Tony Moss described:
>..snip... a "Portable Heliograph Set' in a pouch. It was
>simply a mirror about four inches across with a sighting
>hole in the middle. A length of cord attached it to a short
>rod with a bead on top. ... snip ... It all seems rather 'iffy'
>but I suppose was intended as an emergency device.
I recall such devices from the WW-2 years, as included in
"survival kits'' placed in rafts and life boats. Often the
mirror coating was on metal (e.g.,brass) rather than on
too easily breakable glass.
There is a another similar device to the heliograph, or
"sun writer." ------ In this case called a heliotrope, ("sun
turner," just like the garden plant.) The heliotrope was
used in surveying, including the "Great Survey of India."
In either of these devices, a second mirror could relay
the beam or ray onto the signaling-, or beacon-, mirror,
so that a full range of bearings might be covered.
There is also the heliostat, familiar to amateur telescope
makers, where a tilted mirror is attached to a polar-aligned
axis and rotated at half solar hour angle rate (to allow for
the doubled angle of reflection.) A second mirror then
directs the light in a convenient direction for viewing
the solar image, or to 'feed' a spectroscope or other
apparatus. There are variations of multiple-mirror
setups that can yield a stationary solar image that does
not rotate within the image plane, or which use special
mechanical linkages to enable a single mirror to
produce the stationary image.
I suppose that if the motion were to be supplied by
the observer as feedback to maintain the image in
relation to a fixed target, the rotation mechanism
for the mirror could carry an indicator for time, and
so one would have a form of "interactive sundial."
Bill Maddux
