Page:Observations on Man 1834.djvu/160

 account of the acuteness and precision of the sense of sight. Upon the same principles it is easy to see, how great and unusual agitations of the body, impressions on the stomach, on the olfactory nerves, on the eye, by the quick transition of objects, on the eye and fancy together, by looking down a precipice, &c. should occasion a temporary giddiness.



already observed, Prop. XXX. that these judgments are to be esteemed true or false, according as they agree or disagree with those made by the touch.

Now the associates of greater tangible magnitude are a larger picture on the retina, the distance being the same; and a larger distance, the picture being the same. The associates of a less tangible magnitude are the opposites to these. And the associates of the sameness of tangible magnitude are the increase or diminution of the picture on the retina, while the distance is diminished or increased suitably thereto. All this appears from optical considerations. Hence it follows, that where the picture on the retina is of a just size, and also the previous judgment concerning the distance just, our estimate of tangible magnitude by sight will be just likewise. But if the picture on the retina be magnified or diminished by glasses, or our previous judgment concerning the distance be erroneous, our estimate of tangible magnitude will be erroneous in like manner. And whether it be just or erroneous, it is entirely founded on association.

The following instances, among many others, confirm these positions. Young children judge rightly of magnitude only in familiar places, or at small distances. At great distances they always judge the objects to be less than the truth, not having learnt to judge rightly of these distances, and make allowance for them. The generality of adults judge far better of magnitude at great distances on level ground, than from above, or from below, on account of their greater experience in the former case. The horizontal moon appears larger than the meridional, because the picture on the retina is of nearly the same size, and the distance esteemed to be greater. And yet the horizontal moon appears far less than the truth, because we can form no conception of its vast distance. A tree referred to the horizon in the dusk of the evening, or a fly to the ground at a distance, through the indistinctness of vision, appears much bigger than the truth. In looking through glasses, which magnify or diminish the picture on the retina, the objects themselves seem to be magnified or diminished, because our judgment concerning the distance is not altered proportionally, &c. &c.

There are, besides these, some other associated circumstances,