We must constantly make sure that our eye has not become diseased and that our body is not full of darkness (shadiness or obscurity) instead of light (luminous, to shine). The bright shining of a candle means glaring or lightning. When our eye is single it is folded together or clear. We can see things very clearly.
http://who-god-is.com/your-eye-is-the-lamp-of-your-body