Thus they are called _colores adventicii_ by Boyle; _imaginarii_ and _phantastici_ by Rizetti; by Buffon, _couleurs accidentelles_; by Scherfer, _scheinfarben_ (apparent colours); _ocular illusions_ and _deceptions of sight_ by many; by Hamberger, _vitia fugitiva_; by Darwin, _ocular spectra_. 3. We have called them physiological because they belong to the eye in a healthy state; because we consider them as the necessary conditions of vision; the lively alternating action of which, with reference to external objects and a principle within it, is thus plainly indicated. 4. To these we subjoin the pathological colours, which, like all deviations from a constant law, afford a more complete insight into the nature of the physiological colours. EFFECTS OF LIGHT AND DARKNESS ON THE EYE. 5. The retina, after being acted upon by light or darkness, is found to be in two different states, which are entirely opposed to each other. 6. If we keep the eyes open in a totally dark place, a certain sense of privation is experienced. The organ is abandoned to itself; it retires into itself. That stimulating and grateful contact is wanting by means of which it is connected with the external world, and becomes part of a whole. 7. If we look on a white, strongly illumined surface, the eye is dazzled, and for a time is incapable of distinguishing objects moderately lighted. 8. The whole of the retina is acted on in each of these extreme states, and thus we can only experience one of these effects at a time. In the one case (6) we found the organ in the utmost relaxation and susceptibility; in the other (7) in an overstrained state, and scarcely susceptible at all. 9. If we pass suddenly from the one state to the other, even without supposing these to be the extremes, but only, perhaps, a change from bright to dusky, the difference is remarkable, and we find that the effects last for some time. 10. In passing from bright daylight to a dusky place we distinguish nothing at first: by degrees the eye recovers its susceptibility; strong eyes sooner than weak ones; the former in a minute, while the latter may require seven or eight minutes. 11. The fact that the eye is not susceptible to faint impressions of light, if we pass from light to comparative darkness, has led to curious mistakes in scientific observations. Thus an observer, whose eyes required some time to recover their tone, was long under the impression that rotten wood did not emit light at noon-day, even in a dark room. The fact was, he did not see the faint light, because he was in the habit of passing from bright sunshine to the dark room, and only subsequently remained so long there that the eye had time to recover itself. The same may have happened to Doctor Wall, who, in the daytime, even in a dark room, could hardly perceive the electric light of amber.