appearance of the horizontal moon is far larger than usual, which greatly
conspires with and strengthens our opinion. Neither would it prove in the
least irreconcilable with what we have said, if the horizontal moon should
chance sometimes to seem enlarged beyond its usual extent, even in more
serene weather. For, we must not only have regard to the mist which
happens to be in the place where we stand; we ought also to take into our
thoughts the whole sum of vapours and exhalations which lie betwixt the
eye and the moon: all which co-operating to render the appearance of the
moon more faint, and thereby increase its magnitude, it may chance to
appear greater than it usually does even in the horizontal position, at a
time when, though there be no extraordinary fog or haziness just in the
place where we stand, yet the air between the eye and the moon, taken
altogether, may be loaded with a greater quantity of interspersed vapours
and exhalations than at other times(360).
72. It may be objected that, in consequence of our principles, the
interposition of a body in some degree opaque, which may intercept a great
part of the rays of light, should render the appearance of the moon in the
meridian as large as when it is viewed in the horizon. To which I answer,
it is not faintness anyhow applied that suggests greater magnitude; there
being no necessary, but only an experimental, connexion between those two
things. It follows that the faintness which enlarges the appearance must
be applied in such sort, and with such circumstances, as have been
observed to attend the vision of great magnitudes. When from a distance we
behold great objects, the particles of the intermediate air and vapours,
which are themselves unperceivable, do interrupt the rays of light, and
thereby render the appearance less strong and vivid. Now, faintness of
appearance, caused in this sort, hath been experienced to co-exist with
great magnitude. But when it is caused by the interposition of an opaque
sensible body, this circumstance alters the case; so that a faint
appearance this way caused does not suggest greater magnitude, because it
hath not been experienced to co-exist with it.
73. Faintness, as well as all other ideas or perceptions which suggest
magnitude or distance, does it in the same way that words suggest the
notions to which they are annexed. Now, it is known a word pronounced with
certain circumstances, or in a certain context with other words, hath not
always the same import and signification that it hath when pronounced in
some other circumstances, or different context of words. The very same
visible appearance, as to faintness and all other respects, if placed on
high, shall not suggest the same magnitude that it would if it were seen
at an equal distance on a level with the eye. The reason whereof is, that
we are rarely accustomed to view objects at a great height; our concerns
lie among things situated rather before than above us; and accordingly our
eyes are not placed on the top of our heads, but in such a position as is
most convenient for us to see distant objects standing in our way. And,
this situation of them being a circumstance which usually attends the
vision of distant objects, we may from hence account for (what is commonly
observed) an object’s appearing of different magnitude, even with respect
to its horizontal extension, on the top of a steeple, e.g. a hundred feet
high, to one standing below, from what it would if placed at a hundred
feet distance, on a level with his eye. For, it hath been shewn that the
judgment we make on the magnitude of a thing depends not on the visible
appearance only, but also on divers other circumstances, any one of which
being omitted or varied may suffice to make some alteration in our
judgment. Hence, the circumstance of viewing a distant object in such a
situation as is usual and suits with the ordinary posture of the head and
eyes, being omitted, and instead thereof a different situation of the
object, which requires a different posture of the head, taking place—it is
not to be wondered at if the magnitude be judged different. But it will be
demanded, why a high object should constantly appear less than an
equidistant low object of the same dimensions; for so it is observed to
be. It may indeed be granted that the variation of some circumstances may
vary the judgment made on the magnitude of high objects, which we are less
used to look at; but it does not hence appear why they should be judged
less rather than greater? I answer, that in case the magnitude of distant
objects was suggested by the extent of their visible appearance alone, and
thought proportional thereto, it is certain they would then be judged much
less than now they seem to be. (Vid. sect. 79.) But, several circumstances
concurring to form the judgment we make on the magnitude of distant
objects, by means of which they appear far larger than others whose
visible appearance hath an equal or even greater extension, it follows
that upon the change or omission of any of those circumstances which are
wont to attend the vision of distant objects, and so come to influence the
judgments made on their magnitude, they shall proportionally appear less
than otherwise they would. For, any of those things that caused an object
to be thought greater than in proportion to its visible extension being
either omitted, or applied without the usual circumstances, the judgment
depends more entirely on the visible extension; and consequently the
object must be judged less. Thus, in the present case the situation of the
thing seen being different from what it usually is in those objects we
have occasion to view, and whose magnitude we observe, it follows that the
very same object being a hundred feet high, should seem less than if it
was a hundred feet off, on (or nearly on) a level with the eye. What has
been here set forth seems to me to have no small share in contributing to
magnify the appearance of the horizontal moon, and deserves not to be
passed over in the explication of it.
