How does Terence Rattigan use imagery in The Browning Version?

Imagery:

There's no need. You were merely telling me what I should have known for myself. Perhaps I did in my heart, and hadn’t the courage to acknowledge it. I knew, of course, that I was not only not liked, but now positively disliked. I had realized, too, that the boys – for many long years now – had ceased to laugh at me. I don’t know why they no longer found me a joke. Perhaps it was my illness. No, I don’t think it was that. Something deeper than that. Not a sickness of the body, but a sickness of the soul. At all events it didn’t take much discernment on my part to realize I had become an utter failure as a schoolmaster. Still, stupidly enough, I hadn’t realized that I was also feared. The Himmler of the lower fifth! I suppose that will become my epitaph.