The development of phase contrast and interference microscopy has focused attention on the possibility of measuring the refractive indexes of living cells. Difficulties are encountered in applying immersion methods of refractometry to living cells because the choice of immersion media is very limited. These difficulties have now been overcome by the use of protein solutions. When the refractive index of the immersion medium matches that of the cell cytoplasm the latter becomes virtually invisible and phase contrast can be used as a sensitive null indicator to determine the match position. The refractive index is directly related to the concentration of cell solids and refractometry has been extensively used for measuring such concentrations in a variety of living cells. The concentration of water also follows immediately. Examples of the application of this method to retinal rods and cones, to the study of cell populations such as red blood cells, and to the investigation of the osmotic reactions of living cells are given. Other applications such as contrast variation in living cells, demonstration of chromosomes, and the spectrophotometry of clarified cell suspensions are briefly discussed.

Interference microscopy can be used for the direct measurement of optical path difference and it is shown that this quantity is directly related to the dry mass per unit area. By combining interference microscopy and immersion refractometry it becomes possible to measure the cell thickness and hence to calculate dry mass and wet mass, volume, and water content of living cells. Examples of these methods are given and some of the limitations indicated.