Sir Isaac Newton brought his powerful intellect to bear on these facts, and as a preliminary step invented an instrument for measuring the exact thickness of those transparent substances that afforded colour, and the apparatus displaying Newton's rings is still a favourite optical experiment. It consists of a plano-convex lens, A. (Fig. 314), a slice, as it were, from a globe of glass twenty-eight feet in diameter, or the radius of whose convex surface is fourteen feet. This plano-convex lens is placed on another double convex lens, B., whose convex surfaces have a radius of fifty feet each, consequently the lenses are very shallow, and the space (C C) included between them being filled with air, can of course be accurately measured. (Fig. 314.) It is usual to mount the lenses in brass rings which are brought together with screws, when the most beautiful coloured rings are apparent, and are produced by the extreme thinness of the film or plate of air enclosed between the two lenses; and [Page 329] the relative thicknesses of the plates of air at which each coloured light is reflected are as follows:-- Red 133 10 millionths of an inch. Orange 120 " " Yellow 113½ " " Green 105½ " " Blue 98 " " Indigo 92½ " " Violet 83½ " " By dividing an inch into ten millions of parts, and by taking 133 of such parts, the thickness of the film of air required to reflect the red ray is obtained, and in like manner the other colours require the minute thicknesses of air recorded in the table above. When the thickness of the film of air is about 12/178,000dths of an inch, the colours cease to become visible, owing to the union of all the separate colours forming white light, but if the Newton rings are produced in mono-chromatic light, then a greater number of rings are apparent, but of one colour only, and alternating with black rings, _i.e._, a dark and a yellow succeeding each other; this fact is of great importance as an illustration of the undulatory theory, and demonstrates the important truth, that _two rays of light may interfere with each other in such a manner as to produce darkness_. Sir David Brewster remarks that, "From his experiments on the colours of thin and of thick plates, Newton inferred that they were produced by a singular property of the particles of light, in virtue of which they possess, at different joints of their paths, _fits_ or dispositions to be reflected from or transmitted by transparent bodies. Sir Isaac does not pretend to explain the origin of these _fits_, or the cause which produces them, but terms them _fits of transmission_ and _fits of reflexion_."