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CHAPTER XIII.

GLASSES FOR SHORT-SIGHTED PERSONS.

I HAVE met with several persons of 30 and 40 years of Age who had no notion that they were Near-sighted, until they accidentally looked through my Spectacles at a distant object; when they exclaimed with surprise, "Bless me, how clearly I see! I never saw any Glasses before, that I could see so well with as with my naked Eye, and therefore had no idea that Glass could improve my sight." any

"I can see to read a small print, as well as any body I believe, but I have sometimes suspected that I did not see any thing across the street, or at a Theatre, quite so plainly as I have heard other people say that they did; and I suppose that the Spectacles which I tried before were not suitable to my Sightand so I had no idea that any Glass could improve my Sight."

For such Eyes I have procured a No. 1, or No. 2, Concave and they have been delighted —and said, “Well, I see now that I have never

before discerned the distinct outline of any object which has been further than a few feet from me."

Being a Short-sighted mortal myself, I write this Chapter with confidence, from my own experience of upwards of 31 years, and hope to be able to give some good advice to those who are unfortunately what is called Near-sighted — by briefly narrating "the History of my own Case of Spectacles."

I was about 15 years old, when I first discovered that I could not discern distant objects so distinctly as people who have common Eyes usually do.

Mr. WARE, whose paper on Shortsightedness I had not seen till after I had written this Chapter, has remarked, (see Appendix,) that Young People seldom find out that they are remarkably Short-sighted, until about the time that I did; which is true, and perhaps for this reason, that Young Folks seldom attend to any thing in earnest before they attain to that Age

- when seeing, that I could not see what persons with common Eyes frequently pointed out to me as well deserving my attention, I paid a

visit to an Optician and purchased a Concave Eye-glass No. 2.

After using this some little time, I accidentally looked through a Concave No. 3, and finding my Sight much sharper with this, than with No. 2-had my Spectacles glassed with No. 3, which appeared to afford my Eye as much assistance as it could receive.

After using No. 3 for a few Months, I chanced to look through No. 4, and again found the same increase of Sharpness, &c. which I perceived before when I had been using No. 2 and first saw through No. 3—therefore concluded that I had not yet got Glasses sufficiently Concave, and accordingly procured No. 4:- however, this soon became no more stimulus to the Optic Nerve-than its predecessors Nos. 2 and 3 had been.

I then began to think that the Sight is subject. to the same laws which govern the other parts of our System; i. e. an increased Stimulus by repetition soon loses its power to produce an increased effect- therefore, I refused my Eye any further assistance than it received from Spectacles glassed with No. 2, which I have

worn for upwards of 31 years, and it is very nearly, if not quite, as sufficient help to me now, as it was when I first employed itgiving me a Sight (for objects at a moderate distance), as I find, by comparison, about upon a par with common Eyes:- without my Spectacles, I am quite as Short-sighted as some of my acquaintance who use Nos. 6 and 7 concave; i. e. we read at the same distance.

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Soon after I passed my 40th year I found my Sight become rather Shorter as to distant objects and rather Longer with respect to near objects formerly, I usually wore Spectacles for Reading, Writing, &c.— but lately, the power of my Eye to adapt itself to various distances is so diminished, that when I read, &c. I am obliged to take off my Glasses and objects that are more than 70 feet from me, I see better with one number deeper than that I commonly wear. See a similar case in the Appendix.

The gradations of Concavity, in the Concaves of the common Spectacles which are marked Cheap in the windows of Sale-shops, Toy-shops, &c.—who pretend to undersell the regular Opticians, (Read CHAPTER XIX, and

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See the Appendix,) are not always worked to a certain standard, and what one person calls No. 5, another rates as No. 3, or 4, or 6, or 7.

Mr. PIERCE informed me that the late Mr. JESSE RAMSDEN made the first No. of his Concaves to be equivalent to a Convex of 24 inches focus, -i. e. if a Convex of that focal length be united to a Concave No. 1-the combination will form a Plane, and objects appear through it neither larger nor smaller than they really are.

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The following is a very important fact, which the Short-sighted cannot be too fully sensible of:

"I shall mention a fact with which I was made acquainted by Mr. GEORGE ADAMS the late Mathematical Instrument Maker, who was not only well skilled in the theory of Vision, but, from his situation as an Artist, had better opportunities than most persons of learning such matters.

"The fact is this, that he does not know a

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