The Physics 212 Puzzler

Here's a golden oldie refraction trick. I first struggled with this one 30 years ago. (I was pretty annoyed when I finally figured it out.) A glass cylinder is placed over the text CARBON DIOXIDE. The image of the text created by the glass cylinder inverts the word CARBON but not the word DIOXIDE. Why is this?

Although I have created a graphic below, this really happens with a real glass cylinder. If you actually perform this experiment with pen, paper and the stem of a wine glass, you'll figure it out.

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