This stereoscope, or Holmes Stereoviewer, was most likely made near the end of the 19^th Century.  Cards with two photographs of the same thing were put into the holder (as seen here) then you would move it back or forth until it was clear for the viewer.  The images are slightly off from each other, thus giving the viewer a feeling of depth, or even 3-D, to the picture.  While the concept of this method of looking at pictures dates back to 1838 in England it wasn’t made popular until Oliver Wendell Holmes created this viewer in 1881.  No, not the Supreme Court Justice Oliver Wendell Holmes Jr., but his father Sr.  

Oliver Wendell Holmes Sr. was already a well known poet, professor, and doctor when he developed this handheld viewer.  This style stereopticon (as it was called) made stereo photography extremely popular, so popular that even today I was able to buy that card in the viewer for about a buck.  This method of photography remained in vogue up into the 1930s, and I would guess only faded because of other forms of entertainment like movies and radio.  Holmes never patented his stereoviewer but instead intentionally gave away the design to society. 

My viewer, while being over a century old, is overall in pretty good shape.  Sadly the lens is missing along with the handle (that would go underneath), but the wood veneer looks really nice and the overall quality is good.  This viewer should seem pretty familiar to most of you.  That is if you ever had a View-Master.