Comment by fwlr

Your words are not unclear! You are doing a fantastic job of using words that work very easily with mental imagery, and but not at all without. I said I didn’t understand horizontal because without mental imagery I have no “floor” for the ray to be parallel to - I could have found something to be the referent for the horizontal if I had to, but once I thought I had explained why “horizontal” was mentioned I stopped trying to figure it out further.