  We analyze the depletion interaction between two hard colloids in a
hard--sphere solvent and pay special attention to the limit of large size ratio
between colloids and solvent particles which is governed by the well--known
Derjaguin approximation. For separations between the colloids of less than the
diameter of the solvent particles (defining the depletion region), the solvent
structure between the colloids can be analyzed in terms of an effective
two--dimensional gas. Thereby we find that the Derjaguin limit is approached
more slowly than previously thought. This analysis is in good agreement with
simulation data which are available for a moderate size ratio of 10. Small
discrepancies to results from density functional theory (DFT) at this size
ratio become amplified for larger size ratios. Therefore we have improved upon
previous DFT techniques by imposing test particle consistency which connects
DFT to integral equations. However, the improved results show no convergence
towards the Derjaguin limit and thus we conclude that this implementation of
DFT together with previous ones which rely on test particle insertion become
unreliable in predicting the force in the depletion region for size ratios
larger than 10.
