Realist Radex Viewer

The "Realist format" viewer will view stereo slides from Nimslo (half-frame), Realist, and European format. It is a steal the light viewer which means you hold it up to the light, there is no bulb. This viewing method ensures a sharp, high-resolution image and prevents glare or reflections from spoiling the picture. It's a great way to preserve your memories and share them with others in the highest quality.

These come in black or milky white.  

The milky white is the same as the black but is made entirely (except for the lens of course) with a white translucent plastic that lets in light.  These are used by people who print their stereo images on small cards the same size as a realist mount.  The incidental light allows the image to be seen.  It can also be used by stamp collectors to spot small printing offsets in otherwise identical pairs of stamps.  Many two-color stamps issued in the 50s and before were made with two press runs, and a pair of them could give a 3D effect.