Sight and touch : an attempt to disprove the received (or Berkeleian) theory of vision / by Thomas K. Abbott.
- Thomas Kingsmill Abbott
- Date:
- 1864
Licence: Public Domain Mark
Credit: Sight and touch : an attempt to disprove the received (or Berkeleian) theory of vision / by Thomas K. Abbott. Source: Wellcome Collection.
Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.
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![That we have even with one eye a tolerably correct per- ception of distance, which cannot be compared with the impression produced by a picture, is evident, even from the consideration of the stereoscope. For while the effect of the combination of the two pictures is so strikingly different from the appearance of a single picture, it must be confessed that there is not such a difference in the appearance of the natural objects themselves as seen by one and by both eyes. The perception of relief in the two cases differs in degree only, not in kind. Now when one eye only is used, our percep- tion of distance and magnitude is dependent in great part on the adjustment employed. Objects distinctly seen appear nearer in proportion as the effort of adjustment required is greater; those which are sensibly indistinct are generally referred to a distance less determinate, but depending on the amount of indistinctness and other circumstances. Thus, if we are looking through a window at a distant view, a fly on the glass is mistaken for a bird at a distance. The image on the retina is only an indistinct dark spot, of a magnitude which would correspond equally well with either, but too indistinct to suggest its own distance. The eye, however, being adjusted for distant vision, the dark spot conveys the idea of a distant object. Again, if we hold a pencil between the eye and a window twenty or thirty feet distant we shall observe that when we look at the pencil the window will by experiment to be the average minimum sensible interval. His obser- vations were made with a series of parallel lines one millim. broad, and at intervals of one millim.; and he found that in order that the direction of the lines should be perceived the retinal interval should be that now men- tioned. The observations of H. Muller and of Volkmann give a similar result. Cf. Funke, Physiologie, p. 852; Cornelius, Theorie des Sehens, pp. 340, 341 ; [Bergmann, in Henle and Pfeuffer's Zeitschrift, vol. i., p. 83,94] ; H. Muller, Quarterly Journal of Microscop. Science, vol. i.; Volkmann, Art Sehen, in Wagner's Handwb., p 331. It may be remarked that the chromatic dispersion of the eye amounts to 019' (Helm- holtz, Physiol. Opt., p. 331).](https://iiif.wellcomecollection.org/image/b21037930_0107.jp2/full/800%2C/0/default.jpg)