74. If we attentively consider the phenomenon before us, we shall find the
not discerning between the mediate and immediate objects of sight to be
the chief cause of the difficulty that occurs in the explication of it.
The magnitude of the visible moon, or that which is the proper and
immediate object of vision(361), is no greater when the moon is in the
horizon than when it is in the meridian. How comes it, therefore, to seem
greater in one situation than the other? What is it can put this cheat on
the understanding? It has no other perception of the moon than what it
gets by sight. And that which is seen is of the same extent—I say, the
visible appearance hath the very same, or rather a less, magnitude, when
the moon is viewed in the horizontal than when in the meridional position.
And yet it is esteemed greater in the former than in the latter. Herein
consists the difficulty; which doth vanish and admit of the most easy
solution, if we consider that as the visible moon is not greater in the
horizon than in the meridian, so neither is it thought to be so. It hath
been already shewn that, in any act of vision, the visible object
absolutely, or in itself, is little taken notice of—the mind still
carrying its view from that to some tangible ideas, which have been
observed to be connected with it, and by that means come to be suggested
by it. So that when a thing is said to appear great or small, or whatever
estimate be made of the magnitude of any thing, this is meant not of the
visible but of the tangible object. This duly considered, it will be no
hard matter to reconcile the seeming contradiction there is, that the moon
should appear of a different bigness, the visible magnitude thereof
remaining still the same. For, by sect. 56, the very same visible
extension, with a different faintness, shall suggest a different tangible
extension. When therefore the horizontal moon is said to appear greater
than the meridional moon, this must be understood, not of a greater
visible extension, but of a greater tangible extension, which, by reason
of the more than ordinary faintness of the visible appearance, is
suggested to the mind along with it.
75. Many attempts have been made by learned men to account for this
appearance(362). Gassendus(363), Des Cartes(364), Hobbes(365), and several
others have employed their thoughts on that subject; but how fruitless and
unsatisfactory their endeavours have been is sufficiently shewn in the
_Philosophical Transactions_(366) (Numb. 187, p. 314), where you may see
their several opinions at large set forth and confuted, not without some
surprise at the gross blunders that ingenious men have been forced into by
endeavouring to reconcile this appearance with the ordinary principles of
optics(367). Since the writing of which there hath been published in the
_Transactions_ (Numb. 187, p. 323) another paper relating to the same
affair, by the celebrated Dr. Wallis, wherein he attempts to account for
that phenomenon; which, though it seems not to contain anything new, or
different from what had been said before by others, I shall nevertheless
consider in this place.
76. His opinion, in short, is this:—We judge not of the magnitude of an
object by the optic angle alone, but by the optic angle in conjunction
with the distance. Hence, though the angle remain the same, or even become
less, yet, if withal the distance seem to have been increased, the object
shall appear greater. Now, one way whereby we estimate the distance of
anything is by the number and extent of the intermediate objects. When
therefore the moon is seen in the horizon, the variety of fields, houses,
&c. together with the large prospect of the wide extended land or sea that
lies between the eye and the utmost limb of the horizon, suggest unto the
mind the idea of greater distance, and consequently magnify the
appearance. And this, according to Dr. Wallis, is the true account of the
extraordinary largeness attributed by the mind to the horizontal moon, at
a time when the angle subtended by its diameter is not one jot greater
than it used to be.
77. With reference to this opinion, not to repeat what has been already
said concerning distance(368), I shall only observe, _first_, that if the
prospect of interjacent objects be that which suggests the idea of farther
distance, and this idea of farther distance be the cause that brings into
the mind the idea of greater magnitude, it should hence follow that if one
looked at the horizontal moon from behind a wall, it would appear no
bigger than ordinary. For, in that case, the wall interposing cuts off all
that prospect of sea and land, &c. which might otherwise increase the
apparent distance, and thereby the apparent magnitude of the moon. Nor
will it suffice to say, the memory doth even then suggest all that extent
of land, &c. which lies within the horizon, which suggestion occasions a
sudden judgment of sense, that the moon is farther off and larger than
usual. For, ask any man who from such a station beholding the horizontal
moon shall think her greater than usual, whether he hath at that time in
his mind any idea of the intermediate objects, or long tract of land that
lies between his eye and the extreme edge of the horizon? and whether it
be that idea which is the cause of his making the aforementioned judgment?
