§31.17Διὰ τί εἰς τὸ πλάγιον κινοῦσι τὸν ὀφθαλμὸν οὐ φαίνεται δύο τὸ ἕν;
Why is it that, when people move their eye to the side, one thing does not appear as two?
ἢ ὅτι ἐπὶ τῆς αὐτῆς γίνεται γραμμῆς ἡ ἀρχή;
Is it because the origin of the sight-line remains on the same line?
δύο δὲ φαίνεται ταύτης μεταβαλλούσης ἄνω ἢ κάτω.
But it appears as two when this line is changed upward or downward.
εἰς δὲ πλάγιον οὐδὲν διαφέρει, ἐὰν μὴ ἅμα καὶ ἄνω.
In moving to the side, it makes no difference, unless it is at the same time also moved upward.
τί δὴ ἐπὶ μὲν τῆς ὄψεώς ἐστιν ὥστε φαίνεσθαι τὸ ἓν δύο, ἄν πως τεθῶσιν οἱ ὀφθαλμοὶ πρὸς ἀλλήλους, ἐπὶ δὲ τῶν ἄλλων αἰσθήσεων οὐκ ἔστιν;
Why indeed is it that in the case of sight, if the eyes are placed in a certain relation to each other, one thing appears as two, whereas this does not happen in the case of the other senses?
ἢ καὶ ἐπὶ τῆς ἁφῆς γίνεται τῇ ἐπαλλάξει τῶν δακτύλων τὸ ἓν δύο;
Or does it also happen in the case of touch, by the crossing of the fingers, that one thing appears as two?
ἐπὶ δὲ τῶν ἄλλων οὐ γίνεται, ὅτι οὔτε ἔξω ἀποτεινομένων αἰσθάνεται, οὐδὲ δύο.
But it does not happen in the case of the other senses, because they neither perceive when stretched outward, nor do they perceive two.
γίνεται δὲ [διὰ] τοῦτο διόπερ καὶ ἐπὶ τῶν δακτύλων· μιμεῖται γὰρ τὴν ὄψιν.
And this happens for the same reason as in the case of the fingers; for touch mimics sight.
§31.18Διὰ τί τοῦ μὲν ἄλλου σώματος τὰ ἀριστερὰ ἀσθενέστερα, τῶν δὲ ὀφθαλμῶν οὔ, ἀλλ’ ὁμοίως ὀξύ;
Why is it that while the left parts of the rest of the body are weaker, those of the eyes are not, but are equally sharp?
ἢ ὅτι τὰ μὲν δεξιὰ τῷ ποιητικὰ εἶναι διαφέρουσιν, τῷ δὲ παθητικὰ οὐ διαφέρουσιν;
Is it because the right parts differ in being active, but in being passive they do not differ?
αἱ δὲ ὄψεις παθητικαί.
And sights are passive.
§31.19Διὰ τί τῇ ὄψει πρὸς μὲν τὰ ἄλλα ἀτενίζοντες χεῖρον διατιθέμεθα, πρὸς δὲ τὰ χλωρὰ καὶ ποώδη, οἷον λάχανα καὶ τὰ τούτοις ὅμοια, βέλτιον;
Why is it that when we gaze intently at other things we are in a worse state with respect to our sight, but we are in a better state with respect to green and herbaceous things, such as vegetables and the like?
ἢ ὅτι πρὸς μὲν τὸ λευκὸν καὶ μέλαν ἥκιστα δυνάμεθα ἀτενίζειν (ἄμφω γὰρ λυμαίνεται τὴν ὄψιν), τὰ δὲ τοιαῦτα τῶν χρωμάτων μέσον ἔχει τούτων·
Is it because we are least able to gaze intently at white and black (for both damage the sight), whereas such colors hold the middle between these?
διὸ μετρίως τῆς ὄψεως διατιθεμένης οὐδὲ ἐξαδυνατοῦμεν αὐτῇ, βέλτιον δὲ διατιθέμεθα.
Therefore, since the sight is moderately affected, we do not exhaust it, but are in a better state.
τάχα δὲ ἴσως καθάπερ ἐπὶ τῶν σωμάτων σφοδρότερον πονοῦντες χεῖρον ἔχομεν, τὸ μέσον δὲ βέλτιστα διατίθησιν, τὸν αὐτὸν τρόπον καὶ τὴν ὄψιν.
Or perhaps, just as in the case of our bodies we are in a worse state when we labor too violently, while the middle state puts us in the best condition, in the same way it is also with our sight.
πρὸς μὲν γὰρ στερεὰ ἀτενίζοντες πονοῦμεν αὐτήν, πρὸς δὲ τὰ ὑγρὰ μηδενὸς ἀντιφράττοντος οὐ διαπονοῦμεν.
