Try this one:

(look at the two dots for some time....then close your eyes and hold your fingers over them....then try to blink (even though your eyes are closed...you'll understand) one or two times....and, voila...

(i never got the "64" to appear, but a, well, maybe you do

)
-edit- never mind..looking at the roof works just as good

Oh, and the pic is a bit too lengthy since the eyes can only concentrate on a point of 3-5 degrees....gotta sit a bit from the monitor to get the whole pic into that, but it's possible...and put your contrast on the monitor on top....