Abstract: An optical device can be fitted in front of a patient's eyes and comprises a holder (1) bearing two movable screens (2, 3) of transparent polarizing material. The two screens (2, 3) are moved in front of the eyes, and their position relative to the holder (1) of the device is determined. A target is separated from the holder and is adapted to be placed in front of the patient so as to be visible to the latter, a central part of the target having a polarization axis perpendicular to that of the polarizing screens (2, 3). Each polarizing screen (2, 3), near its lateral edge (2a, 3a) nearest the axis of symmetry (Y, Y') extending between the two screens, has a slot (5a, 6a) substantially parallel to the aforementioned axis of symmetry, whereas near its bottom edge (2b, 3b) it has a slot (5b, 6b) substantially perpendicular to the previously-mentioned slot. The two screens (2, 3) are moved in such a way that the slots (5a, 6a; 5b, 6b) in the screens remain parallel to themselves when the screens move.