The "Corollarium II" to the Proposition XXIII of Saccheri's Euclides

This "Corolarium" of the Euclides (1733) contains an original proof of propositions 1.27 and 1 .28 of Euclid's Elements. In the same corollary Saccheri explains why he dispenses "not only with the propositions 1 .27 and 1.28, but also with the very propositions 1.16 and 1.17, except when it is clearly dealt with a triangle circumscribed by alls sides" ; and also why he rejects Euclid's proof. Moreover, the Corolarium has implications for confirmation of Saccheri's method; and also for his concept of straight line, which leads him to his paralogism of ignorantia elenchi in proposition XXXIII