Stereo Viewer? Eye Doctor Tool?

I actually have two of these...one was from a thrift store and this one, which is in better condition came from an antique store. The antique store owner had no idea what this was and neither do I?! I've had this for 10 years and never known exactly what it is and what it does. I thought maybe it was some sort of a stereo viewer or possibly an eye doctor's tool of some sort. Any help from you experts out there?