  How often do bright optical transients occur on the sky but go unreported? To
constrain the bright end of the astronomical transient function, a systematic
search for transients that become bright enough to be noticed by the unaided
eye was conducted using the all-sky monitors of the Night Sky Live network. Two
fisheye continuous cameras (CONCAMs) operating over three years created a data
base that was searched for transients that appeared in time-contiguous CCD
frames. Although a single candidate transient was found (Nemiroff and Shamir
2006), the lack of more transients is used here to deduce upper limits to the
general frequency of bright transients. To be detected, a transient must have
increased by over three visual magnitudes to become brighter than visual
magnitude 5.5 on the time scale of minutes to years. It is concluded that, on
the average, fewer than 0.0040 ($t_{dur} / 60$ seconds) transients with
duration $t_{dur}$ between minutes and hours, occur anywhere on the sky at any
one time. For transients on the order of months to years, fewer than 160
($t_{dur} / 1$ year) occur, while for transients on the order of years to
millennia, fewer than 50 ($t_{dur}/1$ year)$^2$ occur.
