Kevin McMurtrie wrote:

You don't need a 'scope or a signal generator.
Stick a contact microphone on the glass base, plug it into a low-level
input on a guitar amp, turn it up to 11 and let feedback do its work.


I've tried that and it didn't work. The feedback loop finds many odd
local resonances based on them having much better phase alignment than
the primary resonance frequency. It would take a lot of fussing with
the microphone and speaker positions to make it work.

In that case, plug the microphone into your computer.
Use a free frequency measurement program such as:
http://www.aptuner.com/cgi-bin/aptuner/apmain.html
to measure the resonant frequency of the glass when you tap it.

Then it is easy enough to generate a sinewave on the computer
and either cut a CD or plug the computer's line out into your amp.

--
Eiron.