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Letters on England

16. On Sir Isaac Newton's Optics
The philosophers of the last age found out a new universe; and a circumstance
which made its discovery more difficult was that no one had so much as
suspected its existence. The most sage and judicious were of opinion that it was
a frantic rashness to dare so much as to imagine that it was possible to guess
the laws by which the celestial bodies move and the manner how light acts.
Galileo, by his astronomical discoveries, Kepler, by his calculation, Descartes (at
least, in his dioptrics), and Sir Isaac Newton, in all his works, severally saw the
mechanism of the springs of the world. The geometricians have subjected infinity
to the laws of calculation. The circulation of the blood in animals, and of the sap
in vegetables, have changed the face of Nature with regard to us. A new kind of
existence has been given to bodies in the air-pump. By the assistance of
telescopes bodies have been brought nearer to one another. Finally, the several
discoveries which Sir Isaac Newton has made on light are equal to the boldest
things which the curiosity of man could expect after so many philosophical
Till Antonio de Dominis the rainbow was considered as an inexplicable miracle.
This philosopher guessed that it was a necessary effect of the sun and rain.
Descartes gained immortal fame by his mathematical explication of this so
natural a phenomenon. He calculated the reflections and refractions of light in
drops of rain. And his sagacity on this occasion was at that time looked upon as
next to divine.
But what would he have said had it been proved to him that he was mistaken in
the nature of light; that he had not the least reason to maintain that it is a globular
body? That it is false to assert that this matter, spreading itself through the whole,
waits only to be projected forward by the sun, in order to be put in action, in like
manner as a long staff acts at one end when pushed forward by the other. That
light is certainly darted by the sun; in fine, that light is transmitted from the sun to
the earth in about seven minutes, though a cannonball, which were not to lose
any of its velocity, could not go that distance in less than twenty-five years. How
great would have been his astonishment had he been told that light does not
reflect directly by impinging against the solid parts of bodies, that bodies are not
transparent when they have large pores, and that a man should arise who would
demonstrate all these paradoxes, and anatomise a single ray of light with more
dexterity than the ablest artist dissects a human body. This man is come. Sir
Isaac Newton has demonstrated to the eye, by the bare assistance of the prism,
that light is a composition of coloured rays, which, being united, form white
colour. A single ray is by him divided into seven, which all fall upon a piece of
linen, or a sheet of white paper, in their order, one above the other, and at
unequal distances. The first is red, the second orange, the third yellow, the fourth
green, the fifth blue, the sixth indigo, the seventh a violet-purple. Each of these