_MADAM_
I perceive you are not fully satisfied with my former Letter concerning
Eccho, and a figure presented in a Looking-glass; for you say, how is
it possible, if Eccho consists in the ears patterning out of a voice
or sound, but that it will make a confusion in all the parts of the
air? My answer is, that I doe not say that Eccho is onely made by the
patterning out of the voice or sound, but by repeating the same voice
or sound, which repetition is named an Eccho, for millions of ears in
animals may pattern out a voice or words, and yet never repeat them,
and so may millions of parts of the air; wherefore Eccho doth not
consist in the bare patterning out, but in the repetition of the same
sound or words, which are pattern'd out; and so some parts of the air
may at one and the same time pattern out a sound and not repeat it,
and some may both pattern out, and repeat it, but some may neither
pattern out, nor repeat it, and therefore the Repetition, not the
bare Patterning out is called Eccho: Just as when two or more men do
answer or mock each other, and repeat each others words, it is not
necessary, if there were a thousand standers by, that they should all
do the same. And as for the figure presented in a Looking-glass, I
cannot conceive it to be made by pressure and reaction; for although
there is both pressure and reaction in nature, and those very frequent
amongst natures Parts, yet they do neither make perception nor
production, although both pressure and reaction are made by corporeal
self-motions; Wherefore the figure presented in a Looking-glass, or
any other smooth glassie body, is, in my opinion, onely made by the
motions of the Looking-glass, which do both pattern out, and present
the figure of an external object in the Glass: But you will say, why do
not the motions of other bodies pattern out, and present the figures
of external objects, as well as smooth glassie bodies do? I answer,
they may pattern out external objects, for any thing I know; but the
reason that their figures are not presented to our eyes, lies partly
in the presenting subject it self, partly in our sight; for it is
observed, that two things are chiefly required in a subject that will
present the figure of an external object; first it must be smooth, even
and glassie, next it must not be transparent: the first is manifest
by experience; for the subject being rough and uneven, will never be
able to present such a figure; as for example, A piece of steel rough
and unpolished, although it may perhaps pattern out the figure of an
external object, yet it will never present its figure, but as soon as
it is polished, and made smooth and glassie, the figure is presently
perceived. But this is to be observed, that smooth and glassie bodies
do not always pattern out exterior objects exactly, but some better,
some worse; like as Painters have not all the same ingenuity; neither
do all eyes pattern out all objects exactly; which proves that the
perception of sight is not made by pressure and reaction, otherwise
there would be no difference, but all eyes would see alike. Next I
say, it is observed, that the subject which will present the figure
of an external object, must not be transparent; the reason is, that
the figure of Light being a substance of a piercing and penetrating
quality, hath more force on transparent, then on other solid dark
bodies, and so disturbs the figure of an external object pattern'd out
in a transparent body, and quite over-masters it. But you will say, you
have found by experience, that if you hold a burning Candle before a
Transparent-glass, although it be in an open Sun-light, yet the figure
of light and flame of the Candle will clearly be seen in the Glass. I
answer, that it is an other thing with the figure of Candle-light, then
of a duskish or dark body; for a Candle-light, though it is not of the
same sort as the Suns light, yet it is of the same nature and quality,
and therefore the Candle-light doth resist and oppose the light of the
Sun, so that it cannot have so much power over it, as over the figures
of other bodies patterned out and presented in Transparent-glass.
Lastly, I say, that the fault oftentimes lies in the perceptive
motions of our sight, which is evident by a plain and Concave-glass;
for in a plain Looking-glass, the further you go from it, the more
your figure presented in the glass seems to draw backward; and in a
Concave-glass, the nearer you go to it, the more seems your figure
to come forth: which effects are like as an house or tree appears to
a Traveller; for, as the man moves from the house or tree, so the
house or tree seems to move from the man; or like one that sails upon
a Ship, who imagines that the Ship stands still, and the Land moves;
when as yet it is the Man and the Ship that moves, and not the House,
or Tree, or the Land; so when a Man turns round in a quick motion,
or when his head is dizzie, he imagines the room or place, where he
is, turns round. Wherefore it is the Inherent Perceptive motions in
the Eye, and not the motions in the Looking-glass, which cause these
effects. And as for several figures that are presented in one glass,
it is absurd to imagine that so many several figures made by so many
several motions should touch the eye; certainly this would make such
a disturbance, if all figures were to enter or but to touch the eye,
as the eye would not perceive any of them, at lead not distinctly;
Wherefore it is most probable that the glass patterns out those
figures, and the sensitive corporeal motions in the eye take again a
pattern from those figures patterned out by the glass, and so make
copies of copies; but the reason why several figures are presented in
one glass in several places, is, that two perfect figures cannot be in
one point, nor made by one motion, but by several corporeal motions.
Concerning a Looking-glass, made in the form or shape of a Cylinder,
why it represents the figure of an external object in an other shape
and posture then the object is, the cause is the shape and form of the
Glass, and not the patterning motions in the Glass. But this discourse
belongs properly to the Opticks, wherefore I will leave it to those
that are versed in that Art, to enquire and search more after the
rational truth thereof. In the mean time, my opinion is, that though
the object is the occasion of the figure presented in a Looking-glass,
yet the figure is made by the motions of the glass or body that
presents it, and that the figure of the glass perhaps may be patterned
out as much by the motions of the object in its own substance, as the
figure of the object is patterned out and presented by the motions of
the glass in its own body or substance. And thus I conclude and rest,
Madam,
_Your Faithful Friend_
_and Servant._