Commentary on Proposition I.14¶
Back to Proposition I.14.
Proclus observes (p. 297) that two straight lines on the same side of another straight line and meeting it in one and the same point may make with one and the same portion of the straight line terminated at the point two angles which are together equal to two right angles, in which case however the two straight lines would not be in a straight line with one another. And he quotes from Porphyry a construction for two such straight lines in the particular case where they form with the given straight line angles equal respectively to half a right angle and one and a half right angles. There is no particular value in the construction, which will be gathered from the annexed figure where CE, CF are drawn at the prescribed inclinations to CD.