Score image of principal component (PC)2 (inset), showing high scores for the corneal facet walls in specimen MHM-18a (Fig. 3), and a diagram comparing the positive loadings for PC2 (n = 4 analyses) with a synthetic eumelanin reference spectrum (n = 6 measurements). Note the close similarity between the distribution of the PC2 loadings and peak intensities in the eumelanin spectrum, which indicates that the chemical properties of the facet walls (bright areas in the PC2 score image) are directly comparable to this pigment. Negative PC2 loadings were obtained for O− and SiOx− ions, which represent the adjacent mineral matrix. The analyses included 141 negative-ion images at nominal masses between m/z 10 and m/z 150.