cellent Artists,* who were naturally extremely

anxious about their Eyes

(their Eyes are their Estate, the mainspring of their Fame and of their Fortune) to wear Spectacles before they wanted them.

MEM.The premature use of Spectacles, is as pernicious to the Sight, as Physic to the Stomach of a Man in Health, and as absurd, and as uncomfortable, as it would be to put on a Fur pelisse at Midsummer, as a Preserver against your feeling the Cold of Christmas.

* See Chapter xiii.

FOCAL LENGTH OF CONVEX GLASSES, &c. 33

CHAPTER VII.

TABLE

Of the Focal Length of the Convex or Magnifying Glasses commonly required at Various Ages.

The above Scale was given to me by an eminent Optician, as the average results of upwards of 50 Years' very extensive Experi

• These three last deep Lenses are very rarely required except for Couched Eyes. "One focus is seldom sufficient

ence:- and I believe it is as good a General Rule as can be written; but, as I have ob

to enable those who have undergone the operation of Couching, to see objects at different distances who generally require one pair of Spectacles for near, and another for distant, objects. The foci which are used lie between 6 and 11 inches."—G. ADAMS on Vision, 8vo. 1792, p. 126.

If you are a Laughing Philosopher, gentle Reader, You will not be very angry with the Author for inserting the following Anecdote:

"In the city of Leyden, in Holland, a young woman lost her sight from a cataract; the operation of couching was successfully performed upon her Eyes, and she recovered the use of them; but it appeared that the Visual Organ (as is usual in such cases) was not completely restored to its primitive condition. Some very singular and unaccountable anomalies in her Vision presented themselves, which not a little puzzled the curious in Physiology and Optics.

"It was ascertained that her Eye was able to define a certain class of very minute objects with abundant accuracy, such as the Eye of a needle, for example, which she could thread as well as ever; but on presenting her with a Book, it was evident that she could not distinguish a single letter, but complained that she could see nothing but a heap of odd marks.

"These facts, no less strange than true, naturally excited an intense interest among the Medical Professors and Students; every one was anxious to distinguish himself by

served in the Introduction to this work No General Rule has more Exceptions.

"No regular estimate can ever be established as an absolute criterion, either of the want of, or for the change of Spectacle Glasses; because, the failure or the strength

affording a satisfactory elucidation of these inexplicable phenomena.

66 A hundred theories were framed every one more ingenious than the other. The Professors VON KRACBRANer, and PUZZLEDORF, favoured their pupils with most excellent lectures on the subject, with which they were greatly edified. However, none of the disputants succeeded in establishing a Theory which met with universal approbation. Many of the vulgar still chose to think that all the said Theories might be liable to the old objection (however satisfactory and plausible they might appear), viz. —'That they were not True.'

"Matters were in this state, when a mischievous rogue of an Irish student, who took a singular delight in ridiculing every thing learned and philosophical, contrived to insinuate himself into the confidence of a younger brother of the Patient's by a present of an extra portion of Doublegilt Gingerbread, which so entirely won the Youngster's heart, that he confessed (though with some difficulty) that to the best of his belief, his Sister" Sarah had never learned to Read," but unwilling to acknowledge her ignorance, had made him and all the Family - promise not to tell."

of the Sight, varies so considerably with different people: :- several youths under 20 years of Age, have applied to me, who could not see either to read or write, without very strong Magnifiers of 6 or 8 inches focus while I have met with other persons who have arrived at 80, able to read a small print without any."

"That celebrated Preacher, the REV. MR. ROMAINE, Rector of St. Ann's Blackfriars, who died in the Year 1795, having attained the age of 81, could read the small print in a pocket Bible, unassisted by Glasses*, even to the last. He never wore Spectacles, nor wanted any."

"I knew a Gentleman who took the assistance of Glasses at about 40 years of Age; these after some time he exchanged for older ones; and although he lived to be 84, they were never afterwards altered, and his sight continued sound and healthy. Knowing this circumstance, I had the curiosity to measure the focus of the Glasses, and found it was 14 inches, which he had been using quite satisfactorily for upwards of 40 years."

"Another Gentleman, now living, with