1. We naturally place these colours first, because they belong altogether, or in a great degree, to the subject9—to the eye itself. They are the foundation of the whole doctrine, and open to our view the chromatic harmony on which so much difference of opinion has existed. They have been hitherto looked upon as extrinsic and casual, as illusion and infirmity: their appearances have been known from ancient date; but, as they were too evanescent to be arrested, they were banished into the region of phantoms, and under this idea have been very variously described.
2. 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.