Acuity is the ability of the retina to resolve discrete lines.
The link in the notes will provide more information on the structure of the eye, should one wish to pursue it further.
Suffice it to say that before we consider perception we should have obtained information on at least three functions of the eye.
These include: Acuity, Accommodation and Tracking. There are, of course, many other considerations outside the scope of our discussion.
These would include such things as peripheral vision, and the structure of the visual field etc.
Acuity: This refers to the ability of the retina to resolve discrete lines. If the lines are fused, one may not be able to recognize such things as letters, regardless of figure-ground, closure and/or other perceptual abilities.
NOTES: See the anatomy of the eye.