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.
TRANSPARENCY.
A transparent body, is one through which the visual line is uninterruptedly continuous from an object to the sense. The materials for glass-making are opaque, and the natural opacity of their elementary atoms is unalterable. Hence in some novel arrangement of the atoms towards promoting the direct continuity of the medium of space through them, consists the object of vitrifying and principle of transparency.
OPACITY.
The principal obstacle to transparency is interposed electric matter. In the earliest stages of glass-making an immense volume of electric matter is got rid of by means of the furnace fire, which becomes sooty smoke while ascending and passing through the furnace funnel; and to prevent all return of the like, it is, that solid oxygen is added to the materials when fused, the interstices of which, in the vitrified mass, secure the direct continuity of the visual medium. Priestley made black wax and brass filings transparent, by only removing all interposed electric matter. The body of a living man, by being de-electrised, has been made transparent. In these instances the transparency is of short continuance, and the opacity is restored by returning electric matter. Fire, in de-electrising gems and crystals, destroys all partial opacity. The clearest water is made cloudy on receiving the charge from the electrifying jar; by uncustomary electric matter, the atmosphere is made foggy, and is transparent again when the electric matter becomes a constituent of rain-water. These instances show, that, electric matter lying in the way of the medium of space and vision, interrupts its regular continuity, consequently, its direct pressure; yet not wholly,—clairvoyance and sound make manifest that the continuity is maintained through the most opaque bodies. The principle bears strongly on the physiology of clairvoyance.