Drift Sight US Navy Mark VII
This is a WWII-era Drift Sight Mark VII 88-S-900 as used in multi-crewed aircraft of the US Navy. A drift sight is used to determine the degree of drift from the intended heading due to cross winds left or right, to permit correction of the course by flight controls.
The instrument would either be installed onto an angled periscope installed in the side of the fuselage pointed 90deg to the ground. As the aircraft travels over the earth's surface, terrain as viewed through the drift sight should be traveling parallel with the direction of the aircraft if there are no crosswinds. With crosswinds, the terrain would appear to travel at an angle to the heading of the aircraft, which is measured by comparing the linear scale nearest the eye to the scale at the bottom of the sight. By turning the knob, the scale aligns and drift angle can be measured, and course correction determined.
This example has two pieces, the main instrument and a viewing extension. The extension would have had a mirror affixed to the angled cut to provide a view of the earth's terrain while in flight. The extension would have had another fitting between the two pieces, but it is not included.
You may see other Mark VII drift sights of a slightly different configuration. We do not know the reasons for different designs of the same sight designation.
Measures ~ 12 in x 10 in. Viewing through the lens appears cloudy.