If you click the link there are more details on the construction but basically there is a slit in the mask where the "eyeball" meets the rim of the glasses. Since the glasses are black it hides the slit.
Here is a closeup: http://www.instructables.com/files/deriv/FSX/Z3VR/H8C5TCGZ/FSXZ3VRH8C5TCGZ.MEDIUM.jpg
Here is a closeup: http://www.instructables.com/files/deriv/FSX/Z3VR/H8C5TCGZ/FSXZ3VRH8C5TCGZ.MEDIUM.jpg