Weil gives three variant readings for this sentence:
2) σαφές τὰ (Change that accent on σαφές to a grave b/c I'm too font challenged.)
3) σαφῶς τὰ
As for the final word in the sentence. I think we all agree that it is nom. sing. Now the question is whether it is transitive first aorist or intransitive second aorist. Here I make use of Mastronarde's online paradigms to derive the first aorist: http://socrates.berkeley.edu/~ancgreek/ ... verbpdgm59
For the participle without prefix, in the present, the masc. sing. would be ἱστάς.
For the participle without the prefix, in the aorist, the masc. sing. would be στάς.
For the participle with the prefix, in the aorist, the masc. sing. would be ἐπι-στὰς.
Now in Polybius, we have ἐπι-στήσας. This I take to be the second aorist intransitive participle form. And I take the meaning to be as Qimmik indicated:
V. [select] fix one's mind on, give one's attention to, “σφαγῇ” E.Andr. 547; “τῇ τρύγῃ” PFlor.236.4 (iii A. D.); “ἐπί τι” Isoc.10.29, D.18.60; “τοῖς πράγμασιν . . ἐπιστάντες” Id.4.12; ἐπιστάς abs. (sc. τοῖς πράγμασι), Id.18.233; “διὰ ταῦτ᾽ ἐγρήγορεν, ἐφέστηκεν” Id.6.19.
The problem with this line is that LSJ's Demosthenes Med. intransitive example uses what according to my account is the transitive ἐπιστάντες.
Qimmik, do you have any reference for your claim that "ἐπιστήσας is the 1st aorist participle"? Mastronarde, despite spending a fair bit of ink on this verb and its various forms in the aorist and in the perfect, avoids the issue of variant forms for the aorist participle, so on that basis alone, I strongly suspect that there might be some wayward uses, especially in a later author like Polybius. I'm too much of a coward to look at the LSJ forms right now.