All transparent bodies show a beautiful play of color, when they are reduced to sufficiently thin layers. This is observed most easily in soap-bubbles and in the thin films of glass which a glass-blower produces when he expands the glass ball he is blowing, until it bursts. The same is seen when a drop of any ethereal oil, say oil of turpentine, is placed upon water so as to expand upon it and make a very thin layer. It is also seen when a bright piece of metal, say steel, is heated for the purpose of tempering; the film of oxid formed shows colors varying with the thickness of the film. So too when a heavy piece of glass is cracked, the thin film (if or in the crack will show the same appearance of various colors.
The best way to produce these colors in regular order, and the method of observing the thickness of the films producing them, was invented by Sir Isaac Newton,who, by placing a curved lens upon a flat glass table, produced colored rings, which have been named after him — the rings of Newton. In order to do this successfully the lens must have is very feeble curvature, or, in other words, a long focus, say from 40 to 60 feet; or a convex lens with a curvature of say 20 inches, may be laid on a concave lens of 21 inches, which gives results about equivalent to a lens of 50 feet focal length laid upon a plane surface. When pressed down by proper appliances, such as screws along the edges, it series of concentric colored rings are seen, of which the adjoined illustration gives some idea, while the following details may be observed.
In the center where the contact takes place, reflected light shows a dark spot; around this is a ring of bluish-white, then yellowish-white, brownish-orange, red, another ring of violet, blue, yellowish-green, yellowish-red; then purple-red, blue, yellowish-green, red, carmine-red; then greenish-blue, pale green, yellowish-green, red, etc.
The exterior rings become narrower and narrower, as seen in the figure, and are alternately pale green and pale red—they become more and more faint, so that as a rule only eight or nine rings can be distinguished. The details here described can only he seen when the lenses used are such as to produce large rings; when lenses of short focus, such as spectacle-glasses, are pressed upon a piece of flat plate-glass, the rings form so small a spot that details cannot well be observed with the naked eye; but then a magnifying-glass forty be used and they may be thus seen enlarged.
Instead of rings, the same succession of colors may be observed in bands produced by two pieces of thin plate-glass from 5 to 6 inches long and 1 to 2 inches wide. At. one end they are separated by a strip of gold leaf, at the other end they touch. When pressed together the colors appear in the regular order above described, the dark spot at. the point of contact at one end, then the bluish and yellowish white, orange-red, etc., in regular succession.
Newton measured, with the greatest accuracy, the thickness of the layers producing these colors, which is very simple when the curvatures of the lenses are known. His results for the center of the first and brightest ring are embodied in the following table:
Name of Color - Thickness of the layer of Air.
Extreme red ... 6.344 inches
Limit between red and orange ... 6.868 "
" " orange and yellow ... 5.615 "
" " yellow and green ... 5.237 "
" " green and blue ... 4.841 "
" " blue and indigo ... 4.513
" " indigo and violet ... 4.823 "
Extreme violet ... 3.947 "
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