## Abstract

The hyperfine structures of the two visible resonance lines of the arc spectrum of indium 115 have been measured in the absorption of a multiple atomic beam of high collimation by means of a double Fabry-Perot interferometer; the combined instrumental and Doppler width of the absorption lines was less than 5 mK (0$\cdot $005 cm$^{-1}$). By taking the necessary precautions to avoid systematic errors and by measuring a large number of different interferograms to reduce the random error it has been possible to measure the h.f.s. of the lines 4101 and 4511 angstrom with an accuracy sufficient to allow the intervals of the terms 5 $^{2}$P$_{\frac{3}{2}}$, 5 $^{2}$P$_{\frac{1}{2}}$ and 6 S$_{\frac{1}{2}}$ to be determined with a probable error of 0$\cdot $07 mK. The ratios of the three h.f.s. intervals of the term 5 $^{2}$P$_{\frac{3}{2}}$ are in agreement, within the limits of the probable error, with those required on the assumption that the h.f.s. is due to nuclear magnetic dipole and electric quadrupole moments; the value found for the dipole coupling factor, A, is 8$\cdot $083 $\pm $ 0$\cdot $01 mK and for the quadrupole coupling factor, B, is 15$\cdot $10 $\pm $ 0$\cdot $1 mK. A comparison is made between the values of the h.f.s. intervals found in this work and those found by microwave measurements, since one of the principal aims of this research is to test the agreement between the results found by optical spectroscopic methods and by microwave methods.