Infrared and optical star counts in the plane of the galaxy.

Infrared and optical star counts have been obtained to investigate the stellar distribution in the plane of the Galaxy. The discovery of an obscured infrared cluster of sources is presented and a search technique for finding such obscured sources using visible and near-infrared plates is discussed. The techniques and mathematical treatment of star counting are described and, as an introduction, a literature review of the galactic structure is given. Optical star counts in a strip across the dark rift of the Galaxy, containing the line of sight to the galactic centre, indicate that the extinction in this direction starts to increase significantly at a distance of about 1500 pc, approximately at the distance of the Sagittarius arm. Counting the stars into reseau squares shows that the direction of the thickest extinction in the visible, for star counts to 18m .0, is not coincident with the plane of the Galaxy but about +1° in latitude away. Infrared star counts at 2.2 microns have been obtained for seven regions, spaced at 10o intervals, in the galactic plane. Away from the nucleus the star counts seem to fit a simple exponential disc model. At a longitude of 30o where the line of sight is tangential to a ring of molecular clouds, the disc population appears to be supplemented by an increase in density of late-type giants. Towards, the galactic centre the extinction at 2.2 microns is low enough for the dense core to be seen in the star counts, these dominating the star counts from the disc population.