The Frankfurter Illusion

A more Detailed Explanation on This Opticall Illusion on 

Optical Illusions & Visual Phenomena

131 of them – by Michael Bach 

in the meantime:

What to do & see

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.

Source: http://www.michaelbach.de/ot/sze-Frankfurter/index.html