A diffuse transmittance spectroscopy method that utilizes a large area photodetector in contact with the sample is a simple and useful method to probe a weak light absorption by molecular species incorporated in a strongly scattering medium. The method successfully uncovered the electronic transition centered at 455 nm involving what are identified as diatomic Ag nuclei (Ag-2), which were size-selectively formed on a 0.45 mu m octahedral AgBr microcrystal in a photographic emulsion in a surprisingly high density (many thousands on each microcrystal surface) through a heterogeneous chemical reduction. The estimated oscillator strength for the 455 nm absorption, 0.5 +/- 0.1, compares favorably to the theoretical prediction (0.64) reported for the lowest-energy electronic transition of an isolated Ag-2 molecule, which is known to be centered at 435 nm. A possible mechanism underlying the remarkable size selectivity in the chemistry of Ag-2 formation on AgBr is also discussed in conjunction with a hypothetical (111) surface structure of the octahedral AgBr grain.