The limiting surface of one thing is the beginning of another.

A single and distinct luminous body causes stronger relief in the objects than a diffused light; as may be seen by comparing one side of a landscape illuminated by the sun, and one overshadowed by clouds, and illuminated only by the diffused light of the atmosphere.

Learning never exhausts the mind.

The mind that engages in subjects of too great variety becomes confused and weakened.

I have solved what color is, however ; I still have no idea about what line is.

The truth of things is the chief nutriment of superior intellects.

He who loves practice without theory is like the sailor who boards ship without a rudder and compass and never knows where he may cast.