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. * * * * *