Theoretical and experimental treatments of the shape of Fizeau fringes
crossing a fibre of trilobal-shaped cross-section assumed to consist
of a single medium are presented. A new mathematical expression is der
ived and used to estimate the refractive indices and the birefringence
of Perlon (polycaprolactam) fibre. The resulting data are used to cal
culate the polarizability per unit volume. Cauchy's dispersion constan
ts, the isotropic refractive index and the variation of refractive ind
ex with temperature are reported. The results are in agreement with pr
eviously published data The data are illustrated by microinterferogram
s.