The frequency separation between transverse modes of a ruby laser is measured using a Fabry-Perot interferometer and a high-speed camera. The results are compared with the theoretical predictions based on the curved-reflector theory. Satisfactory agreement is obtained. The measurements are verified by studying the output of the laser after application of transverse mode selection. The mode selection technique is based on the control of the reflector size, and results in single-transverse-mode operation at high pumping levels. Time dependence of the longitudinal mode spectrum of a single-transverse mode laser is studied. Particularly orderly mode-hopping sequences from shorter to longer wavelengths are observed. The spectrum of a relatively high-output laser which shows regular pulsations is similarly examined. The spectrum shows some interesting features, which include a near continuum during regular pulsations and repetitive shifts of the high intensity portion of the spectrum from short to long wavelengths.