NGC 1851 velocity dispersion computed binning the data as in Fig. 4. Error bars along the X axis represent the dispersion of the data within the bin; along the Y axis they give the 1-σ uncertainty in the dispersion. The central velocity dispersion as given in Harris (1996) is also shown (diamond). The vertical line marks the radius where the acceleration is a₀. Overplotted on the data is a Gaussian plus a constant, which is not a fit to the data, but is meant to highlight the flattening of the velocity dispersion that occurs well within the cluster tidal radius of 41 pc (Harris 1996). The dotted line falls as r ^− 1/2.