We point out the relatively clean geometrical aspect of the Lorentz-Lorenz effect and use it to extract the strength of the effect from an analysis of the elastic scattering of 50-MeV ${\pi }^{+}$ on ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$. We find that for a finite-range $\pi -N$ interaction there is strong evidence for the effect with a value $0.2<~\xi <~0.6$.

• Received 27 September 1977

DOI:https://doi.org/10.1103/PhysRevLett.39.1316