mmmm, not exactly. you cannot see atomic brownian motion with an optical microscope, what you can see is visible brownian motion of otherwise visible particles caused by their collisions with molecules/atoms. this says as much about the momentum/energy of the collisions as it does about the mass (which bears some relationship to the size which bears direct relation to optical visibility)

You are correct, but that is what I meant, even if the way I wrote it is either incorrect or somewhat unclear.