Physics for Geologists, Second edition

56 Optics
Figure 4.9 The mirrors in a stereoscope.
Figure 4.10 These figures can be viewed stereoscopically. Concentrate on the
black rings, one to each eye. It may help to separate the images with
a card in front of your nose. Note that the differences of parallax
create the impression of three dimensions.
Topography seems exaggerated. Knowing the height of the aircraft above
the ground, the focal length of the lens of the camera, and the horizontal
distance between the principal points (the intersection of the optic axis of
the camera with the ground and with the film), quite accurate maps can be
drawn including contours and the heights of hills and buildings. The top of
a tall factory chimney, for example, will be closer together in the two pho-
tographs than the bottom, that is, the distances between them on the table
when viewed stereoscopically will be different. An area is covered by runs
and the runs must also overlap for a good map to be drawn.
In Figure 4.10, see if you can fuse the two images into a single stereoscopic
image. If necessary, try placing a card vertically between the two circles to
encourage each eye to look at one circle only, concentrating on the dark
circles. You will see a right pyramid below the thick circle, and a thin circle
above the thick circle, and various triangles and points.
Air photographs differ from maps in two main respects. A map is of con-
stant scale, and the position of all points is the vertical projection of each
Copyright 2002 by Richard E. Chapman