  We measured the resonance spectra of two stadium-shaped dielectric microwave
resonators and tested a semiclassical trace formula for chaotic dielectric
resonators proposed by Bogomolny et al. [Phys. Rev. E 78, 056202 (2008)]. We
found good qualitative agreement between the experimental data and the
predictions of the trace formula. Deviations could be attributed to missing
resonances in the measured spectra in accordance with previous experiments
[Phys. Rev. E 81, 066215 (2010)]. The investigation of the numerical length
spectrum showed good qualitative and reasonable quantitative agreement with the
trace formula. It demonstrated, however, the need for higher-order corrections
of the trace formula. The application of a curvature correction to the Fresnel
reflection coefficients entering the trace formula yielded better agreement,
but deviations remained, indicating the necessity of further investigations.
