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.
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.
I
EFFECTS OF LIGHT AND DARKNESS ON THE EYE.
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.