“…Thus, when I look upon a music book, and, shutting my left eye, direct the right eye to a point of the middle line of the five which compose the staff of music, the middle line appears dim indeed at the point to which the eye is directed, but straight; at the same time, the two lines above it and the two below it appear to be bent outwards, and to be more distant from each other and from the middle line, than at other parts of the staff to which the eye is not directed (Reid, 1801).…”