$\phi =0.545$. Particle size polydispersity was 0.069, measured from the apparent angle dependence of the diffusion coefficient in a dilute suspension [@measurePolydisp].

The colloidal volume fraction was calibrated by measuring the amount of crystal phase in the coexistence region and taking the fluid and crystal volume fraction to be at 0.494 and 0.545 respectively. Samples of the volume-fraction-calibrated stock were also weighed and dried in a vacuum oven to determine the effective density of the particles, which was then used in subsequent calculations of the volume fraction of samples prepared from the stock. The stock volume fraction was also re-calibrated from time to time by drying and weighing.

To induce attraction between the colloids, we added non-adsorbing linear polystyrene. This well-characterized model colloid-polymer mixture has been studied extensively over the last decade [@poon_review]. The polymer used was purchased from Polymer Laboratories and had a molecular weight of $M_w=370000$ daltons. Its radius of gyration in [*cis*]{}-decalin at $20^{\circ}$C was calculated from the data in [@PSdata] to be $r_g = 17.8$ nm. Thus the dimensionless range of the depletion attraction is $\xi \sim
r_g/R=0.09$.

Colloid-polymer mixture samples were prepared by mixing stocks of colloids and polymers with known concentration and solvent by 