The B&S CF 28A is a porro II prism monocular with 7 times magnification and a 42mm objective lens. Its field of view is 127yds/1000ft. or 7°18'. The patent number 572537 engraved at the top of the prism box tells us that there is " a thin sheet of a transparent flexible material" between the prism elements (s. W.Reid's book "We're certainly ....", p. 168). Also the field lens is cemented to the prism.