Upon their separating after coming to this understanding each of them must first have on the spot a telescope with two tubes, so that with the one he can observe the space on the right of the man who is going to signal back and with the other that on the left.
The tablets must be set straight up in order next the telescope,
and there must be a screen before both spaces, as well the right as the left, ten feet in length and of the height of a man so that by this means the torches may be seen distinctly when raised and disappear when lowered.
When all has been thus got ready on both sides, if the signaller wants to convey, for instance, that about a hundred of the soldiers have deserted to the enemy, he must first of all choose words which will convey what he means in the smallest number of letters, e.g. instead of the above "Cretans a hundred deserted us," for thus the letters are less than one half in number, but the same sense is conveyed.
Having jotted this down on a writing-tablet he will communicate it by the torches as follows:
The first letter is kappa. This being in the second division is on tablet number two, and, therefore, he must raise two torches on the left, so that the receiver may know that he had to consult the second tablet.
He will now raise five torches on the right, to indicate that it is kappa, this being the fifth letter in the second division, and the receiver of the signal will note this down on his writing tablet.
The dispatcher will then raise four torches on the left as rho belongs to the fourth division, and then two on the right, rho being the second letter in this division. The receiver writes down rho and so forth. This device enables any news to be definitely conveyed.
Walbank Commentary