According to the popular opinion, which governs the philosopher,
and with which the established philosophy agrees, vision is an act
performed by the eye, which is said to be endowed with the faculty of
sight, by which it is enabled to look into, through space, and see
external bodies made visible when covered with solar or day light;
nothing of which is true. The eyeball is not possessed of sight; to
see is not the function of the sense; externals are not visible; there
is no material light; light is a sensible or mental effect consequent
on the chromatic organ of the brain being excited by the fluid of the
optic nerve. All we know by means of the optic sense, consists in the
sensation of light or coloured light, accompanied with the idea of
form. The object which promotes the sensation being, seemingly, the
place of the sensation, all imagine the sensation is the colour of the
object to which the eye is directed, and hence, that the object or
body is seen by the eyes. These general mistakes are made evident and
stand corrected by reference to the sense itself, its physiology and
function, as previously stated and advised.
The medium of space is the visual medium; not, however, for looking
through, as is supposed, but by reason of it forming the link or
intermediate means by which the object is connected with the sense.
Now, as the medium of space is present everywhere, and as it promotes
visual or optic perception, the question naturally arises, why do we
not see in the night as well as day, in all places and at all times; in
a word, why do we not see in the dark? The clairvoyant does "see" in
the dark.
The nervous fluid excites the sensation of colour; the medium of space
connects mediately the object with the nervous fluid, which fluid
acts on the optic cerebral organ by pressure and degrees of pressure.
The nervous fluid, nor anything else, acts essentially, that is, by
means of properties and qualities; and its acting on the brain is
caused by external agency, the fluid itself being _inert_. It may
well be supposed that the exquisite construction of the brain, from
being competent to produce psychologic effects, although excited by
material agency, requires but the most simple means, such as a simple
impulse or impression, to be actuated into excitement; and as the
portion or line of the medium of space which is continuous from the
external object, through the pupil, to the nervous fluid within the
retina, is that which puts the nervous fluid into functional action on
the brain, it is fairly assumable that only by pressure, degrees, and
changes of pressure, the nervous fluid can by possibility act on and
excite the brain; which equally applies to the nervous fluid of all
the senses. Taking, then, the maximum of optic pressure as productive
of no sensation; so, from there being no object to perceive, it is
imagined we are surrounded with darkness; and taking the minimum as
exciting the sensation recognised as luminous, light, or white, to
intermediate degrees of cerebral pressure are to be attributed the
sensations of red, yellow, blue and of colours generally. According to
these terms of the colorific scale, all optically-excited perceptions
are consequent on the cerebral pressure being in degrees on the scale
of descent from the maximum.
For the reduction of optic pressure, there are different minus-pressure
means, namely, the sun, flame, electricity, phosphoric substances;
and the daily electric matter, which is constant in the atmosphere at
the eastern hemisphere of the globe, and which keeps pace with the
sun; because the rarest elements of the atmosphere will be in greater
quantity on the side facing the sun. As this daily electric matter
emerges before the sun is above the horizon, the general optic pressure
excites the sensation supposed to be the light of day-break; and while
following, after sunset, the sensation is known as twilight. Any such
minus-pressure matter lying in the visual direction, shortens the
visual line, and intercepts the continuity of that line of the medium
of space which makes one with the axis of the eye, and thus effects the
reduction of optic pressure.
* * * * *
_Note._--The terms here made use of, from being unknown in the olden
philosophy, need explanation.--_Axis line_: that line of the medium
of space which is as the axis of the eye produced to, and terminated
by the external object. _Visual line_, the same. _Visual continuity_;
the line which is continuous _angularly_ with the termination of the
axis line. From the termination of this _continuous_ line, there may
be another angular continuity or _line_, as from mirror to mirror.
All lines continuous from the axis line and terminated by _the
object_ supposed to be seen, and however irregular, are _lines_ of
_vision_: the angular point, _the point of_ (first, second, or third)
_continuity_. The reader should make a diagram for each case as he
proceeds.
* * * * *
Within the window-closed room, a lighted candle is supposed to fill
the entire space with light radiated from the flame: the perception is
named light, and is thus wise excited. When the axis line is terminated
by the flame, the pressure on the nervous fluid is lessened to the
degree which promotes the sensation of luminousness, which seems to be
the physical appearance of the flame itself. Again; when, in the same
room, the eye is directed to a mirror the like perception is excited,
because the visual line is continuous from the point of continuity, or
termination of the axis line, to the flame as before. When the axis
line is terminated by a piece of furniture, the point of continuity
being imperfect and the visual continuity thence to the flame irregular
or indirect, the optic pressure on the brain by the axis line excites
the sensation of colour, which is imputed to the object, chair, or
table.
