A Hebrew expression used (e.g. Ps. 17: 8) for the eyeball, the part to be most carefully guarded.
Citation for apple of the eye
Citation styles are based on the Chicago Manual of Style, 15th Ed., and the MLA Style Manual, 2nd Ed..
MLA
"apple of the eye." In A Dictionary of the Bible. Ed. W. R. F. Browning. Oxford Biblical Studies Online. May 23, 2013. <http://www.oxfordbiblicalcstudies.com/article/opr/t94/e136>.
Chicago
"apple of the eye." In A Dictionary of the Bible. , edited by W. R. F. Browning. Oxford Biblical Studies Online, http://www.oxfordbiblicalcstudies.com/article/opr/t94/e136 (accessed May 23, 2013).