Thank you @Ric and @sabbamitta. 
I think I may have found it, thanks to @sabbamitta’s pointer in another thread.
Here’s what Mil 3.1.15 says in T.W. Rhys Davids’ translation (1890):
‘[wisdom’s (paññā) characteristic mark of enlightenment (obhāsanalakkhaṇā)] is like a lamp, O king, which a man might introduce into a house in darkness. When the lamp had been brought in it would dispel the darkness, cause radiance to arise, and light to shine forth, and make the objects there plainly visible. Just so would wisdom in a man have such effects as were just now set forth.’
I wonder where Tolstoy got the notion of “perserverance, persistance” (упорствуй) from? Maybe he read another translation?