“Good eyesight recognizes masses of troops at 1,700 yards: beyond this distance the glitter of arms may be observed. At 1,300 yards infantry may be distinguished from cavalry, and the movement of troops may be seen; the horses of cavalry are not, however, quite distinct but that the men are on horseback is clear. A single individual detached from the rest of the corps may be seen at 1,000 yards but his head does not appear as a round ball until he has approached up to 700 yards at which distance white cross belts and white trousers may be seen. At 500 yards the face may be observed as a light colored spot; the head, body, arms and their movements, as well as the uniform and the firelock (when bright barrels) can be made out. At between 200 and 250 yards all parts of the body are clearly visible, the details of the uniform are tolerably clear, and the officers may be distinguished from the men.”

The Artillerist’s Manual and British Soldier’s Companion (1839—1859)