A hollow sphere, $0\mathrm{.}5cm$ in diameter, with a mass of $0\mathrm{.}05g,$ is released in a column of liquid and has a terminal velocity of $0\mathrm{.}5c\frac{m}{s}.$ The liquid density is $0\mathrm{.}9\frac{g}{c}{m}^3.$ The acceleration of gravity is $980c\frac{m}{s^2}.$ Calculate the viscosity of the liquid.