Peculiar properties of specially treated flat glass make it transparent to light rays and vision at an angle of 90 degrees, but looked at from above at a 45-degree angle, it becomes opaque. Used as window glass, it is in keeping with the present air-raid precautions campaign. To an observer on the ground, or on the same level as the window, the glass is clear. However, to the warbirds overhead, a factory’s windows would give no visible light to mark it as a target.