The sentence that correctly uses the word "principal" is the second sentence. The word "principal" refers to the person who has the highest authority in an institution, particularly as referred here, which is a school. On the rest of the sentences, the right word to be used is "principle" and not principal.
The author explains all the splendors of nature whilst the cage imprisons him, preventing him from fully taking in the splendor that is around him. Hence creating slight hyperbole that displays his yearn to experience it all firsthand.
