1. The cross-cylinder tests, especially in relation to the astigmatic axis;CRISP, W.H.;Trans. Obhthal. Soc. U. K., Vol. LI,1931
2. Fundamental differences between crossed cylinder and line chart astigmatic tests;PASCAL, J.I.;Arch. Ophthal., Vol. XXLV,1940
3. Further notes on the Jackson cross-cylinder;COATES, W.R.;The Refractionist,1938