Stokes Law, Viscosity. (very simple)

Main Question or Discussion Point

I'm trying to find the viscosity of some glycerol that we dropped various steel balls down using the equation:

V = [2r^2 (p – σ) g] / 9η

I put in these values:
p = 7800 kg m-3
σ = 1200 kg m-3
g = 9.8 m s-2

And ended up with the equation. η = 129360r^2 / 9V

My problem is that I thought viscosity is a constant. When I put my values in the viscosity decreases as radius decreases.