Re: Why do the numbers on a digital clock seem to wave when you trill your tong

   What you are seeing here is a kind of strobe effect, an interference 
pattern.  Your eyes constantly make very rapid movements (called saccades) 
when you look at something.  LCD displays, monitors and TV screens don't 
show a continuous picture, but refresh their displays frequently, doing so 
fast enough that you can't see it happen when you just look at them 
normally.  When you trill your tongue against the roof of your mouth this 
transfers movement to your eyes, interrupting the normal saccade patterns, 
which then enables you to see the refresh lines on the screen.  These 
lines travel from top to bottom of the screen, not side to side.  So I 
expect that if you put your monitor on its side, or hold your head at 
right angles to it when you trill the effect should disappear.  On your 
digital clock the numbers possibly seem to move independently because they 
are 'refreshed' from left to right.  The effect you see may vary on 
different computer monitors that have a different refresh rate.
   Thank you for an interesting question.