Stereoscopes, also known as stereopticons or stereo viewers, were one of America's most popular forms of entertainment in the late 1800s and early 1900s. 

Claims that there was a stereoscope in every parlor in America came as early as the 1860s (Darrah, 2), but in their second wave of popularity in the 1880s-1910s, the availability of stereographs could be quantified: Underwood & Underwood, one of the three major stereographic companies in this period, produced over 25,000 images per day (Darrah, 47), and an estimated 300 million stereographs were issued between 1854 to 1920 (Wadja, 112). Selling at six for a dollar, most stereographs captured the interest of middle class consumers, but a few companies catered to the working class, providing similar views at 3 cents a piece or 85 cents per 100 (DeLeskie, 69). Found in drugstores, distributed through mail-order catalogs, given away as premiums by cereal and tea companies, and canvassed cross-country by college students (including a young Carl Sandburg), it is no wonder that many scholars consider the stereoscope as the first mass photographic medium prior to cinema or television (see Trachtenberg, Reading, 17).

Around the turn of the century, the stereoscope began to move, along with education in general, from the parlor to the public school. Underwood & Underwood and the other company with the widest distribution of the period, the Keystone View Company, actively marketed sets of 600 or more views to schools for instructional purposes.

Along with the educational sets, the immensely popular “Tours of the World” and “Underwood Travel System” as well as individually sold cards worked to give Americans homogenized ways of seeing labor in America and in foreign lands. In a time of conflicting messages about industrialization and an increasing amount of visual and statistical information, the “highly structured presentation of visual and text-based information provided viewers with a familiar framework with which to approach change” (DeLeskie, 121). Stereographs standardized the way images were constructed, what was worth looking at and how to look at images.

Stereoscopes continued to be widespread in America until the 1930s.  Then stereoscope production declined, likely due to the new interest in motion pictures.  However, the stereoscope continues to offer viewers something that no ordinary photograph or movie can offer, namely a sense of depth and image realism.  A descendant of the stereoscope, the Viewmaster, became a popular children's toy.

Further Reading: http://xroads.virginia.edu/~MA03/staples/stereo/stereographs.html