In this work, we demonstrate that the spectral-frequency composition of a wavelength-resolved image registered by a reflected-light, bright-field microscope can be analyzed to independently obtain two explicit physical measures of the RI distribution within weakly scattering samples such as biological cells and tissues: the standard deviation and the spatial correlation length. Since the local mass density is a linear function of RI within biomaterials (Gladstone–Dale relation),^{13} these measures of RI distribution directly translate into statistics of mass density distribution inside biological cells and tissues: the correlation length of mass density is exactly the same as that of RI, and the standard deviation of mass is that of RI divided by the RI-mass proportionality coefficient $\alpha =0.18\u2009\u2009ml/g$. In biological terms, variance of local mass density $\sigma \rho 2(x,y)$ quantifies the compaction degree of macromolecular complexes (folded proteins, chromatin aggregates, etc.) contained within the volume underneath a diffraction-limited area surrounding each pixel $(x,y)$.^{21} In turn, $lc(x,y)$ is the characteristic size of macromolecular complexes within that same volume. Hence, measurement of $\sigma \rho 2(x,y)$ and $lc(x,y)$ is an important tool in studies of structure–functional relationship in crucial biological processes including cancer initiation and progression (epigenetic changes observed in fixed-cell nucleus,^{22}^{,}^{23} cytoplasm,^{24} extracellular matrix,^{25}^{,}^{26} etc.), cell proliferation,^{20}^{,}^{27} as well as genome dysregulation and potential therapy.^{28}^{,}^{29}