SYNOPSISUnder certain conditions the relationship between the optical‐path difference and the light intensity in an interference system is identical for both even field and interference fringes. This principle has been utilised to determine the absolute value of optical‐path differences in interference contrast (even field) for estimation of the dry‐mass concentration of tissue sections. A Leitz double‐beam interference microscope has been employed and the intensities were evaluated by photographic photometry. The method adopted consists of the following steps: (a) tissue sections are immersed in a medium which completely penetrates into the object; (b) the specimen is first examined in interference fringes using monochromatic light and a photograph is taken comprising the object and part of the free background; (c) even field is then adjusted and a second photograph is taken under the same conditions. (d) the photographs are quantitatively processed and the densities of selected areas of the object in even field and of the background in both even field and fringes are recorded by a microdensitometer; (e) the optical‐path differences are measured by comparing the densities in the object with those of the sinusoidal reference curve of the interference fringes. Experimental data are presented to validate the various steps of the procedure. The error of the method was found to vary from 2 to 9 p.c. when applied to sections of hyaline