In Newton's "Principia Mathematica" Book 1, Section 1 ("Of the Motion of Bodies") there is the following Lemma 6:
If any arc ACB, given in position, is subtended by its chord AB, and in any point A, in the middle of the continued curvature, is touched by a right line AD, produced both ways; then if the points A and B approach one another and meet, I say, the angle BAD, contained between the chord and the tangent, will be diminished in infinitum, and ultimately will vanish.
For if that angle does not vanish, the arc ACB will contain with the tangent AD an angle equal to a rectilinear angle; and therefore the curvature at the point A will not be continued, which is against the supposition."
Noting, that the line
rbd is parallel to the line
RBD and the arc
Acb appears to have a smaller curvature than the arc
ACB on Newton's diagram, which of the following animated diagrams correctly depicts his Lemma ?
Note, that in the latter diagram, I have added the red colored depictions of the angles
ABD. These angle depictions do not appear on Newton's diagram (...but he writes about the angle
BAD in the text of his 6th Lemma).