The central line of sight and how two eyes double an object
Notebook D: the axial line that alone sees clearly, and the crossing rays of binocular vision
Leonardo explains that the eye holds one privileged line, the central line, set amid infinite adjacent lines: species arriving along it are seen perfectly, while those arriving off-axis are the weaker the farther they lie from it. This axial line, K g, is the straight, uncrossed 'mistress' that determines what the eye truly recognizes, so peripheral things are seen but not known. He then treats binocular vision: where the two central lines of the eyes meet fixes a point whose angle grows or shrinks with the object's distance, and objects off the fixation point are seen doubled because the right eye works with its right line and the left with its left. Several right-hand diagrams show the eye with its median line, an eye struck by many rays, and pairs of converging lines meeting on objects.
On this page
The central line that alone sees clearly
The eye has one line, the central line, set amid infinite adjacent lines; all species arriving by it are perfectly seen unless great distance impedes them. The adjacent lines are of greater or lesser strength as they lie nearer to or farther from that central line, which is the straight, uncrossed mistress of the rest.
Two eyes see an off-point object doubled
If the two central lines meet at object x, the lower adjacent lines S v and r y see object t occupy two places, v and y, on the wall n m. But if the central lines end at t, then x is seen by the two outer adjacent lines r x and S x, because the right eye sees with the right line and the left eye with the left.
The visual angle grows and shrinks with distance
The concurrence of the two central lines is always at a point, where an angle is generated of greater or lesser magnitude according as the thing seen is at a lesser or greater distance from the eye.
Seeing without recognizing at the periphery
The eye sees, though it does not recognize, lights at the edge of vision, because the light of the eye receives what those objects imprint. But things farther from the middle of that light, where the median line K g ends, are less known; that line is directed at whatever must be truly recognized.