We present experimental observations of evanescent waves in a Smith-Purcell free-electron laser (FEL). These waves, predicted by both theory and simulations, have wavelengths longer than the Smith-Purcell radiation, group velocity antiparallel to the electron beam, and for sufficiently high current, provide feedback to bunch the electron beam. This feedback is the basis of oscillator operation of the SmithPurcell FEL. The wavelengths observed agree with theoretical predictions, and strong radiation from the upstream end of the grating confirms the negative group velocity. Radiation observed at the second harmonic may indicate electron bunching by the evanescent wave.

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Summay.-For 37 participants, 16 men and 21 women, scores on the Tellegen Absorption Scale significantly correlated with liking for abstract paintings but not with liking for representational ones. After completing the scale, each participant viewed 20 paintings in quasirandom order, 10 of representational works and 10 of abstract ones. Each was rated on a scale from 1 for strongly disliked to 5 for strongly liked. The Pearson correlation of .48 for abstract paintings was significant, while that of .16 for representational ones was not.

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