As the bubble is rising steadily the net force acting on it will be zero.

(Because of density of air is negligible, so the value of mg can be neglected)
So, the force of buoyancy B is equal to the weight of the displaced liquid acting upward is equal to the viscous force acting downward, i.e., B=F.
⇒34πR3ρg=6πηRv
Putting R=1mm=10−3m, ρ=1.75×103kgm−3, g=10ms−2 and v=0.35×10−2ms−1
η=910=1⋅11
In SI unit, η=11 poise (CGS).