Now imagine a Priest, whose mouth is at M , and whose front semicircle ( AMB ) is consequently coloured red, while his hinder semicircle is green; so that the diameter AB divides the green from the red. If you contemplate the great man so as to have your eye in the same straight line as his dividing diameter ( AB ), what you will see will be a straight line ( CBD ), of which one half ( CB ) will be red, and the other ( BD ) green . The whole line (