  Waldmeier [1971] found a very tight relationship between the F10.7 solar
radio flux and the sunspot number and suggested using the flux for an objective
calibration of the sunspot number. He suggested that if this relationship
changed later on, the sunspot number should be re-calibrated, assuming that the
calibration must have drifted with time. I repeat his analysis using data up to
the present and it is, indeed, clear that the relationship has changed
significantly. This could be due to a drift of the calibration or to a secular
change in the visibility of sunspots, or both.
