A Treatise on the Eye/Book 5/Chapter 6
CHAP VI.
Of the Situation of Objects.
Sect.
Pag.
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| 1. | The Mind perceives the Situation of Bodies from the Situation of their Pictures on the Retina, and how, | 411 |
| 2. | But we can never determine how far Objects are distant from one another, or from the Axis of Vision, unless we know their Distance from the Eye, and why, | 412 |
CHAP. VI.
Of the Situation of Objects.
Sect. 1. THE Sixth Phænomenon of Vision, which now falls to be explained, is the Situation of Bodies. This the Mind easily perceives, from the Situation of their Image painted on the Retina; for if this Image be in the Axis of the Eye, we conclude that the Object is straight before if it be on the Left of the Retina, we judge that the Object is on the Righthand; if it be on the superior Part of the Retina, we thence perceive that the Object is below; and the same of all other Situations: For we have before demonstrated, that the Images of Objects are always inverted upon the Retina, and therefore towards whatever Side of the Axis of the Eye any Object is situated, its Picture will always fall on the opposite Part of the Retina. We have also demonstrated, that every Object and Part of an Object is seen, not in the Retina, but in Right-lines drawn from their Image to the Centre of the Eye, and thence continued outwards; and therefore, because of the Intersection of those Lines at the Centre of the Eye, we must necessarily perceive that Objects are situated towards that Side of the Axis of the Eye that is opposite to the Place of the Retina on which their Image falls.
§ 2. Thus in general, we easily judge of the Situation of Objects, from the Situation of their Pictures on the Retina; but we can never determine how far they are distant from each other, or from the Axis of Vision, that is, how far they are above or below, or on either hand, unless we also know their Distance from the Eye; but this being known, we can judge with sufficient Exactness of the true Situation. of Objects, and of all their Parts, by tracing back the Impressions they make on the Retina, along the above-mentioned Right-lines drawn from that Place of the Retina on which their Picture is projected, through the Centre of the Eye; for these Lines continued outwards, lead to the Object itself, and the several Parts of it, which must therefore all appear in the Place where they are; whence their Situation will be known to us, as has been fully explained before, to which the Reader may have recourse.