For when we gaze intently at solid things we fatigue it, but when we gaze at moist things we do not fatigue it, as nothing blocks the sight.
τὰ δὲ χλωρὰ στερεά τε μετρίως, καὶ ὑγρὸν ἐν αὐτοῖς ἱκανόν.
And green things are moderately solid, and there is sufficient moisture in them.
διὸ βλάπτει τε οὐθέν, καὶ διαναγκάζει τὴν ὄψιν πρὸς τούτοις εἶναι διὰ τὸ τὴν τοῦ χρώματος κρᾶσιν σύμμετρον ἔχειν πρὸς τὴν ὄψιν.
Therefore, it does no harm, and it compels the sight to remain on them because the mixture of the color is commensurate with the sight.
§31.20Διὰ τί τὰ μὲν ἄλλα ἀμφοτέροις τοῖς ὀφθαλμοῖς μᾶλλον ὁρῶμεν, τὰ δὲ εὐθὺ τὸ ἐπὶ τῶν στίχων τῷ ἑνὶ προσάγοντες πρὸς τὰ γράμματα μᾶλλον καθορῶμεν;
Why is it that we see other things better with both eyes, but we discern the straight line on written rows better by bringing one eye close to the letters?
ἢ ἀμφότεραι μὲν αἱ ὄψεις συμπίπτουσαι, καθάπερ λέγουσιν οἱ περὶ τὰ ὀπτικά, ταραχὴν παρέχουσιν, ἐπειδὰν δὲ τῇ μιᾷ θεωρῶμεν, πρὸς εὐθεῖαν τὴν ὄψιν, ὥσπερ πρὸς κανόνα, μᾶλλον φαίνεται τὸ εὐθύ.
Is it because, as writers on optics say, both sights falling together produce confusion, but when we look with one eye, the straight line is more apparent because the sight is directed in a straight line, as if against a ruler?
§31.21Διὰ τί ὁ καπνὸς τοὺς ὀφθαλμοὺς μᾶλλον δάκνει;
Why does smoke sting the eyes most?
ἢ ὅτι μόνοι ἀσθενέστατοι;
Is it because they alone are the weakest?
ἀεὶ γὰρ τὰ ἔσω τοῦ σώματος ἀσθενέστατα.
For the inner parts of the body are always the weakest.
σημεῖον δὲ ὅτι καὶ τὸ ὄξος καὶ ἕκαστον τῶν δριμέων τὴν μὲν ἔξω σάρκα οὐ δάκνει, τὴν δὲ ἐντός, ὅτι ἀραιότατον τοῦ σώματος καὶ μάλιστ’ ἔχει πόρους·
And a sign of this is that both vinegar and each of the pungent substances do not sting the outer flesh, but they sting the inner flesh, because it is the most porous part of the body and has the most passages.
αἱ γὰρ ὄψεις διά τινων πόρων ἐκπίπτουσιν, ὥστε τὸ ἔσω δηκτικώτατον ἀπὸ τῆς σαρκὸς ἀποπίπτει.
For the sights pass out through certain passages, so that the inner part, being most sensitive to stinging, is exposed apart from the flesh.
ὁμοίως δὲ καὶ τὸ κρόμμυον, καὶ ὅσα ἄλλα δάκνει τοὺς ὀφθαλμούς.
It is similar also with the onion, and all other things that sting the eyes.
τὸ δὲ ἔλαιον μάλιστα τῶν ὑγρῶν, ὅτι λεπτομερέστατον· τοιοῦτον δ’ ὂν εἰσδύνει διὰ τῶν πόρων·
And oil does so most of all liquids, because it consists of the finest parts; and being such, it penetrates through the passages.
τὸ δ’ ὄξος ἐν φαρμάκῳ τῇ ἄλλῃ σαρκί.
Whereas vinegar acts on the other flesh in the form of a drug.
§31.22Διὰ τί ὁ ὀφθαλμὸς μόνον τοῦ σώματος, ἀσθενέστατος ὤν, οὐ ῥιγοῖ;
Why is it that the eye alone of the body, though it is the weakest, does not feel the cold?
ἢ ὅτι πίων ἐστὶν ὁ ὀφθαλμός, σαρκὸς δὲ οὐθέν;
Is it because the eye is fat and has no flesh?
τὰ δὲ τοιαῦτα ἄριγά ἐστιν.
And such things do not feel the cold.
οὐ γὰρ δὴ ὅτι γε πῦρ ἐστὶν ἡ ὄψις, διὰ τοῦτο οὐ ῥιγοῖ· οὐ γὰρ τοιοῦτόν γέ ἐστι τὸ πῦρ ὥστε θερμαίνειν.
For it is certainly not because the sight is fire that it does not feel the cold; for that fire is not of such a kind as to warm.