ιθ΄.

Ἐν τοῖς ἐπιπέδοις ἐνόπτροις τὰ δεξιὰ ἀριστερὰ φαίνεται καὶ τὰ ἀριστερὰ δεξιὰ καὶ τὸ εἴδωλον ἴσον τῷ ὁρωμένῳ, καὶ τὸ ἀπόστημα τὸ ἀπὸ τοῦ ἐνόπτρου ἴσον ἐστίν. ἔστω ἐπίπεδον ἔνοπτρον τὸ ΑΓ, ὄμμα δὲ τὸ Β, ὄψεις δὲ αἱ ΒΑ, ΒΓ ἀνακλώμεναι ἐπὶ τὰ Ε, ∠,ὁρώμενον δὲ ἔστω τὸ Ε ∠,καὶ ἀπὸ τῶν Ε, ∠ἐπὶ τὸ ἔνοπτρον κάθετοι ἤχθωσαν αἱ Ε Ζ, ∠Θ καὶ ἐκβεβλήσθωσαν, ἐκβεβλήσθωσαν δὲ καὶ αἱ ΒΓ, ΒΑ ὄψεις καὶ συμπιπτέτωσαν ταῖς καθέτοις κατὰ τὰ Κ, Λ, καὶ ἐπεζεύχθω ἡ ΛΚ. οὐκοῦν φαίνεται τὸ μὲν Ε ἐπὶ τοῦ Κ, τὸ δὲ ∠ἐπὶ τοῦ Λ· τοῦτο γὰρ προεδείχθη. τὰ ἄρα ἀριστερ δεξιὰ φαίνεται καὶ τὰ δεξιὰ ἀριστερά. καὶ ἐπεὶ ἴσ ἐστὶν ἡ ὑπὸ τῶν ΚΓΖ γωνία τῇ ὑπὸ τῶν ΖΙΕ, κ εἰσιν ὀρθαὶ αἱ πρὸς τῷ Ζ, ἴση ἂν εἴη καὶ ἡ ΖΚ τ ΖΕ. διὰ τὰ αὐτὰ καὶ ἡ ∠Θ τῇ ΘΑ. ἴσον ἄρα τὸ ἀπόστημα, ὃ ἀπέχει ἀπὸ τοῦ ἐνόπτρου τὸ Ε∠, τῷ, ὃ ἀπέχει τὸ εἴδωλον τὸ Κ Λ. καὶ ἴσον τὸ ὁρώμενον τὸ Ε∠ τῷ εἰδώλῳ τῷ Κ Λ διὰ τὸ ἴσην εἶναι τὴν μὲν ΕΖ τῇ ΖΚ, τὴν δὲ ∠Θ τῇ ΘΛ, κοινὴν δὲ καὶ πρὸς ὀρθὰς τὴν ΘΖ.