§ 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, which may stand for Newton's image of the
sun. Causing the beam (from the aperture L, fig. 7) which produces the
disk to pass through a lens (E), we form a sharp image of the
aperture. Placing in the track of the beam a prism (P), we obtain