Following on from Larry's comments about Horace Fords writing on the subject, I think this extract might be of interest written in 1859 ...
... " When both eyes are directed to any single object, say the gold of the target, their axes meet at it, and all other parts of the eyes, having perfect correspondence as regards that object, give the sensation of direct vision; but images at the same time are formed of other objects nearer or farther to the right or the left, as the case may be, which may be called the indirect vision; and any object embraced by this indirect vision will be seen more or less distinctly, according to its remoteness or otherwise from either of the axes in any part of their length; and it will be, or at any rate naturally should be, clearest to the indirect vision of that eye to the axis of which it most approximates.
Now, in aiming with the bow, to arrive at anything like certainty, it is necessary to obtain a view of three things, namely, the mark to be hit (which is the gold of the target), the arrow in its whole line and length, (otherwise its real course cannot be appreciated), and the point of aim.
It may, perhaps, be as well to explain here, that by the point of aim is meant the spot apparently covered by the point of the arrow. This, with the bow, is never identical with the gold, excepting at one particular distance to each individual Archer, because the arrow has no adjusting sights to make it always so, as is the case with the rifle. As an example, let us suppose an Archer shooting in a side wind, say at eighty yards, and that this distance is, to him, that particular one where, in calm weather, the point of his arrow and the gold are identical. It is clear if he now makes them so, the effect of the wind will carry his arrow to the right or the left, according to the side from which it blows. He is, therefore, obliged to aim to one side of his mark, and the point of his arrow, consequently, covers a spot other than that of the gold. And this spot, in this instance, would be to him his point of aim. Under the parallel circumstances of a long range and a side wind, the rifle even would be subject to the same rule.
Now I shall be understood when I repeat, that it is necessary for the Archer to embrace within his vision the gold, the point of the aim, and the true line in which the arrow is directed.
Direct vision, however, can only be applied to one object at a time, and as that object must never in any case be the arrow, I will first proceed to show in what way this must be held, in order to enable the Archer, by means of his indirect vision, clearly to appreciate the true line in which it points at the time of aiming, leaving for after discussion the question as to whether the gold or the point of aim should be directly looked at.
Now it is at once asserted, as an incontrovertible axiom in Archery, that this true line can never be correctly appreciated by the shooter, excepting when the arrow lies in its whole length directly beneath the axis of the aiming eye. (The indirect vision of both eyes can never be used here, as, if it were, according to the law of optics, two arrows would he seen; but this is never the case with the habitual shooter, though both eyes be open, habit and the wonderful adapting power of the eye preventing such an untoward effect equally as well as if the second eye were closed-which, indeed, with many Archers is the case.)
I have said, then, that the arrow, in its whole length, must be directly beneath the axis of the aiming eye (which I shall here assume to be the right one, as in ninety-nine instances out of one hundred is the case,) and it must do so, because otherwise, the shooter will be deceived as to its true line; for so long as the point intersects the axis of the aiming eye, the arrow will appear to that eye to be pointing in a straight line with the object looked at, though in reality directed far away to the right or the left of it".
"Now, as to whether the direct vision should be applied to the mark or the point of aim, the argument is all in favour of the latter. For the point of aim must, necessarily, be in relation to the mark, either in a perpendicular line with it or outside that line: if outside, then the direct vision must certainly be upon the point of aim, otherwise the arrow cannot be directly beneath the line of the axis of the eye, which has already been shown to be necessary; therefore, the only remaining question to be decided is, when the point of aim falls in a perpendicular line with the mark, which of the two should be directly looked at? Here again an argument can be adduced to determine the choice in favour of the former; for when the point of aim is above the mark, the latter will be concealed from the right, or aiming eye, by the necessary raising of the bow-hand (as may be proved by the experiment of shutting the left eye); therefore, the direct vision cannot be here applied to the mark, though it may be to the point of aim. There remains then but one other case, namely, when the point of aim falls in the perpendicular line below the mark; and here (though either of them may in this case be viewed with the direct vision), as no reasoning or argument can be put forward for violating the rule shown to be necessary in the other cases, and as it is easier to view the point of aim directly, and the mark indirectly, than the contrary, and as uniformity of practice is highly desirable, I strongly recommend that in all cases the direct vision be upon the point of aim, This is contrary to the usual received opinion, which is that the eye should always be intently fixed upon the mark to be hit; but I am very much inclined to think that even those Archers that imagine they do so, will find, as I have done, upon careful experiment, that the point of aim is directly looked at, and not the mark, this being only seen indirectly, except as before stated, when the aim is point-blank; and this is exactly analagous to that part of rifle-shooting where allowance must be made for a strong side wind, at a long range.
