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.