The notorious Frankfurter Illusion
If you have two roughly equivalent eyes you will see a ‘sausage’ floating in front of you in mid air, by following these steps:
1. Hold your hands in front of you, at 20–30 cm distance from you, at eye level.
2. Point your index fingers against each other, leaving about 2 cm distance between them.
3. Now look “through” your fingers, into the distance behind them.
4. The sausage should appear now, and you can change its length by varying the distance between the finger tips.
5. For most observers, the sausage will look blurred, at least initially.
6. If you try to look at the sausage, it will disappear, it is only present if you look at something more distant than your fingers.
7. It helps if the background is rather homogenous and has a color very different from your fingers.