To all who value their sight : a few practical suggestions and illustrations, intended briefly to awaken the attention of every individual to the condition of his sight; and enable him to promote the improvement and preservation of that invaluable faculty / By R.B. Bate.

  • Bate, R. B.
Date:
1830
Catalogue details

Licence: Public Domain Mark

Credit: To all who value their sight : a few practical suggestions and illustrations, intended briefly to awaken the attention of every individual to the condition of his sight; and enable him to promote the improvement and preservation of that invaluable faculty / By R.B. Bate. Source: Wellcome Collection.

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    The distance at which we read the best; experiment upon this dis¬ tance : should be maintained. 1. The focal length of the eye increas¬ ing with years It may be observed, that persons having ordinary sight read the small print of a newspaper at a distance of ten or twelve inches from the eye: if such persons attempt to read this print at the distance of twenty-four inches, they will, in many cases, find it impossible; and when they succeed in reading it, they find that it requires a considerable effort. The necessity for this effort principally arises from the image of the print being actually reduced to ONE quarter OF ITS FORMER size!—To be satisfied of this fact we may take a sheet of paper, and having divided it into two equal parts, place one upright before us at two or three feet dis¬ tance, and having folded the other into half its height, first convince ourselves that it is so by holding its left edge in contact with the tall one, then move it slowly forward until the short and tall appear of equal height*: it will be found that the shorter is only half its former distance from the eye. In the next place, further doubling the folded leaf into half the width of the other, it will become one quarter of its size or surface; and moving it forward in like manner until it just hides the first, it will be again found to be half its distance from the eye.—It is therefore obvious that when the print is held at double its former distance, our eyes are compelled to make the same exertion as if the whole page with its lines of type had been crowded into that diminished space. A little consideration will now satisfy us of the advantage of maintaining this desirable distance, at which we see the best, instead of suffering the print to be gradually di¬ minished in size by its removal from us: if the leadei feel at once the force of this suggestion, it is not necessary for him to read one line further; but if he do not perceive it, and especially if he consider that he has, without aid, a full controul over the choice of the distance at which lie reads, his attention is earnestly invited to the following short! considerations, which will convince him ol his eiioi, and. when well understood, will sufficiently illustrate the prac¬ tical parts of the subject. 1. There is a point or distance at which each person se¬ verally sees a small object more distinctly than at any, other: this distance is the focal length, or (as it is com¬ monly called) the focus of that person’s eye. There is also * This will be best observed with one eye.