We have made precision measurements of the turbidity of S${\mathrm{F}}_6$ near its critical point using an optical analog of the Wheatstone bridge. From the measurements we obtain values for the compressibility of ${\kappa }_T=(1.39\mathrm{}\pm 0.19){(T-T_C)}^{-1.225\mathrm{}\pm 0.02}=(1.18\mathrm{}\pm 0.16)\times {10}^{-3\mathrm{}}\times \frac{{\epsilon }^{-1.225\mathrm{}\pm 0.02}}{\mathrm{atm}}$, and for the correlation length of $\xi =(72\mathrm{}\pm 11){(T-T_c)}^{-0.67\mathrm{}\pm 0.07}=(1.5\mathrm{}\pm 0.23){\epsilon }^{-0.67\mathrm{}\pm 0.07}$ Å.

• Received 13 February 1970

DOI:https://doi.org/10.1103/PhysRevLett.25.143