the base of the cone changes with that of its apex. And here we have
no difficulty in answering a question often asked--namely, whether a
rainbow is ever seen reflected in water. Seeing two bows, the one in
the heavens, the other in the water, you might be disposed to infer
that the one bears the same relation to the other that a tree upon the
water's edge bears to its reflected image. The rays, however, which
reach an observer's eye after reflection from the water, and which
form a bow in the water, would, were their course from the shower
uninterrupted, converge to a point vertically under the observer, and
as far below the level of the water as his eye is above it. But under
no circumstances could an eye above the water-level and one below it
see the same bow--in other words, the self-same drops of rain cannot
form the reflected bow and the bow seen directly in the heavens. The
reflected bow, therefore, is not, in the usual optical sense of the
term, the _image_ of the bow seen in the sky.

Sec. 7. _Analysis and Synthesis of Light. Doctrine of Colours_.

In the rainbow a new phenomenon was introduced--the phenomenon of
colour. And here we arrive at one of those points in the history of
science, when great men's labours so intermingle that it is difficult
to assign to each worker his precise meed of honour. Descartes was at
the threshold of the discovery of the composition of solar light; but
for Newton was reserved the enunciation of the true law. He went to
work in this way: Through the closed window-shutter of a room he
pierced an orifice, and allowed a thin sunbeam to pass through it. The
beam stamped a round white image of the sun on the opposite wall of
the room. In the path of this beam Newton placed a prism, expecting to
see the beam refracted, but also expecting to see the image of the
sun, after refraction, still round. To his astonishment, it was drawn
out to an image with a length five times its breadth. It was,
moreover, no longer white, but divided into bands of different
colours. Newton saw immediately that solar light was _composite_, not
simple. His elongated image revealed to him the fact that some
constituents of the light were more deflected by the prism than
others, and he concluded, therefore, that white light was a mixture of
lights of different colours, possessing different degrees of
refrangibility.

Let us reproduce this celebrated experiment. On the screen is now
stamped a luminous disk