Following on from my post on heliographs one of my regular readers observed:
“Aiming a signalling mirror with the sun behind you could be tough but basically if you have the sun behind you and a plane in front, the problem (conceptually) is put the mirror flat between the two.”
He is correct. During my research I came across this informative illustration:
Note that the signaller is using his free hand to aim by directing the reflected spot of light on to it.