Sight and the visual organ / by A. von Graefe.

  • Graefe, Albrecht von, 1828-1870.
Date:
1871
Catalogue details

Licence: Public Domain Mark

Credit: Sight and the visual organ / by A. von Graefe. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    eye, the images on the retina will therefore stand in reversed proportion to the distance of the objects. The image of a pencil held a foot from your eye, covers the trunk of the tree before your window, that of a pea at a like distance, the moon in the sky. If we notwithstanding, think the moon bigger than the pea, and the tree than the pencil, the reason is that, apart from our being well-acquainted with the tree at least, our judgment is a combination of the size of the image on the retina, which is directly impressed, and the distance of the object. Now as consciousness is for the most part founded on experience, so just and correct perspective sight is in the main something we acquire. A child will assuredly not appreciate the differ- ence between the pencil and the trunk of the tree in the same degree as an individual, who by experience has learned to know the value of his impressions. What the child first knows of the moon is, that he cannot reach it with his hand; but,'1 as I once heard a child say, mother can reach it down. Other inferences have helped him to this conclu- sion. We are so accustomed to merely play with children, that we are easily blinded to the full seriousness of such requests. We cannot however, break off these reflections on the image on the retina, before making mention of a remark- able spot in the background of the eye,— the yellow spot marking the entrance of the optic nerve. From the moderate distance of the hollow of the retina, and according to the above-discussed principles of excentric vision, the images fall- ing on this spot, ought to produce tolerable, if not altogether perfect visual impressions. Instead of which however, all per- ception whatever, is arrested within the bounds of this spot — it is a blind point in our field of view. Most of you (75)