Abstract: A stereo slide viewer comprising a stereo slide holder, first and second projection lenses held in a lens holder, first and second focusing plates held by respective first and second focusing plate holders. The lens holder is located between the stereo slide holder and the first and second focusing plate holders. Distance adjusting means are also provided for moving the focusing plates perpendicular to optical axes of the system.

Abstract: A stereo slide mount is divided into a base frame 21 and a cover frame 22 in order to facilitate the operation for mounting the stereo slide and to improve correctness. The size of the windows of the base frame is equal to the size of the pictures of the films, and pins 25 are provided at four corners of the windows to engage with perforations of the film. The cover frame 22 has holes 26 that correspond to the pins 25 and can be fitted to the base frame. The cover frames 22 are prepared in plural kinds (#0, #1, --) in which the width C.sub.1 between the windows is equal to the width between the windows in the base frame, and the width W of the windows decreases stepwisely. Plural kinds of masking-amount guide devices 31 having the same size of windows as that of the cover frames and having a collimation pattern at the same position on the right and left windows, are successively and temporarily fitted to the base frame 21, and the image of the stereo slide films is observed using a stereo slide viewer.