Adam Clarke on Psalms 17:8

Keep me as the apple of the eye - Or, as the black of the daughter of eye. Take as much care to preserve me now by Divine influence, as thou hast to preserve my eye by thy good providence. Thou hast entrenched it deeply in the skull; hast ramparted it with the forehead and cheek-bones; defended it by the eyebrow, eyelids, and eyelashes; and placed it in that situation where the hands can best protect it.

Hide me under the shadow of thy wings - This is a metaphor taken from the hen and her chickens. See it explained at large in the note on Mat 23:37 (note). The Lord says of his followers, Zac 2:8 : "He that toucheth you, toucheth the apple of mine eye." How dear are our eyes to us! how dear must his followers be to God!