7141. RAINBOWS, Formation of.—
An
Abbé here [Paris] has shaken, if not destroyed,
the theory of Dominis, Descartes and Newton,
for explaining the phenomenon of the rainbow.
According to that theory, you know, a cone of
rays issuing from the sun, and falling on a
cloud in the opposite part of the heavens, is
reflected back in the form of a smaller cone, the
apex of which is the eye of the observer; so that
the eye of the observer must be in the axis of
both cones, and equally distant from every part
of the bow. But he observes that he has repeatedly
seen bows, the one end of which has
been very near to him, and the other at a very
great distance. I have often seen the same
thing myself. I recollect well to have seen the
end of a rainbow between myself and a house,
or between myself and a bank, not twenty yards
distant; and this repeatedly. But I never saw,
what he says he has seen, different rainbows
at the same time interesting each other. I
never saw coexistent bows, which were not concentric
also. Again, according to the theory,
if the sun is in the horizon, the horizon intercepts
the lower half of the bow; if above the
horizon, that intercepts more than half, in proportion.
So that, generally, the bow is less
than a semi-circle, and never more. He says
he has seen it more than a semi-circle. I have
often seen the leg of the bow below my level.
My situation at Monticello admits this, because
there is a mountain there in the opposite direction
of the afternoon's sun, the valley between
which and Monticello, is five hundred feet deep.
I have seen a leg of a rainbow plunge down on
the river running through the valley. But I do
not recollect to have remarked at any time that
the bow was more than half a circle. It appears
to me that these facts demolish the Newtonian
hypothesis, but they do not support that in its
stead by the Abbé. He supposes a cloud between
the sun and the observer, and that
through some opening in that cloud, the rays
pass, and form an iris on the opposite part of
the heavens, just as a ray passing through a
hole in the shutter of a darkened room, and
falling on a prism there, forms the prismatic
colors on the opposite wall. According to this,
we might see bows of more than the half circle,
as often as of less. A thousand other objections
occur to this hypothesis. * * * The result
is that we were wiser than we were, by having
an error the less in our catalogue.—
To Rev. James Madison. Washington ed. ii, 430.
(P.
1788)
