Sir Richard C. Jebb, Commentary on Sophocles: Trachiniae

ἔνθα μή τις εἰσίδοι would usu. mean, ‘in any place where no one beheld,’— oblique of “ἔνθ᾽ ἂν μή τις εἰσίδῃ”. But here the sense is final; ‘where no one should behold.’ The normal Attic for this would be, “ἔνθα μή τις ὄψεται” (cp. 800),—not “ὄψοιτο”, since, in a final relat. clause, the fut. indic. was usu. kept even after a secondary tense.

In Homeric Greek, a final relat. clause can take the subjunct. (usu. with “κε”) after a primary tense, and the optat. (without “κε”) after a secondary tense. But this is not an Attic construction. Thus the Homeric “ἄγγελον ἧκαν ὃς ἀγγείλειε” ( Od.15. 458) would in Attic be “ἄγγελον ἔπεμψαν ὃς ἀγγελεῖ”: it could not be, “ὃς ἀγγείλειε”. The constr. “ἔνθα μή τις εἰσίδοι” —a very rare one in Attic—has grown out of the ‘deliberative’ constr. “οὐκ οἶδεν ἔνθα μή τις εἰσίδῃ”, by steps which have changed the interrogative clause into a final relative clause. A like instance is Ph.281“οὐχ ὅστις ἀρκέσειεν”, (seeing no one) to aid. See Appendix.