An ocular device for a surgical instrument having an ocular mount for an optical assembly. The ocular device including: an optical flat; and a holder for accommodating the optical flat, wherein the holder is configured to be connected to the ocular mount; wherein the optical flat having a widened side edge provided with a first contact surface for the ocular mount and a second contact surface for the holder, and a surface normal of the optical flat and the first contact surface of the optical flat facing the ocular mount form an angle different than 0° relative to each other.