A camera includes a main unit which has an arc-shaped groove and a fixed hook formed on and projecting from the groove; and a bayonet ring which has, on the back face thereof, an arc-shaped projection to be engaged in the groove and a stop hook to go under the fixed hook and stopped by the fixed hook when the projection is engaged in the groove and rotated. The technique promotes improvement of visual acuity in the weaker eye. The detector detects an abnormal condition of the polishing pad. The barrel comprises a needle mount retainer that comprises a plurality of fingers that engage the retractable needle mount to prevent inadvertent retraction. The indicators may have distinct levels of intensity for each level of supervisory control required to safely control a vehicle. The second section is generally cylindrical in shape.