of the Production of Colours. 391 
that of the colours of halos ; but, on a more minute examination, 
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 production of these fringes ; for, the light trans- 
mitted 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 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. All these circumstances are sufficient to satisfy us with 
respect to the truth of the explanation ; and it is still more con- 
firmed by the effect of inclining the plates to the direction of 
. the light; for then, instead of dilating, like the colours of thin 
plates, these rings contract : and this is the obvious consequence 
of an increase of the length of the paths of the light, which 
now traverses both mediums obliquely ; and the effect is every 
where the same as that of a thicker plate. 
3 E 2 
