1. Occulus Hoc est: fundamentum opticum;Scheiner,1619
2. Novo instrumento de esploracao da cornea;Placido;Periodico d'Ofthalmologica Practica, Lisbon,1880
3. Advances in the analysis of corneal topography;Wilson;Survey Ophthalmol.,1991
4. Evaluation of corneal topography: past, present and future trends;Morrow;Can. J. Ophthalmol.,1992