Abstract
Experiments on the stability of the circular interface formed in a Hele-Shaw cell when air displaces a viscous oil have shown that available theories underestimate the wavelength of the most unstable wave when the capillary number (Nca) is large. Apparently this is caused by a modification of the interface boundary condition by three-dimensional effects. At small values of Nca the results overlap several of the theories and we are unable to choose among them on the basis of the present experiments.