He will, without doubt, reply in the negative, and declare the horizontal
moon shall appear greater than the meridional, though he never thinks of
all or any of those things that lie between him and it. [And as for the
absurdity of any idea’s introducing into the mind another, whilst itself
is not perceived, this has already fallen under our observation, and is
too evident to need any farther enlargement on it(369).] _Secondly_, it
seems impossible, by this hypothesis, to account for the moon’s appearing,
in the very same situation, at one time greater than at another; which,
nevertheless, has been shewn to be very agreeable to the principles we
have laid down, and receives a most easy and natural explication from
them. [(370)For the further clearing up of this point, it is to be
observed, that what we immediately and properly see are only lights and
colours in sundry situations and shades, and degrees of faintness and
clearness, confusion and distinctness. All which visible objects are only
in the mind; nor do they suggest aught external(371), whether distance or
magnitude, otherwise than by habitual connexion, as words do things. We
are also to remark, that beside the straining of the eyes, and beside the
vivid and faint, the distinct and confused appearances (which, bearing
some proportion to lines and angles, have been substituted instead of them
in the foregoing part of this Treatise), there are other means which
suggest both distance and magnitude—particularly the situation of visible
points or objects, as upper or lower; the former suggesting a farther
distance and greater magnitude, the latter a nearer distance and lesser
magnitude—all which is an effect only of custom and experience, there
being really nothing intermediate in the line of distance between the
uppermost and the lowermost, which are both equidistant, or rather at no
distance from the eye; as there is also nothing in upper or lower which by
necessary connexion should suggest greater or lesser magnitude. Now, as
these customary experimental means of suggesting distance do likewise
suggest magnitude, so they suggest the one as immediately as the other. I
say, they do not (vide sect. 53) first suggest distance, and then leave
the mind from thence to infer or compute magnitude, but suggest magnitude
as immediately and directly as they suggest distance.]
78. This phenomenon of the horizontal moon is a clear instance of the
insufficiency of lines and angles for explaining the way wherein the mind
perceives and estimates the magnitude of outward objects. There is,
nevertheless, a use of computation by them(372)—in order to determine the
apparent magnitude of things, so far as they have a connexion with and are
proportional to those other ideas or perceptions which are the true and
immediate occasions that suggest to the mind the apparent magnitude of
things. But this in general may, I think, be observed concerning
mathematical computation in optics—that it can never(373) be very precise
and exact(374), since the judgments we make of the magnitude of external
things do often depend on several circumstances which are not proportional
to or capable of being defined by lines and angles.
79. From what has been said, we may safely deduce this consequence, to
wit, that a man born blind, and made to see, would, at first opening of
his eyes, make a very different judgment of the magnitude of objects
intromitted by them from what others do. He would not consider the ideas
of sight with reference to, or as having any connexion with, the ideas of
touch. His view of them being entirely terminated within themselves, he
can no otherwise judge them great or small than as they contain a greater
or lesser number of visible points. Now, it being certain that any visible
point can cover or exclude from view only one other visible point, it
follows that whatever object intercepts the view of another hath an equal
number of visible points with it; and, consequently, they shall both be
thought by him to have the same magnitude. Hence, it is evident one in
those circumstances would judge his thumb, with which he might hide a
tower, or hinder its being seen, equal to that tower; or his hand, the
interposition whereof might conceal the firmament from his view, equal to
the firmament: how great an inequality soever there may, in our
apprehensions, seem to be betwixt those two things, because of the
customary and close connexion that has grown up in our minds between the
objects of sight and touch, whereby the very different and distinct ideas
of those two senses are so blended and confounded together as to be
mistaken for one and the same thing—out of which prejudice we cannot
easily extricate ourselves.
80. For the better explaining the nature of vision, and setting the manner
wherein we perceive magnitudes in a due light, I shall proceed to make
some observations concerning matters relating thereto, whereof the want of
reflection, and duly separating between tangible and visible ideas, is apt
to create in us mistaken and confused notions. And, _first_, I shall
observe, that the _minimum visibile_ is exactly equal in all beings
whatsoever that are endowed with the visive faculty(375). No exquisite
formation of the eye, no peculiar sharpness of sight, can make it less in
one creature than in another; for, it not being distinguishable into
parts, nor in anywise consisting of them, it must necessarily be the same
to all. For, suppose it otherwise, and that the _minimum visibile_ of a
mite, for instance, be less than the _minimum visibile_ of a man; the
latter therefore may, by detraction of some part, be made equal to the
former. It doth therefore consist of parts, which is inconsistent with the
notion of a _minimum visibile_ or point.
81. It will, perhaps, be objected, that the _minimum visibile_ of a man
doth really and in itself contain parts whereby it surpasses that of a
mite, though they are not perceivable by the man. To which I answer, the
_minimum visibile_ having (in like manner as all other the proper and
immediate objects of sight) been shewn not to have any existence without
the mind of him who sees it, it follows there cannot be any part of it
that is not actually perceived and therefore visible. Now, for any object
to contain several distinct visible parts, and at the same time to be a
_minimum visibile_, is a manifest contradiction.
