Apparatus for Observing Polarized Light through Glass, by Tisley & Spiller, c. 1875

The form of this device is closely similar to the design of a simple instrument for holding a thick piece of glass in polarized light. If the glass has been uniformly heated it will cool down more quickly at the edges creating iridescent colours. Equally, if the frame of the instrument is heated and the glass piece inserted, the edges will heat up first, with the differential expansion creating colours in polarised light. See A. Daguin, Traité de physiqie théorique et expérimentale, Paris-Toulouse, 1861. page 567 (fig. 1732) - (http://gallica.bnf.fr/ark:/12148/bpt6k91985r/f568.image.r=flexion.langEN)

But note that the device shown in Daguin is described as having a metal (heat conducting) frame. The square frame of this instrument is wooden, raising a doubt about the identification.