Two ways to make a flash
A signal lamp carries its own light, so it works at night and in fog, and the shutter makes the dots and dashes. A heliograph carries none: it borrows the sun and flicks it at a distant hill, which means it is silent, weighs almost nothing, and stops working the moment a cloud arrives.
Both were standard army equipment for a century. A heliograph message has been read at over a hundred miles, which no shout, drum or flag will ever manage.
Why armies loved light
A runner can be caught. A wire can be cut, and following it tells the enemy where both ends are. Light needs no wire, leaves nothing to find afterwards, and a lamp pointed carefully is hard to see from the side.
Ships still carry a signal lamp for exactly this reason. Two vessels can talk without a radio transmission that anyone within a hundred miles could pick up.
And the catch is the same as always. Everyone in the beam can read it. Directing the light narrows who receives, but it never encrypts anything — which is why lamp messages have always been sent in code as well as in morse.
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