Paula Fleming has aptly answered the question, I believe. I would just
like to make a plea for accurate terminology and spelling, however.
Assuming that you were asking about stereoscopes or stereo viewers for
3-dimensional pictures (stereographs), the word "stereoptican" is
incorrect. It's a misspelling of stereopticon, which is not a stereo
viewer at all, but a magic lantern (slide projector), usually with
double or triple lenses, used to produce dissolve effects. I'm not sure
why people got into the habit of calling stereoscopes stereopticons, but
museums which collect them should take care to maintain correct
nomenclature. When someone mentions "stereopticon slides" to me I always
have to request clarification--do they mean glass lantern slides or do
they mean stereographs (to be viewed in stereoscopes)? --David
Haberstich