§29## κθ΄.
## 29.
Δυνατόν ἐστιν ἔνοπτρον κατασκευασθῆναι ὥστε ἐν τῷ αὐτῷ φαίνεσθαι πλείω πρόσωπα, τὰ μὲν μείζονα, τὰ δὲ ἐλάσσονα, καὶ τὰ μὲν ἔγγιον, τὰ δὲ πορρώτερον, καὶ τῶν μὲν τὰ δεξιὰ δεξιά, τὰ δὲ ἀριστερὰ ἀριστερά, τῶν δὲ τὰ ἀριστερὰ δεξιά, τὰ δὲ δεξιὰ ἀριστερά.
It is possible for a mirror to be constructed so that in the same mirror several faces appear, some larger, some smaller, and some closer, some further away, and of some the right appears right, the left left, and of others the left right, the right left.
ἔστω γὰρ ἐπίπεδον τὸ ΑΜ. οὐκοῦν ἐν τούτῳ γένοιτʼ ἂν κυρτὰ μὲν ἔνοπτρα οἷα τὰ ΑΟΓ, ΘΡΚ, κοῖλα δὲ οἷα τὰ Γ∠Ε, ΖΗΘ, ἐπίπεδα δὲ οἷα τὰ ΕΖ, ΛΜ. τεθέντος οὖν τοῦ προσώπου, ὅπου τὸ Ν, φαίνεται ἀπὸ μὲν τῶν ἐπιπέδων ἴσα τὰ εἴδωλα καὶ ἴσον ἀπέχοντα, ἀπὸ δὲ τῶν κυρτῶν ἐλάσσονα καὶ ἔλασσον ἀπέχοντα, ἀπὸ δὲ τῶν κοίλων παντοδαπῶς, καθάπερ δέδεικται.
For let AM be a plane. Therefore, in this there could be made convex mirrors such as AOG, ThRK, and concave ones such as GDE, ZETh, and plane ones such as EZ, LM. Therefore, when the face is placed where N is, from the plane mirrors there appear equal images at an equal distance, from the convex ones smaller images at a smaller distance, and from the concave ones in all kinds of ways, just as has been shown.
§30## λ΄.
## 30.
Ἐκ τῶν κοίλων ἐνόπτρων πρὸς τὸν ἥλιον τεθέντων πῦρ ἐξάπτεται.
From concave mirrors placed towards the sun, fire is kindled.
ἔστω κοῖλον ἔνοπτρον τὸ ΑΒΓ, ἥλιος δὲ ὁ ΕΖ, κέντρον δὲ τοῦ κατόπτρου τὸ Θ, καὶ ἀπό τινος σημείου τοῦ ∠ ἐπιζευχθεῖσα μὲν ἐπὶ τὸ Θ κέντρον ἡ ∠Θ ἐκβεβλήσθω ἐπὶ τὸ Β, προσπεπτωκέτω δὲ ἡ ∠Γ ἀκτὶς καὶ ἀνακεκλάσθω ἐπὶ τὸ Κ. ἀνακλασθήσεται δὴ ἐπάνω τοῦ Θ κέντρου· ἡ γὰρ γωνία ἡ πρὸς τῇ περιφερείᾳ ἡ Π ἐλάσσων ἐστὶ τῆς πρὸς τῇ περιφερείᾳ λοιπῆς τῆς ὑπὸ ΒΓ∠.
Let the concave mirror be ABG, the sun EZ, and the center of the mirror Th, and let the line DTh joined from some point D to the center Th be extended to B, and let the ray DG fall upon it and be reflected to K. It will indeed be reflected above the center Th; for the angle at the circumference P is smaller than the remaining angle at the circumference, BGD.
καὶ ἔστω ἡ Α Β περιφέρεια ἴση τῇ ΒΓ, καὶ ἀπὸ τοῦ ∠ ἄλλη τις ἀκτὶς προσπιπτέτω ἡ ∠Α. φανερὸν οὖν, ὅτι ἀνακλωμένη ἡ Α ∠ ἀκτὶς πεσεῖται ἐπὶ τὸ Κ διὰ τὸ ἴσην εἶναι τὴν ΑΒ περιφέρειαν τῇ ΒΓ. ὁμοίως δὲ δειχθήσεται, ὅτι πᾶσαι αἱ ἀπὸ τοῦ ∠ προσπίπτουσαι πρὸς τὸ ἔνοπτρον καὶ ἴσας ἀπολαμβάνουσαι εἰς τὸ αὐτὸ συμπεσοῦνται τῇ Θ ἀνώτερον τοῦ Θ.
ἔστω πάλιν κοῖλον ἔνοπτρον τὸ ΑΒΓ, ἥλιος δὲ ὁ ∠ΕΖ, καὶ ἀπό τινος σημείου τοῦ Ε διὰ τοῦ Θ κέντρου ἔστω ἡ ΕΘΒ, καὶ ἀπʼ ἄλλων [διὰ] τῶν ∠, Ζ αἱ ∠ΘΓ ΖΘΑ. οὐκοῦν προδεδείχαμεν, ὅτι αἱ ἀπὸ τοῦ Ε ἀκτῖνες συμπεσοῦνται εἰς ἑαυτὰς διὰ τὰς Π, Ρ γωνίας ἴσας οὔσας· διάμετροι γάρ εἰσιν· αἰ δὲ ἀπὸ τοῦ Ζ διὰ τὰς Κ, Λ γωνίας, αἱ δὲ ἀπὸ τοῦ ∠ ἐπὶ τὴν ∠Γ διὰ τὰς Ν, γωνίας ἴσας οὔσας.
And let the arc AB be equal to BG, and let another ray DA fall from D. It is therefore manifest that the reflected ray DA will fall upon K because the arc AB is equal to BG. In like manner it will be shown that all rays falling from D upon the mirror and cutting off equal arcs will meet in the same [line] above Th. Let the concave mirror again be ABG, the sun DEZ, and let EThB be from some point E through the center Th, and let DThG, ZThA be from the other points D, Z. Therefore, we have shown before that the rays from E will meet themselves because the angles P, R are equal; for they are diameters; and those from Z because of the angles K, L, and those from D upon DG because of the equal angles N.
ὅτι δὲ πᾶσαι αὐταὶ εἰς ἑαυτὰς ἀνακλῶνται, δῆλον· ἐκ τοῦ γὰρ κέντρου οὖσαι ἡμικύκλια ποιοῦσιν, αἱ δὲ τῶν ἡμικυκλίων γωνίαι ἴσαι εἰσίν· διʼ ἴσων ἄρα γωνιῶν αἱ ἀνακλάσεις γίγνονται· εἰς ἑαυτὰς οὖν ἀνακλῶνται.
And that all these are reflected into themselves is clear; for being from the center they make semicircles, and the angles of semicircles are equal; therefore the reflections take place through equal angles; therefore they are reflected into themselves.
πᾶσαι ἄρα συμπεσοῦνται ἀπὸ πάντων τῶν σημείων ἐπὶ τὰς διὰ τοῦ κέντρου καὶ ἐν τῷ κέντρῳ [ἀκτῖνας].
Therefore all rays from all points will meet upon the [rays] through the center and at the center.
τούτων οὖν τῶν ἀκτίνων ἐκθερμαινομένων περὶ τὸ κέντρον πῦρ ἀθροίζεται.
Therefore, when these rays are heated around the center, fire is collected.
ὥστε ἐνταῦθα στύππιον τεθὲν ἐξαφθήσεται.
So that tinder placed there will be kindled.