An Account of Some Cases of the Production ofColours, Not Hitherto Described. Thomas Young.

                                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.