The Abbe refractometer in its usual form with compensating prisms is restricted to reading index of the test liquid for but one wave length, usually &lgr;D. Although the setting of the compensating prisms for achromatization permits, by the aid of either a numerical or a graphical chart supplied with the instrument, an evaluation of the dispersionnF—nCof the liquid, the value so obtained is claimed to be no more than approximate. Data given in the present paper indicate that the degree of this approximation is variable withnDof the liquid. The agreement is better (about 5%) with a liquid of higher index such asCS2(nD=1.627), but with a liquid of lower index such as methyl alcohol (nD=1.330) the discrepancy may be as high as 30% and above. These conclusions appear to be valid for two particular instruments of quite recent manufacture, used with the charts accompanying them.More reliable values of the dispersion were obtained by calibrating the instrument to read index for any wave length. Then from individual values ofnF, nCthus obtained (which may be calculated from measurements with two other &lgr;'s if the colorsC, Fare not available, by use of the simpler Cauchy formula) the average deviation of the dispersion from book values for five different liquids (1.330<nD<1.627) was less than 5%. In this manner the value of the dispersion of a test sample as determined by achromatization with compensating prisms with the use of the customary chart, can be checked on the same instrument.A method of so calibrating the refractometer to read index for any wave length and for any setting of the compensating prisms has been worked out. The derivation of the necessary equations and the procedure are given in this paper.