Is it a Spice Rack?
No, it’s a DIY Michelson interferometer by the Celtic Mad Scientist.
In a standard Michelson interferometer, the beamsplitter would actually go at 90º to the shown orientation, so that each beam hits a mirror, but it’s all good. If your light is polarized, you’ll want to make sure that’s vertical, lest you be near Brewster’s [...]