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