moisture was intermixed with portions of air, producing an appearance similar to dew. I supposed that the origin of colours was <s>[text]<\s> the same as that of the colours of halos; but on a more minute examina= tion, I found that the magnitude of the portions of air and water was by no means uniform, and that the explanation was therefore inadmissible. It was however easy to find two portions of light sufficient for the pro- duction of these fringes; for the light transmitted through the water, moving in it with a velocity different from that of the light passing through the interstices filled only with air, the two portions would interfere with each other, and produce effects of colour according to the general law. The ratio of the velocities in water and in air is that of 3 to 4, the fringes ought therefore to appear where the thickness is <s>[text]<\s> 6 times as great as that which corresponds to the same colour in the common case of thin plates: and, upon making the experiment with a plane glass and a lens slightly convex, I found the sixth dark circle actually of the same diameter as the first in the new fringes. The colours are also very easily produced, when butter or tallow is substituted for water; and the rings then become smaller, on account of the greater refractive density of the oils: but, when water is added, so as to fill up the interstices of the oil, the rings are very much enlarged; for here the difference only of the velocities in water and in oil is to be considered, and this is much smaller than the difference between air and water.
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An Account of Some Cases of the Production ofColours, Not Hitherto Described. Thomas Young., 1800. From The Royal Society, L&P/12/32
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