9 x 18 format

The 9 x 18 cm format probably originated as a “slightly larger” version of the 8.5 x 17 cm stereoview. The format developed into a size that was adopted by manufacturers. In general, stereoscopes for the 8 x 19 cm format can also be used for 8.5 x 17 cm stereoviews. A summarised quotation from Mattey's 1922 catalogue:

Stereoscopes for 8.5 x 17 are divided into two very distinct categories: the old models, series A, and the new models, series B. The models of series A are built to support 8.5 x 17 and 9 x 18 views; that is to say, the rear part is wide enough for 9 x 18; the 8.5 x 17 therefore fits easily into place, but centring can be obtained only by trial and error. The models of this series are therefore built rather for examining 9 x 18 views. For here there is an anomaly: the format for paper stereoviews is 9 x 18, while the format for stereoviews on glass is 8.5 x 17. We have always fought against this oddity, but unfortunately without success. The stereoscopes of series B are built specially for a specific size and can receive only one format. 

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