Bagnall's screens

Bagnall's screens are collodion-coated, dye-treated glass plates that were invented by Oscar Bagnall, based on the earlier work of Walter J. Kilner (and his Kilner screens), for the intended development of certain visual faculties by a user that would enable them to see the aura of other beings and objects, eventually to the point that they would not require the use of the screens. Bagnall experimented with replacing dicyanin, Kilner's preferred dye, with pinacyanol in the fabrication of blue screens, finding they were as effective for the purpose, leading him to speculate "that dyes other than dicyanin have the same effect upon the nerves of the retina," suggesting that it is the screen's colour (wavelength of the light) rather than any particular chemical property that made them effective.