In the celebrated OPTICS, the visual lines are mistaken for rays of
light radiated from the flame, and reflected from the other objects;
which rays are supposed to enter the eye, and (as if possessed of
intelligence) arrange themselves on the back of the eye or on the
retina, in the precise form, but of a different size, of the object
to which the eyes are directed, as the means by which externals are
seen before the face. In cases wherein the visual line is indirect,
as when lying through media of unequal density, the supposed rays are
said to be refracted: and, because the curtained iris excludes the
visual medium, except through the pinhole pupil, thence along the axis
through the lenses of the eyeball, the _optics_ inculcate, that the
eye has been formed to see only in straight lines. Finally, by Dr.
Reed it is taught, that the use of the sensation and of the image on
the _back_ of the eye, is to make the external object _opposite the
face_ be seen; all which has to be rejected and forgotten in being
guided by the natural, real function of the sense, against which there
is no appeal. There are no rays concerned; the medium of vision is
quiescent; there can be neither radiation, reflection, nor refraction
effected by passive inert bodies; there is no image on any part of the
eye or retina; and externals could not be made visible, or seen by
their images. Such absurdities, all of which are maintained in modern
philosophy, have prevented, more than any thing else, the science and
phenomena of Mesmerism being understood.
According to the interstitial composition of the surface of a body,
so is the point of visual continuity at or beneath the surface; which
determines the degree of pressure on the axis line; which determines
what shall be the resulting sensation, or apparent colour of the
surface of the object to which the pupil of the eyeball is directed.
Through a pane of glass, or through the clear atmosphere, the axis line
may be said to be uninterruptedly continuous, and the perception is as
if the glass were away. Through an ignited sheet of iron the visual
continuity is imperfect, and may be said to be continuous only halfway
through the sheet. An ignited bar, at first, is said to be brown, then
ignited to redness: colours are sensations. Within the bar the axis
line is continuous in zig-zag order, which causes the optic pressure
to excite the sensation of red: it is a prismatic case. The _spectra_,
by means of the prism, are only in the sensorium; the skreen itself is
unseen. When the direct axis line terminates at the apparent red on the
skreen, the continuity thence is maintained through some particular
part of the prism; when terminated by the yellow, through a different
part; when by the blue, through another different part; and through
each part the continuity is somewhat curvilinear, hence the pressures
and perceptions are different. Through the air, when the perception is
of the many-coloured rainbow, the visual continuity is as through the
prism: there is no coloured bow out of the sensorium.
Where there are no minus-pressure means for lessening the optic
pressure, as in mines, caves, and window-closed rooms, there can be no
perceptions of light and colour. From the sensation ceasing the same
instant the last window-shutter is closed, it would seem, that, the
_daily_ minus-pressure matter is in constant flow eastward through the
globe. The rheumatic sufferer fears sun-down, as if the daily matter
enters and protects the nerves from the nightly. The meteorologist has
to resolve the problem for the philosopher in tracing the magnetic
meridian.
The objection is unfounded against pressure being the cerebral exciting
cause. It is objected, that, from two stars equally distant, one
considered red, the other blue, the pressure cannot be changed along
the visual lines in the small space of time the eye takes to direct
itself from one to the other star. There is no changing of pressure on
either line. The existing pressure on the sense by each is different,
and what it is, depends on the constitution of the external object, as
in every other instance, and just as on that of the ignited bar already
stated. The imputed colours of the stars being different, so is the
continuity of axis line beneath the surface of the atmosphere of each
star, also the degree of pressure and the sensitive result.
Neither is it maintainable that the medium of space cannot be the
medium of vision, because "from being all-pervading, it should excite
vision through all kinds of bodies, as through a block of rock crystal,
but does not through so thin a substance as a leaf of blotting-paper."
By clairvoyance it is proved that the visual continuity is maintained
through stone walls; and by reason of the _visual and auditory_ medium
being the same, that is, medium of space, the "hearing" through
stone walls, makes the "seeing" possible. The bell must be connected
mediately with the auditory sense, as is the object with the visual
sense; and through stone walls there is nothing continuous but the
medium of space. Sound is no more a transmissible object than colour;
neither belongs to the external object. In all such cases of sensations
which are different, although the promoting means are the same for all
the senses, that the organs of sense may not be equally susceptible,
or capable of being put into functional service by the same degree of
cerebral pressure, should be held in mind, or else it might be asked
why all the senses are not excited at the same time.