35.  Secondly, suppose in the adjacent figures NP represent an eye duly framed and retaining its natural figure.  In Fig. 1 the rays falling nearly parallel on the eye, are by the crystalline ab refracted, so as their focus or point of union F falls exactly on the retina:  but if the rays fall sensibly diverging on the eye, as in Fig. 2, then their focus falls beyond the retina:  or if the rays are made to converge by the lens QS before they come at the eye, as in Fig. 3, their focus F will fall before the retina.  In which two last cases it is evident from the foregoing section that the appearance of the point Z is confused.  And by how much the greater is the convergency, or divergency, of the rays falling on the pupil, by so much the farther will the point of their reunion be from the retina, either before or behind it, and consequently the point Z will appear by so much the more confused.  And this, by the bye, may show us the difference between confused and faint vision.  Confused vision is when the rays proceedings from each distinct point of the object are not accurately recollected in one corresponding point on the retina, but take up some space thereon, so that rays from different points become mixed and confused together.  This is opposed to a distinct vision, and attends near objects.  Faint vision is when by reason of the distance of the object or grossness of the interjacent medium few rays arrive from the object to the eye.  This is opposed to vigorous or clear vision, and attends remote objects.  But to return.

36.  The eye, or (to speak truly) the mind, perceiving only the confusion itself, without ever considering the cause from which it proceeds, doth constantly annex the same degree of distance to the same degree of confusion.  Whether that confusion be occasioned by converging or by diverging rays, it matters not.  Whence it follows that the eye viewing the object Z through the glass QS (which by refraction causeth the rays ZQ, ZS, etc., to converge) should judge it to be at such a nearness at which if it were placed it would radiate on the eye with rays diverging to that degree as would produce the same confusion which is now produced by converging rays, i.e. would cover a portion of the retina equal to DC (VID.  Fig. 3 supra).  But then this must be understood (to use Dr. Barrow’s phrase) SECLUSIS PRAENOTIONIBUS et PRAEJUDICIIS, in case we abstract from all other circumstances of vision, such as the figure, size, faintness, etc. of the visible objects; all which do ordinarily concur to form our idea of distance, the mind having by frequent experience observed their several sorts or degrees to be conneted with various distances.

37 It plainly follows from what hath been said that a person perfectly purblind (i.e. that could not see an object distinctly but when placed close to his eye) would not make the same wrong judgment that others do in the forementioned case.  For to him greater confusions constantly suggesting greater distances, he must, as he recedes from the glass and the object grows more confused, judge it to be at a farther distance, contrary to what they do who have had the perception of the objects growing more confused connected with the idea of approach.