Dictionary.net

If the eye has been for some time regarding a particular colour, the retina becomes insensible to this colour; and if afterwards, it be turned to a sheet of white paper, the paper will not seem to be white, but will be of the colour that arises from the union of all the rays of the solar spectrum except the one to which the retina has become insensible. Thus, if the eye be directed for some time to a red wafer, the sheet of paper will seem to be of bluish-green, in a circular spot of the same dimensions as the wafer. This bluish-green image is called an ocular spectrum, because it is impressed upon the eye and may be retained for a short time; and the colour bluish-green, is said to be the accidental colour of the red. If this experiment be made with wafers of different colours, other accidental colours will be observed, varying with the colour of the wafer employed.