Subject: I don't know how to count the bacteria.

In my project I grow bacteria in different temperatures.  I do 
this in petri dishes using agar.  What I was not sure of was how 
I could "count" the bacteria.  The only way I can think of is to 
give a general observation.  Such as half full, less than half, 
more than half etc.  I would like a better way to do this so that 
I can possibly put this on to a gragh.  Do you have any ideas 
that I can think about.  Thank you for answering.

Re: I don't know how to count the bacteria.