The B&L exophthalmometer is used to determine the axial position of the eyes.  The instrument is placed on the lateral orbital rim of each side, using as small of a base as possible.  Depending on the device, lines on the prism are placed into position by moving the examiners view point to determine the measurement.  The measurement for the patient's right eye should be determined with the examiner's left eye and vice versa.  The device measures the distance between the lateral orbital rim and the most anterior position of the cornea.  In general, measurements between the two eyes should be equal.  If the measurements are different by 2 or more millimeters, the patient should undergo additional evaluation.