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

I have noticed that if you
trill your tongue while you are looking at a digital clock the numbers seem 
to
wave and even move up and down separately, this works best if you are
relatively far away and if the display is large.
Something to a similar effect happens with a regular computer monitor or 
T.V. I
noticed when I tried this with my laptop that it did not work very well, 
even
at different distances. Also, if you trill your tongue at different speeds 
the
effects on the object change, for instance at lower speeds the computer 
monitor
seems to vibrate but at higher speeds the screen seems to wave (the reason 
that
I used the computer monitor as an example is because my computer monitor 
works
the best for this). I believe that this has to do with the trill vibrating 
your
head so the image appears to move, but I would like to know why with the
digital clock the numbers seem to move independently, or separately up and
down.

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