Effect of Particle Shape on the Spectral Absorption of Colloidal Silver in Gelatin

Small prolate spheroids of silver (between 140 and 400 Å av diam) were produced by photographic development of fine‐grain silver bromide embedded in gelatin. The shape and size distributions of the silver particles were determined from electron micrographs of thin sections. The average axial ratios of these spheroids had values for different preparations between 0.3 and 0.9. The optical absorption was measured and a shift of the main absorption peak (near 400 m$\mu$ ) to shorter wavelengths, and of a secondary peak to longer wavelengths, with elongation of the particles was found to be in good quantitative agreement with the theory of Gans. The position of the main peak can be brought into even better agreement with the theory by taking account of the finite average particle size (by using the Mie theory) and by selecting the most appropriate set of optical constants for chemically prepared silver. Small departures from spherical shape affect the spectral absorption of colloidal silver much more than changes in size or refractive index.