Colour cathode-ray tube comprising a colour selection mask placed on a metal frame, the said frame comprising, on at least two opposite sides, means of damping vibrations of the mask, of the type comprising two masses connected to the surface of the frame by a connection having a mechanical clearance between the facing surfaces of the mask and of the frame. A second diffraction grating is provided for receiving the electromagnetic wave reflected from the air bearing surface. 01 to about 5 mm, and a dental base.