The phrase "the apple of his eye" seems to have a more ancient origin than one might think. From a quick Google search of the phrase, Psalm 17:8 pops up with the following:
"Keep me as the apple of the eye, hide me under the shadow of thy wings, From the wicked that oppress me, from my deadly enemies."
This verse shows that the phrase dates back to Biblical times and is used along with a plea for protection. Then, dating even further back, a verse in Deuteronomy 32:10 uses the phrase a little differently: " He guarded him as the pupil of His eye." Based on the wording of this verse, it seems as if the apple represents and refers to the pupil of one's eye. Again, the plea for protection, or to be guarded, accompanies the phrase.
Therefore, if the king's daughter is the "apple of his eye," then that would mean that she is the only thing that he sees because she would be his pupil figuratively speaking. Another way to say it is...
(The entire section contains 2 answers and 539 words.)