My design is to shew the manner wherein we perceive by sight the distance, magnitude, and situation of objects. Also to consider the difference there is betwixt the ideas of sight and touch, and whether there be any idea common to both senses.
2. It is, I think, agreed by all that distance, of itself and immediately, cannot be seen. For distance being a Line directed end-wise to the eye, it projects only one point in the fund of the eye, which point remains invariably the same, whether the distance be longer or shorter.
3. I find it also acknowledged that the estimate we make of the distance of objects considerably remote is rather an act of judgment grounded on experience than of sense. For example, when I perceive a great number of intermediate objects, such as houses, fields, rivers, and the like, which I have experienced to take up a considerable space, I thence form a judgment or conclusion that the object I see beyond them is at a great distance.
Again, when an object appears faint and small, which at a near distance I have experienced to make a vigorous and large appearance, I instantly conclude it to be far off: And this, 'tis evident, is the result of experience;without which, from the faintness and littleness I should not have inferred anything concerning the distance of objects.
4. But when an object is placed at so near a distance as that the interval between the eyes bears any sensible proportion to it, the opinion of speculative men is that the two optic axes (the fancy that we see only with one eye at once being exploded) concurring at the object do there make an angle, by means of which, according as it is greater or lesser, the object is perceived to be nearer or farther off.
5. Betwixt which and the foregoing manner of estimating distance there is this remarkable difference: that whereas there was no apparent, necessary connexion between small distance and a large and strong appearance, or between great distance and a little and faint appearance, there appears a very necessary connexion between an obtuse angle and near distance, and an acute angle and farther distance. It does not in the least depend upon experience, but may be evidently known by anyone before he had experienced it, that the nearer the concurrence of the optic axes, the greater the angle, and the remoter their concurrence is, the lesser will be the angle comprehended by them.
6. There is another way mentioned by optic writers, whereby they will have us judge of those distances, in respect of which the breadth of the pupil hath any sensible bigness: And that is the greater or lesser divergency of the rays, which issuing from the visible point do fall on the pupil, that point being judged nearest which is seen by most diverging rays, and that remoter which is seen by less diverging rays: and so on, the apparent distance still increasing, as the divergency of the rays decreases, till at length it becomes infinite, when the rays that fall on the pupil are to sense parallel. And after this manner it is said we perceive distance when we look only with one eye.
7. In this case also it is plain we are not beholding to experience:
it being a certain, necessary truth that the nearer the direct rays falling on the eye approach to a parallelism, the farther off is the point of their intersection, or the visible point from whence they flow.
8. Now though the accounts here given of perceiving near distance by sight are received for true, and accordingly made use of in determining the apparent places of objects, they do nevertheless seem very unsatisfactory:
and that for these following reasons.
9. It is evident that when the mind perceives any idea, not immediately and of itself, it must be by the means of some other idea. Thus, for instance, the passions which are in the mind of another are of themselves to me invisible.
I may nevertheless perceive them by sight, though not immediately, yet by means of the colours they produce in the countenance. We often see shame or fear in the looks of a man, by perceiving the changes of his countenance to red or pale.
10. Moreover it is evident that no idea which is not itself perceived can be the means of perceiving any other idea. If I do not perceive the redness or paleness of a man's face themselves, it is impossible I should perceive by them the passions which are in his mind.
11. Now from sect. 2 it is plain that distance is in its own nature imperceptible, and yet it is perceived by sight. It remains, therefore, that it be brought into view by means of some other idea that is itself immediately perceived in the act of vision.
I2. But those lines and angles, by means whereof some men pretend to explain the perception of distance, are themselves not at all perceived, nor are they in truth ever thought of by those unskilful in optics. I appeal to anyone's experience whether upon sight of an object he computes its distance by the bigness of the angle made by the meeting of the two optic axes? Or whether he ever thinks of the greater or lesser divergency of the rays, which arrive from any point to his pupil? Everyone is himself the best judge of what he perceives, and what not. in vain shall any man tell me various ideas of distance, so long as I myself am conscious of no such thing.
13. Since, therefore, those angles and lines are not themselves perceived by sight, it follows from sect. 10 that the mind doth not by them judge of the distance of objects.
14. The truth of this assertion will be yet farther evident to anyone that considers those lines and angles have no real existence in nature, being only an hypothesis framed by the mathematicians, and by them introduced into optics, that they might treat of that science in a geometrical way.
15. The last reason I shall give for rejecting that doctrine is, that though we should grant the real existence of those optic angles, etc., and that it was possible for the mind to perceive them, yet these principles would not be found sufficient to explain the phenomena of distance, as shall be shewn hereafter.
