Here the reflected star does not pass on the direct star in the vertical movement: The reflected star has to pass alternately below and above the direct star (not reflected) : Set the alidade to zero degrees, o bserve the star with the sextant and move the drum (minute scale). We choose a very small star in the sky ( less than magnitude 4.0 or 5.0) t o identify the error. This error appears when the horizon mirror (rounded mirror) is not perpendicular to the frame. If we see a broken line we remove the error using the adjusting screw on the index mirror: We must see a straight line (the arc with the graduated scale) as in this photo: We see two images of the scale: the reflected image on the left side, the direct image on the right side. Look at the index mirror in the direction of arc (graduated scale). Put the sextant on an horizontal plane and move the alidade to a position 35°-45° degrees. This error appears when the index mirror (rectangular mirror) is not perpendicular to the frame (and arc). The sequence has to be respected in detecting and removing of errors : Cassens & Plath and Freiberger attest that ! It is not an “ adjustable sextant error ”.