It should be noted that while stereoviews were arguably more suited to the use of landscapes, buildings, statuary and staged tableaux, rather than portraits, the latter are not completely unknown. Hans Peter Hansen (1868-1943) of Ashbourne was one Derbyshire photographer who experimented with sterescopic portraits, as in this example showing three of his children.
Robert Leggatt provides a brief history of sterescopic photography, as do many others, and I will not bother to to repeat this detail here. The popularity of stereoviews continued to grow in spurts through the 1870s and 1880s, fuelled by the development of steamships and cheap travel.
The stereoview could in many ways be considered the forerunner of the postcard, and of course the latter eventually caused a wane in popularity. During the Great War, however, there was a significant resurgence of interest in the stereoview as a means to portray views of the battlefields to family and friends back home.
This image is one from my own collection, captioned "Entente Cordiale; the Allies fraternizing on a canal boat in Flanders," shows a lighter moment away from the front lines. It was No. 73 in one of many series produced by perhaps the largest of the stereoscopic publishers, Realistic Travels Publishers, of London, Cape Town, Bombay, Melbourne and Toronto.
Post Script - June 2008
David Spahr, who has a very interesting web site Stereoviews.com, offered the following comments on stereoviews and corrections to my article above, for which I'm very grateful:
"Stereoviews on paper actually appeared in the 1850s as both salt and albumen prints. I can't really agree that stereoviews influenced postcards all that much. The creation of the postal service and cameras mass produced in postcard format had more to do with it. Stereoviews actually survived and flourished long after the advent of postcards."