[Illustration: Fig. 316.--No. 1. A model of waves with moveable rods.--No. 2. A model of fixed waves.--No. 3. Intensity of waves doubled by the superposition and coincidence of two equal systems.--No. 4. Waves neutralized by the superposition and interference of two equal systems, the raised part of one wave accurately fitting into and making smooth the hollow of the other, illustrating the fact that two waves of light or sound may destroy each other.] [Illustration: Fig. 317. Appearance of Newton's rings when produced in yellow light, 1, 3, 5, 7, being the yellow rings, and 2, 4, 6, 8, the dark rings. Light by the odd numbers; darkness by the even numbers. The central spot, where the two surfaces are in contact, is dark.] Returning again to the coloured rings, we find that Newton discovered that at whatever thickness of the film of air the coloured ring first appeared, there would be found at twice that thickness the dark ring, at three times the coloured, at four times the dark, and so on, _the coloured rings_ regularly occurring at the _odd numbers_, and the _dark ones_ at the _even numbers_. This discovery is well illustrated by the models (Fig. 316); and it maybe noticed at No. 3 that the highest and the lowest parts of the waves [Page 332] interfere, but coincide and produce a wave of double intensity; the little crosses of the upper model are in a straight line with the numbers 1, 3, 5, 7, and are supposed to represent the coloured rings, whilst in No. 4 the upper series of waves is half an undulation in advance of the lower; and if the eye is again directed from the little crosses downward, the figures 2, 4, 6, 8, even numbers, are apparent, and represent the dark rings, when the waves of light destroy each other. The phenomena of thin plates, such as colours from soap bubbles, and the films of varnish, are well explained by the law of interference. The light reflected from the second surface of the film of air (which must of course, however thin, have two surfaces, viz., a upper and a lower one) interferes with the light reflected from the first, and as they come from different points of space, one set of waves is in advance of the other, No. 4, Fig. 316; they reach the eye with different lengths of paths, and by their _interference_ form alternately the luminous and dark fringes, bands, or circles. Bridge's diffraction apparatus, manufactured only by Elliott Brothers, offers itself specially as a most beautiful drawing-room optical instrument. The purpose of this apparatus is to illustrate in great variety, and in the most convenient and compact form, the phenomena of the diffraction or interference of light. This is attained by the assistance of photography. Transparent apertures in an opaque collodion film are produced on glass, and a point of light is viewed through the apertures. [Page 333] The forms of the apertures are exceedingly various,--triangles, squares, circles, ellipses, parabolas,