By evaluating, with help of Mie's theory, absorption measurements of spherical silver particles — (between 24 and 210 Å in diameter) — suspended as colloid in photosensitive glasses we found that the absorption coefficient at the wavelength of absorption peak (
=0.405
) remains constant, whereas the refractive index increases by about a factor of 5 when the particle size decreases. Within accuracy of measurements and the differences of bulk optical constants of various authors this deviation from the bulk values can be described by the
free path effect
, the influence of conduction electron collisions with particle surfaces. The results are compared with the quantum mechanical theory of
Kawabata and
Kubo.
We are indebted to Prof. Dr. J.Dörr for use of the Electrologica X1 of the Rechenzentrum der Universität des Saarlandes, to Dr. R. D.Maurer and Dr. J. S.Stroud of the Corning Glass Works, New York, for some of the glass samples used, and to Dr. F. J.Schoenes for a part of the computation program.