Prism spectacles joke

The effect of these spectacles on the person who puts them on is most disconcerting. Behind each celluloid eye is a small glass prism. If you hold your arm vertically in front of the person and ask them to reach out and grab your arm, they will be surprised because they miss it by a long way. This is because the prisms through which they are looking will shift the apparent position of your arm sideways. They grab towards this position only to discover that your arm is not there. We have four examples, one of which has D.R.G.M. on it, confirming it was made in Germany.