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plural hagioscopes
(architecture) A small opening in an interior wall of a church, enabling those in the transept to view the high altar. quotations examples
It appears that Drs. Jones and Freeman account for the arch over the hagioscope at St. David's by saying, though without citing authorities, that there was a door there originally.
1886, Miss M. A. R. Tucker, “St. David's”, in Walford's Antiquarian: A Magazine & Bibliographical Review