnish, instead of oil, Mr. Delarue has imparted such tenacity to these films that they may be removed from the water on which they rest and preserved for any length of time. By such films the colours of certain beetles, and of the wings of certain insects, may be accurately imitated; and a rook's feather may be made to shine with magnificent iridescences. The colours of tempered metals, and the beautiful metallochrome of Nobili are also due to a similar cause. [Sidenote: DIFFRACTION.]