Fig. 4-4. Red glass test. When the direction of gaze uses the weak muscle, the image is always displaced away from the macula of the retina so that the brain interprets the abnormal, blurred (off-macula) image as being farther out in the field of gaze than the normal image. The examiner needs only to detemine which eye sees the farthest displaced image to determine the abnormal side. Blurring is less useful because it is usually minimal and not noticed by the patient.