I was curious, so I did a bit of searching. It appears to be a way to use your existing stereoscopic slit-lamp examination thingamajoober to safely observe the spot where you are shining an argon laser into a patient's eyes to clot or close up some blood vessels in there.
So, like, you have this old slit-lamp thingamabob, and you want to start using this newfangled fancy photocoagulator, you can buy an accessory that has this optic in it that lets you integrate it into your existing equipment.
Better links than what I generally spew forth :crackup:
That piece might be useful as some laser goggles, a la pince-nez, or perhaps an argon-blocking pair of opera glasses? Optics look like they were meant to transmit as much visible light while blocking argon wavelengths.