SYNOPSISThe purpose of this paper is to show that refractive‐index measurements made by interference microscopes can provide much varied and interesting information about many of the visible constituents of living cells, and consequently about the cells as a whole.There are two main methods of measuring the refractive indices of cytoplasmic inclusions in living cells; by immersing the cells in non‐toxic isotonic mounting media with the same refractive index as their cytoplasm, and by compressing the cells to a determined thickness. Both these methods, which can under certain circumstances also be applied to measuring the refractive indices of nuclear inclusions, are described in detail.Usually, refractive‐index measurements made on an inclusion in a living cell are used to indicate the percentage of total solid and water present in that region, but, in addition, they can sometimes provide information about its submicroscopic molecular arrangement, its histochemical nature, and its permeability properties; and examples of all these interpretations are desc