What is the purpose of the "Mirror-Thing" over the Mig-29 Cockpit? I noticed that some Mig-29 Fighter Jets have a little mirror-like-thing over the cockpit. I can't imagine a possible usage from it (If it's really a mirror).

Here's a picture from Wikipedia:

  

As you can see on the following picture, not every Mig-29 has this thing:

  


What is this "Mirror"-like-thing supposed to do?
Why can't I find it on every Mig-29?
When is this thing used? In combat, training etc.?
How does it help the pilot / trainer in flight?
Additional details to Marco's selected answer.

On the two-seater version, the instructor seat (aft) is not high enough, contrary to other training aircraft, to see the runway. A forward view mirror replaces the rear view mirror and allows the instructor to see ahead. The runway is projected on a second mirror inside the cockpit, creating a full periscope system:


Photo by Pavel Vanka. Modified.


(Source)

Note how the projected part of the image fits very well into the direct view of the exterior.

An opening is created in the canopy when the top mirror is deployed. From below this is what it looks like (not exactly the same model):


Photo by Burkhard Domk. Modified.

The bottom mirror unfolds down to instructor eyes height:


(Source)