oblique Racket, describe such a curve line. For, a circular as well as a progressive motion being
communicated to it by that stroak, its parts on that side, where the motions conspire, must press
and beat the contiguous Air more violently than on the other, and there excite a reluctancy and
reaction of the Air proportionably greater. And for the same reason, if the Rays of light should
possibly be globular bodies, and by their oblique passage out of one medium into another acquire a
circulating motion, they ought to feel the greater resistance from the ambient Æther, on that side,
where the motions conspire, and thence be continually bowed to the other. But notwithstanding this
plausible ground of suspition, when I came to examine it, I could observe no such curvity in them.
And besides (which was enough for my purpose) I observed, that the difference 'twixt the length of
the Image, and diameter of the hole, through which the light was transmitted, was proportionable to
their distance.
The gradual removal of these suspitions, at length led me to the Experimentum Crucis, which was
this: I took two boards, and placed one of them close behind the Prisme at the window, so that the
light might pass through a small hole, made in it for the purpose, and fall on the other board, which I
placed at about 12 feet distance, having first made a small hole in it also, for some of that Incident
light to pass through. Then I placed another Prisme behind this second board, so that the light,
trajected through both the boards, might pass through that also, and be again refracted before it
arrived at the wall. This done, I took the first Prisme in my hand, and turned it to and fro slowly
about its Axis, so much as to make the several parts of the Image, cast on the second board,
successively pass through the hole in it, that I might observe to what places on the wall the second
Prisme would refract them. <(3079)> And I saw by the variation of those places, that the light,
tending to that end of the Image, towards which the refraction of the first Prisme was made, did in
the second Prisme suffer a Refraction considerably greater then the light tending to the other end.
And so the true cause of the length of that Image was detected to be no other, then that Light
consists of Rays differently refrangible, which, without any respect to a difference in their incidence,
were, according to their degrees of refrangibility, transmitted towards divers parts of the wall.
When I understood this, I left off my aforesaid Glass works; for I saw, that the perfection of
Telescopes was hitherto limited, not so much for want of glasses truly figured according to the
prescriptions of Optick Authors, (which all men have hitherto imagined,) as because that Light it self
is a Heterogeneous mixture of differently refrangible Rays. So that, were a glass so exactly figured,
as to collect any one sort of rays into one point, it could not collect those also into the same point,
which having the same Incidence upon the same Medium are apt to suffer a different refraction.
Nay, I wondered, that seeing the difference of refrangibility was so great, as I found it, Telescopes
should arrive to that perfection they are now at. For, measuring the refractions in one of my
Prismes, I found, that supposing the common sine of Incidence upon one of its planes was 44 parts,
the sine of refraction of the utmost Rays on the red end of the Colours, made out of the glass into
the Air, would be 68 parts, and the sine of refraction of the utmost rays on the other end, 69 parts:
So that the difference is about a 24th or 25th part of the whole refraction. And consequently, the
object-glass of any Telescope cannot collect all the rays, which come from one point of an object so
as to make them convene at its focus in less room then in a circular space, whose diameter is the
50th part of the Diameter of its Aperture; which is an irregularity, some hundreds of times greater,
then a circularly figured Lens, of so small a section as the Object glasses of long Telescopes are,
would cause by the unfitness of its figure, were Light uniform.
This made me take Reflections into consideration, and finding them regular, so that the Angle of
Reflection of all sorts of Rays was equal to their Angle of Incidence; I understood, that by their
mediation Optick instruments might be brought to any degree of perfection imaginable, provided a
Reflecting substance could be <(3080)> found, which would polish as finely as Glass, and reflect as
much light, as glass transmits, and the art of communicating to it a Parabolick figure be also
attained. But there seemed very great difficulties, and I have almost thought them insuperable,
when I further considered, that every irregularity in a reflecting superficies makes the rays stray 5 or
6 times more out of their due course, than the like irregularities in a refracting one: So that a much
greater curiosity would be here requisite, than in figuring glasses for Refraction.
Amidst these thoughts I was forced from Cambridge by the Intervening Plague, and it was more
then two years, before I proceeded further. But then having thought on a tender way of polishing,
proper for metall, whereby, as I imagined, the figure also would be corrected to the last; I began to
try, what might be effected in this kind, and by degrees so far perfected an Instrument (in the
essential parts of it like that I sent to London,) by which I could discern Jupiters 4 Concomitants,