What the eye can see at different distances

I have just been re-reading A PRIMER FOR HORSE, FIELD & MOUNTAIN ARTILLERY by Captain T. H. Lloyd, RA. It was published in 1878, and was intended:

'to facilitate the Instruction of N.C. Officers and Gunners of Horse, Field and Mountain Batteries in accordance with the Instructions contained in regimental Orders, Horse Guards, War Office, 1st August, 1876.'

The author stated in the Preface that:

'I have tried to use the simplest language, such, in fact, that it can be understood by a man of the least education.'

Of particular interest was the section that deals with ranges and elevations, where it states the following:

'To good eyesight in clear weather men and horses appear as follows:
At 2000 yards, single men and horses like dots.
At 1880 yards, detached files of cavalry, clear.
At 1300 yards, detached files of infantry, clear.
At 1200 yards, bodies of cavalry and infantry distinct.
At 1000 yards, single men like a narrow oval.
At 850 yards, heads and movements of limbs of detached men clear.
At 500 yards, heads of men in the ranks, as round balls.
At 300 yards, figure of a man distinct.
At 80 yards, eyes like dots on the face.'

The primer also contains diagrams that show how gun limbers, artillery ammunition wagons, the axletree boxes on guns, and artillery mules should be loaded. Of particular interest was the fact that each Field, Horse and Mountain gun – regardless of calibre – carried four case shot and appropriate cartridges in its axletree boxes. One assumes that this was for self-defence of the gun and crew in the event of an ambush at close range.