Do you think that I am worthless? Observe that I am furnished with a glass at both ends of my pipe. Consider that by looking through one of them, you will see whatever object you wish to behold. What, in the whole universe, would you wish to see?

Ha! Now, tell me what I am worth!

The Magic Perspective Glass in Prince Ahmed and the Fairy Paribanou, Blue Fairy Book. Vintage postcard, artist unknown.