Isophotes of a field in the Cygnus loop photographed in the [O III] and [N II]+Hα lines

From interference-filter image-tube photographs of a 9' field in the western part of the Cygnus Loop supernova remnant, taken in the lambda5007 [O III] and lambdalambda 6584, 6563 [N II] + Hα lines, sets of isophotes are derived by an equidensitometry technique based on the Sabattier effect. The emission regions in these lines exhibit a relative displacement, interpreted as evidence for radiative cooling of the gas behind the shock generated in the supernova outburst. An explanation is offered for the differing morphology of the nebular filaments in the [O III] and [N II] + Hα lines. The anomalously high I/sub Otsi/II/I/sub H/β intensity ratio may reflect a spatial separation of the corresponding emission zones