Since very old times nonverbal signs were used in army for communication. This type of signs provided security of message. One of most famous and oldest method of optical military communication is sound of drums before the war. This sound warned allies and confused enemies.
However, by the time humankind developed. By the time people invented electricity. In my opinion invention of electricity was a turning point for humans. Obviously electricity brought many problems but it solved many more. I would consider electricity as accelerator of development for people.
Electricity also has very big impact on optical military communication. Morse code is very good example of it. In nineteenth century in America Samuel Morse built first telegraph line, communication system between Baltimore and Washington. Morse used an electromagnet to raise the signal, so that it was strong enough to make a sequence of dots and lines on the sheet of paper on the other end.
Morse code is plain English alphabet and every single letter transformed into dots and lines. Every letter of the alphabet has its own unique combination of dots and dashes. This alphabet is called Morse alphabet or more often Morse code. Labeling this alphabet as code was mainly because Samuel Morse simply encoded each letter with special combination. By the time Morse code became very used in Europe. This alphabet was very useful in police, enabling the police to capture bandits, providing spread of the news faster and allowing distant companies to make deals.
In military sphere it came later. In early 1900s. It became very useful in military troops. Especially in Navy and Aviation. Telegraph was very heavy and hard to carry it in to the war, so telegraph was substituted with simple lantern or mirror. Any light source could be used. Communication in war zones was crucial for army and Morse code with any source of light provided that. Short intervals between turning on and off considered as dots and dashes represented with turning bulb on and waiting 3 to 5 seconds and then off.
This type of communication became very trendy in military. Governments even started to produce special lamps for that (Figure1).
Figure 1: First signaling oil lamp one of the famous one is Lime light signaling lamp. It worked very easily,
but worked with oil. But after some time oil lamps were changed by Electric lamps.