If anybody is still watching this thread:
Actually, you can see 337 nm, but extreme care is recommended and one can debate whether you're seeing the 337 nm photons or other photons from a florescence in the eye . And extreme care is perhaps not enough when doing so--UV photons are very good at damaging eyes, and the damage can be both delayed and cumulative! Polycarbonate becomes very absorbing at about 390 nm, so cheap safety glasses make excellent nitrogen laser safety glasses.
The way I saw it was to find a white diffuse scattering surface that didn't fluoresce--it has been decades, and I don't remember what it was. Use the safety glasses, and find something white that the spot doesn't show up on. This is not easy--pretty much everything fluoresces when hit with a few MW pulse. Then take the glasses off, and look at the spot from a few meters away. You'll see a diffuse purple spot--the eye doesn't focus 337 nm, so the spot is large. In my case, I also removed my regular glasses--they blocked UV--and that also made the spot big.
A very good TEA laser can lase air, which mitigates the need for a N2 source. But the power will be higher when running on straight nitrogen. We avoided dealing with tanks and regulators by using the boil-off from liquid nitrogen, but that is because I've always had access to LN2.
My partner-in-crime played with a UV-vis spectrometer, staring into it as he dialed the wavelength further and further out. Again, cautious people have strongly dis-recommended experiments like this. I forget the numbers he came up with, but he could see way further than the canonical 700-400 nm range. The interesting thing is that he found that the perceived color changed somewhere in the 300's--far enough out that one would expect that the green receptors had already given up. This was likely the onset of florescence in the eye.
I once accidentally did the experiment in the other direction, and saw 1064 nm. In that case, it appeared greenish-white, due either to second harmonic generation in the eye, or perhaps a thermal process. Afterwords, I wanted to believe that I saw some secondary emission from the laser, but I checked--using 1064 nm blocking glasses and a video camera--and there was no detectable visible emission. Fortunately, the eye doc could find no damage from that accident.
Working with 780 nm lasers, they were not hard to see. The 850 nm lasers were much dimmer, but still visible to most of us--one of my co-workers had a genetic variation and absolutely could not see 850 nm at any power.
