Franken, Hill, Peters and Weinreich put a ruby laser through quartz and looked for light at half the wavelength. The second-harmonic spot on the spectrograph plate was so faint the journal almost cropped it as dirt. The published figure still looks like a smudge with a caption.
That was the first optical second-harmonic generation: 694 nm in, a little 347 nm out. This bench keeps both on the path to the screen — leftover red and the violet that was not in the lamp.