135. A marine sextant has the index arm set at zero and the reflected image of the horizon forms a continuous line with the actual image. When the sextant is rotated about the line of sight the images separate. The sextant has which error?

Side error occurs when the horizon glass is not perpendicular to the sextant frame, causing the reflected and direct horizons to separate when the sextant is rotated about the line of sight; this is corrected with the side-error adjustment screw.