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_ 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 novelties.
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 rays, transmitted afterwards by a
hundred other prisms, will never change the colour it bears; in like
manner, as gold, when completely purged from its dross, will never
change afterwards in the crucible. As a superabundant proof that
each of these elementary rays has inherently in itself that which
forms its colour to the eye, take a small piece of yellow wood, for
instance, and set it in the ray of a red colour; this wood will
instantly be tinged red. But set it in the ray of a green colour,
it assumes a green colour, and so of all the rest.
From what cause, therefore, do colours arise in Nature? It is
nothing but the disposition of bodies to reflect the rays of a
certain order and to absorb all the rest.
What, then, is this secret disposition? Sir Isaac Newton
demonstrates that it is nothing more than the density of the small
constituent particles of which a body is composed. And how is this
reflection performed? It was supposed to arise from the rebounding
of the rays, in the same manner as a ball on the surface of a solid
body. But this is a mistake, for Sir Isaac taught the astonished
philosophers that bodies are opaque for no other reason but because
their pores are large, that light reflects on our eyes from the very
bosom of those pores, that the smaller the pores of a body are the
more such a body is transparent. Thus paper, which reflects the
light when dry, transmits it when oiled, because the oil, by filling
its pores, makes them much smaller.
It is there that examining the vast porosity of bodies, every
particle having its pores, and every particle of those particles
having its own, he shows we are not certain that there is a cubic
inch of solid matter in the universe, so far are we from conceiving
what matter is. Having thus divided, as it were, light into its
elements, and carried the sagacity of his discoveries so far as to
prove the method of distinguishing compound colours from such as are
primitive, he shows that these elementary rays, separated by the
prism, are ranged in their order for no other reason but because
they are refracted in that very order; and it is this property
(unknown till he discovered it) of breaking or splitting in this
proportion; it is this unequal refraction of rays, this power of
refracting the red less than the orange colour, &c., which he calls
the different refrangibility. The most reflexible rays are the most
refrangible, and from hence he evinces that the same power is the
cause both of the reflection and refraction of light.
But all these wonders are merely but the opening of his discoveries.
He found out the secret to see the vibrations or fits of light which
come and go incessantly, and which either transmit light or reflect
it, according to the density of the parts they meet with. He has
presumed to calculate the density of the particles of air necessary
between two glasses, the one flat, the other convex on one side, set
one upon the other, in order to operate such a transmission or
reflection, or to form such and such a colour.
From all these combinations he discovers the proportion in which
light acts on bodies and bodies act on light.
He saw light so perfectly, that he has determined to what degree of
perfection the art of increasing it, and of assisting our eyes by
telescopes, can be carried.
Descartes, from a noble confidence that was very excusable,
considering how strongly he was fired at the first discoveries he
made in an art which he almost first found out; Descartes, I say,
hoped to discover in the stars, by the assistance of telescopes,
objects as small as those we discern upon the earth.
But Sir Isaac has shown that dioptric telescopes cannot be brought
to a greater perfection, because of that refraction, and of that
very refrangibility, which at the same time that they bring objects
nearer to us, scatter too much the elementary rays. He has
calculated in these glasses the proportion of the scattering of the
red and of the blue rays; and proceeding so far as to demonstrate
things which were not supposed even to exist, he examines the
inequalities which arise from the shape or figure of the glass, and
that which arises from the refrangibility. He finds that the object
glass of the telescope being convex on one side and flat on the
other, in case the flat side be turned towards the object, the error
which arises from the construction and position of the glass is
above five thousand times less than the error which arises from the
refrangibility; and, therefore, that the shape or figure of the
glasses is not the cause why telescopes cannot be carried to a
greater perfection, but arises wholly from the nature of light.
For this reason he invented a telescope, which discovers objects by
reflection, and not by refraction. Telescopes of this new kind are
very hard to make, and their use is not easy; but, according to the
English, a reflective telescope of but five feet has the same effect
as another of a hundred feet in length. _
Read next: LETTER XVII - ON INFINITES IN GEOMETRY, AND SIR ISAAC NEWTON'S CHRONOLOGY
Read previous: LETTER XV - ON ATTRACTION
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