Abstract: An improved seal assembly provides a mechanical seal for a housing having a window panel of dissimilar material which prevents moisture from entering the housing or liquid escaping the housing over a wide range of temperature and environmental conditions, without loading the window panel to a stressful condition. The seal member is subject to a reduced level of radial force in a limited portion of the seal member across the seal groove and does not have to be pressed from all sides to maintain a seal while accommodating expansion and contraction. Positioning means stabilizes the panel to provide a uniform seal groove.