We have beautiful natural illustrations of luminous refraction in the rainbow and in the halo: in both cases the rays of light being separated by the refractive power of the falling rain drop, or the vesicles which form the moisture constituting a fog. In the simple toy of the child--the soap-bubble floating upon the air--the philosopher finds subjects for his contemplation; and from the unrivalled play of colours which he discovers in that attenuated film, he learns that the varying thicknesses of surfaces influence, in a most remarkable manner, the colours of the sunbeam. Films of oil floating upon water present similar appearances; and the colours developed in tempering steel are due entirely to the thickness of the oxidized surface produced by heat. There have lately been introduced some beautiful specimens of paper rendered richly iridescent by the following process:--A solution of a gum resin in chloroform is floated upon water, where it forms a film giving all the colours of Newton’s rings. A sheet of paper which has been previously sunk in the water is carefully lifted, and the film thus removed adheres with great firmness to the paper, and produces this rich and curious play of colour. The rich tints upon mother-of-pearl, in the feathers of many birds, the rings seen in the cracks of rock-crystal, or between the unequal faces of two pieces of glass, and produced by many chemical and indeed mechanical operations--are all owing to the same cause;--the refraction of the luminous pencil by the condition of the film or surface. If we take one of those steel ornaments which are formed by being covered with an immense number of fine lines, it will be evident that these striæ present many different angles of reflection, and that, consequently, the rays thrown back will, at some point or another, have a tendency to cross each other. The result of this is, that the quantity of light is augmented at some points of intersection, and annihilated at others.[97] Out of the investigation of the phenomena of diffraction, of the effects of thin and thick plates upon light, and the results of interference, has arisen the discovery of one of the most remarkable conditions within the range of physical science.