Eye

Matthew 6:22-23 

(Also Luke 11:34-36)

The light of the body is the eye: if therefore your eye is good, your whole body shall be full of light.

But if your eye is evil, your whole body shall be full of darkness. If therefore the light that is in you is darkness, how great is that darkness!

Luke adds, If your whole body therefore is full of light, having no part dark, the whole shall be full of light, as when the bright shining of a candle gives you light.