Full details are given in this paper for carrying out the measurement of the refractive index of living cells by the method already discussed in Parts I and II of this series (Barer and Joseph, 1954, 1955). The method of preparing and storing immersion media is described as well as the dialysis technique used for adjusting pH, and in certain cases salt concentration. Methods of measurement applicable to single cells and to cell populations are discussed. It is shown that the numerical results can be used to obtain the concentration of solids, concentration of water, total wet mass and density, and concentrations in terms of wet weight of tissue. The underlying optical principles are discussed in some detail.

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