Note 10 — Chapter IV · THE PROPERTIES OF OBJECTIVES AND MIRRORS
Reading edition
More recently his condition proves to be quite the exact equivalent of Abbé’s sine condition which states that the sine of the angle made with the optical axis by a ray entering the objective from a given axial point shall bear a uniform ratio to the sine of the corresponding angle of emergence, whatever the point of incidence. For parallel rays along the axis this reduces to the requirement that the sines of the angles of emergence shall be proportional to the respective distances of the incident rays from the axis.