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.