"Perspective glass" is defined by the OED as: "a scrying glass, or a magic mirror." However, it is evidently synonymous with a "perspective," which the OED defines as: "An optical instrument for looking through, as a magnifying glass, telescope, monocle, etc. In early use also: any of various devices, such as an arrangement of mirrors, for producing an unusual optical effect, e.g. the distortion of an image." As this definition covers quite a wide range of possibilities, it is difficult to pin down exactly what is being referenced in the text.