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.