82. Of these visible points we see at all times an equal number. It is
every whit as great when our view is contracted and bounded by near
objects as when it is extended to larger and remoter ones. For, it being
impossible that one _minimum visibile_ should obscure or keep out of sight
more than one other, it is a plain consequence that, when my view is on
all sides bounded by the walls of my study, I see just as many visible
points as I could in case that, by the removal of the study-walls and all
other obstructions, I had a full prospect of the circumjacent fields,
mountains, sea, and open firmament. For, so long as I am shut up within
the walls, by their interposition every point of the external objects is
covered from my view. But, each point that is seen being able to cover or
exclude from sight one only other corresponding point, it follows that,
whilst my sight is confined to those narrow walls, I see as many points,
or _minima visibilia_, as I should were those walls away, by looking on
all the external objects whose prospect is intercepted by them. Whenever,
therefore, we are said to have a greater prospect at one time than
another, this must be understood with relation, not to the proper and
immediate, but the secondary and mediate objects of vision—which, as hath
been shewn, do properly belong to the touch.
83. The visive faculty, considered with reference to its immediate
objects, may be found to labour of two defects. _First_, in respect of the
extent or number of visible points that are at once perceivable by it,
which is narrow and limited to a certain degree. It can take in at one
view but a certain determinate number of _minima visibilia_, beyond which
it cannot extend its prospect. _Secondly_, our sight is defective in that
its view is not only narrow, but also for the most part confused. Of those
things that we take in at one prospect, we can see but a few at once
clearly and unconfusedly; and the more we fix our sight on any one object,
by so much the darker and more indistinct shall the rest appear.
84. Corresponding to these two defects of sight, we may imagine as many
perfections, to wit, 1st. That of comprehending in one view a greater
number of visible points; 2dly, of being able to view them all equally and
at once, with the utmost clearness and distinction. That those perfections
are not actually in some intelligences of a different order and capacity
from ours, it is impossible for us to know(376).
85. In neither of those two ways do microscopes contribute to the
improvement of sight. For, when we look through a microscope, we neither
see more visible points, nor are the collateral points more distinct, than
when we look with the naked eye at objects placed at a due distance. A
microscope brings us, as it were, into a new world. It presents us with a
new scene of visible objects, quite different from what we behold with the
naked eye. But herein consists the most remarkable difference, to wit,
that whereas the objects perceived by the eye alone have a certain
connexion with tangible objects, whereby we are taught to foresee what
will ensue upon the approach or application of distant objects to the
parts of our own body—which much conduceth to its preservation(377)—there
is not the like connexion between things tangible and those visible
objects that are perceived by help of a fine microscope.
86. Hence, it is evident that, were our eyes turned into the nature of
microscopes, we should not be much benefitted by the change. We should be
deprived of the forementioned advantage we at present receive by the
visive faculty, and have left us only the empty amusement of seeing,
without any other benefit arising from it. But, in that case, it will
perhaps be said, our sight would be endued with a far greater sharpness
and penetration than it now hath. But I would fain know wherein consists
that sharpness which is esteemed so great an excellency of sight. It is
certain, from what we have already shewn(378), that the _minimum visibile_
is never greater or lesser, but in all cases constantly the same. And in
the case of microscopical eyes, I see only this difference, to wit, that
upon the ceasing of a certain observable connexion betwixt the divers
perceptions of sight and touch, which before enabled us to regulate our
actions by the eye, it would now be rendered utterly unserviceable to that
purpose.
87. Upon the whole, it seems that if we consider the use and end of sight,
together with the present state and circumstances of our being, we shall
not find any great cause to complain of any defect or imperfection in it,
or easily conceive how it could be mended. With such admirable wisdom is
that faculty contrived, both for the pleasure and convenience of life.
‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
88. Having finished what I intended to say concerning the Distance and
Magnitude of objects, I come now to treat of the manner wherein the mind
perceives by sight their Situation(379). Among the discoveries of the last
age, it is reputed none of the least, that the manner of vision has been
more clearly explained than ever it had been before. There is, at this
day, no one ignorant that the pictures of external objects are painted on
the retina or fund of the eye; that we can see nothing which is not so
painted; and that, according as the picture is more distinct or confused,
so also is the perception we have of the object(380). But then, in this
explication of vision, there occurs one mighty difficulty, viz. the
objects are painted in an inverted order on the bottom of the eye: the
upper part of any object being painted on the lower part of the eye, and
the lower part of the object on the upper part of the eye; and so also as
to right and left. Since therefore the pictures are thus inverted, it is
demanded, how it comes to pass that we see the objects erect and in their
natural posture?
[Illustration]
Figure 4