Probably, it is the most "ancient" device for viewing stereo photos. It's principle is simply based on direct placing the left image in front of the left eye and the right image in front of the right eye. Stereoscopes of various modifications are widely distributed and used to view stereo slides (Fig. 11).

Fig. 11. Slide stereoscopes (picture from www.stereoscopy.com)

Wide capabilities are available with stereoscope designed for viewing stereo pairs printed on paper (Fig. 12). This method is convenient because stereo cards can be easily made at home and if you do not have a stereoscope, you can view them as regular photos.

Fig. 12. Stereoscope (picture from www.stamptex.pl)

Due to the fact that the left and right images are distinctly separated and that it is possible to adjust the viewing distance, stereoscopes provide a beautiful stereo effect and comfortable viewing images.