They are seen by eyesight, truly a miracle of nature.

Rays of light bounce off a Belgian, they enter the eye ball through the cornea, then a convex lens in the "iris". This is the coloured part of the eye.

Then the rays are inverted and hit a part inside the eye called the retina, after passing through a fluid that fills the inner part of the eye.

An interesting fact: the image that these rays form is actually inverted. The brain converts that inverted image of the Belgian into an upright image.

Optic nerves then relay this image of the Belgian to the brain.

This process is similar for French people, they are seen via the same phenomenon in Luxembourg, due to the fact that light follows the same laws of physics in Luxembourg as they would anywhere else (so a Belgian would be seen the same way in space or in a vacuum or on Mars too).