Until now, I've been writing about "now" as if it were literally an instant of...

Until now, I've been writing about "now" as if it were literally an instant of time, but of course human faculties are not infinitely precise. It is simplistic to suppose that physical events and mental events march along exactly in step, with the stream of "actual moments" in the outside world and the stream of conscious awareness of them perfectly synchronized. The cinema industry depends on the phenomenon that what seems to us a movie is really a succession of still pictures, running at twenty-five {sic} frames per second. We don't notice the joins. Evidently the "now" of our conscious awareness stretches over at least 1/25 of a second.
In fact, psychologists are convinced it can last a lot longer than that. Take he familiar "tick-tock" of the clock. Well, the clock doesn't go "tick-tock" at all; it goes "tick-tick," every tick producing the same sound. It's just that our consciousness runs two successive ticks into a singe "tick-tock" experience-but only if the duration between ticks is less than about three seconds. A really bug pendulum clock just goes "tock . . . tock . . . tock," whereas a bedside clock chatters away: "ticktockticktock..." Two to three seconds seems to be the duration over which our minds integrate sense data into a unitary experience, a fact reflected in the structure of human music and poetry.