upon the Superficies of the second Diaphanum,
and inclines to the Perpendicular Line b, a, the
Refracted Line will be c, g, and the Angle of
Refraction h, c, g. But if it break, and recede
from it, the Refracted Line will be c, j, and the
Angle of Refraction h, c, j. The same Proporti∣onally
may be seen in the 10th Figure.
Note, That this is ever observed in Refractions,
that when the Ray is struck through by the first
Medium, or inclines from the thinner Diaphanum [ 10]
through the thicker Medium, the Refraction is
made to the Perpendicular. But on the contrary,
if it be produc't from a thicker Diaphanum through
a thinner Medium, the Refraction is made from the
Perpendicular, and this is certain, That the Rays
propagated through a thicker Medium are there∣by
rendred the more strong and active, it being
common to all natural Agents to act the more
vehemently in a thicker Subject; so Combustion is
made in Burning-glasses by the help of this sort [ 20]
of Refraction.
The sight is very much help't by Tubes, Pros∣pectives
and other sorts of Optick Glasses, because
by the means thereof the visual Species pass more
strongly and directly to the Eye, by reason of the
long passage, and have all their virtue, the better
rallied, and united together.
Amongst the Effects of Refraction, these are
principally taken notice of, First, that by this
means Things appear greater, or lesser, according [ 30]
as the Refraction is made from the Perpendicular,
or to the Perpendicular. Thus to us who are in
the Air, Fish appear in clear Water bigger and
nearer, because in that case the Refraction is made
from the Perpendicular, so that the Rays the more
they decline from it, the greater Picture they
make in the Eye; whereas we appear less and
more remote to the Fish, by Reason of the con∣trary
Picture in the Eye. Thus by Refraction
the place is apparently changed; in the same [ 40]
manner as the Stars appear to us as risen, when
indeed they are beneath the Horizon. Thus Re∣fraction
changes the Figure, as when by reason
of the Interjected Clouds, the Morning, or Even∣ing
Sun seems under an Eclipse.
The second thing taken notice of amongst the
Effects of Refraction is, that to any one looking
through a Triangular Prisme, there will appear
Objects tinged with strange Colours like a Multi∣plied
Rain-bow; and those things which are in [ 50]
Plano, seem raised upwards in a strange manner.
The Trigon being in a particular manner disposed
to the Eyes. Vision Reflect is what which is made
by the Species, not streight imprest from the
Object, but rather directed from the Object to
the Speculum, and Reflect from the Speculum to
the Eye. Thus in the 7th Figure, we may observe
the Speculum a, b, receiving the Species from
the Object c, the Line c, d, is called the direct
Radius; in d, is the Point of Incidence, the Line [ 60]
d, c, is the Line of Reflection, making a like
Angle of Reflection c, d, b, with the Angle c, d, a,
which is called the Angle of Incidence.
The Speculum is either perfect, and properly
so called, or imperfect, or improperly so called;
the perfect is that which should remit the Species,
wherefore it is to be Terse, P••lite, Equal, and
Opaque, otherwise it will not remit the Species and
Image. The Imperfect is that from which the
Species are not Reflected to the Eye, but rather are as
it were terminated in the Reflecting Body it self.
Of the perfect Specula some are plain, some
Sphaerical, either Convex, or Concave, some Coni∣cal
either Convex, or concave, and some Cylindrical,
Eliptical, Perabolical, &c.
Note, that in a perfect Speculum, that is not
the Species which seems to be, but rather the Ob∣ject
reflext by the Species from the Speculum, be∣cause
otherwise, the same must needs happen as
in imperfect Speculums; for Eample, in Paper
wherein through the Hole of a Chamber, shut
close from Light, the Species are received, and
that apparent Image would be seen in the Specu∣lum,
every way in like manner as the Species
received, whereas it is not seen but in the Line
of the Reflexion.
But then some will say every Speculum ought
to represent the Object under the same Form, and
Magnitude, whereas Experience shews that a
smaller Speculum shews it in a smaller Form
and Magnitude. To which may be answered,
That a plain Speculum, whether little or
great, attributes the same Magnitude to the Ob∣ject,
only with this difference, that a small Ob∣ject
may be seen whole in a plain Speculum, when
a great one cannot. Moreover it may be said,
that the Speculum being divided into parts, the
Object would not be manifoldly represented, for
as much as not the Image in the Speculum, but
only the thing objected by the Speculum, is seen.
To which it may be answered, That the breaking
of the Speculum, if it change the Representation
of the Object; for when it doth change it, it happens
by reason of the changed Situation of its Parts.
It is lastly Objected, that the Speculum being
moved, and the Object unmoved, the Object could
not appear during the Motion; for as the Ob∣ject
only, not the Species is seen in the Speculum,
We answer that the Object unmoved seems then
to be moved, when in this or that part of the
Eye, it is successively Painted; but that the
Motion of the Speculum doth, whence it Repre∣sents
the Object as it were in Motion.
Concave Speculums render the Images greater,
the Convex shew them lesser, whereas the Plain
ones Represent them as they are.
The 11, 12, 13, and 14, Figures make appear
according to Rules of Reflection already delivered,
how an Image perfectly deformed, is represen∣ted
in its true shape in the Cylindricum Speculum,
that is, a Division being made by divers Quadran∣gles
at pleasure, these parts which are designed
in each of these sort of Quadrangles inscribed
in the hidden Circles about the Cylindricum Spe∣culum.