This handheld mirrored stereoscope has a very special twist.  It can view both stereocards in small formats, normal Holmes cards and large format stereo prints.  It works by using an adjustable converging/diverging mirror arrangement.  The central knob causes the internal mirrors to angle inward or outward depending on how far apart the centers are on the stereo image one is viewing.  It has high quality optics which include 4 heavy glass front surface mirrors and all elements are sealed by glass optical windows.  It is used for viewing: