rotateOnWorldAxis( axis, angle ) {

		// rotate object on axis in world space
		// axis is assumed to be normalized
		// method assumes no rotated parent

		_q1.setFromAxisAngle( axis, angle );

		this.quaternion.premultiply( _q1 );

		return this;

	}
Rotates the 3D object along an axis in world space.

@param {Vector3} axis - The (normalized) axis vector.
@param {number} angle - The angle in radians.
@return {Object3D} A reference to this instance.