The Stéréo-Mon­o­cle is a chain-based stere­o­scope that allows the view­ing of both stere­oviews and panoram­ic pho­tographs. The device was intro­duced to com­ple­ment the Stéréo-Panoramique cam­era of Lucien Leroy and was described in his cat­a­logue as Sys­tème L. Leroy. The instru­ment was designed in col­lab­o­ra­tion with Mat­tey and man­u­fac­tured exclu­sive­ly by Mat­tey for Leroy.
The Stéréo-Mon­o­cle incor­po­rates a rotat­ing lens pan­el. This pan­el con­tains both stereo­scop­ic lens­es and a wide lens for view­ing panoram­ic pho­tographs. By rotat­ing the pan­el, the appro­pri­ate lens­es can be brought into position.
(from: Com­pendi­um of Stereoscopes)