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To measure the Focal Length of Spectacle Glasses, see Chapter xv.
To give my Readers all the satisfaction in my power, I have added an APPENDIX
which contains sufficient corroboration of what I have asserted on certain important points, because by the time Persons want Spectacles, they have generally become wise "in their own conceit," and have picked up a parcel of silly prejudices concerning them, which unless completely counteracted, and rectified by the invariable standard of irresistible Truth,-will prevent their deriving that benefit from this publication, which the Author heartily wishes that they should receive to the utmost extent.
I have given Rules for the choice of Glasses as relates to their degrees of Magnifying, and have also pointed out those criterions by which All may judge if the Glasses are good and of the right focus and readily discover those imperfections in them which are so common, and so injurious to the Eyes.*
* See Chapter xvii. “ Of the Quality of Spectacle Glasses," and the Note at the foot of page 40.
To effectually eradicate erroneous opinions, and to establish the Truth beyond all doubt, I know is no easy task therefore, I have given an Appendix of Quotations from the best Authors on the subject-the correctness of whose Judgment has been established by Experiment, and has been pronounced indubitable, by universal acceptance—i. e. from the Writings of Dr. Smith;- Dr. Jurin; — Dr. Porterfield;-Dr. Blagden ;—Dr. Wells ;—Dr. Herschell;- Mr. G. Adams, the Optician;the late Mr. Ware, the Oculist; and Mr. Stevenson, the Oculist; - and for many valuable facts, I am indebted to that experienced Optician, MR. SAMUEL PIERCE, who was upwards of Thirty Years with Mr. Jesse Ramsden.
"Palmam qui meruit, ferat."、
There could not be a more useful Charitythan that of providing proper
SPECTACLES FOR THE POOR.
The Best Glasses, set in Single-Jointed Steel Frames, may be purchased wholesale at the rate of 18s. per dozen Pairs; if a Singlejointed frame is fastened round the head with
a Riband, it may be kept on, almost as steadily and comfortably as a Double-Jointed Frame.
For the small sum of 18 Pence the Benevolent may enjoy the gratifying reflection of giving an industrious Workman the power of long continuing his labour with undiminished Ability, and of earning a subsistence till extreme Old Age.
In no way-can so much Good be done with so little Money!
"Qui Visum, Vitam dat."
The greatest part of the Disorders of the Eyes of Poor People who are upwards of 45 years of Age, are occasioned by their straining their Sight for want of Spectacles, or by looking through Bad Glasses, or those of a Focus not suitable to their Eyes: - I hope when this is considered by THE OVERSEERS OF THE POOR, THE DISTRICT SoCIETIES FOR BETTERING THE CONDITION OF THE POOR, and the Patrons of THE EYE INFIRMARIES, that they will make the distribution of Spectacles a part of their Bounty.
ON READING GLASSES,
For Persons whose Eyes are impaired by Ageand Single Eye-Glasses for the Short-Sighted. WHEN Would-be-thought-young Persons, first feel the necessity of giving their Eyes Optical assistance, they are, nevertheless, shy of mounting Spectacles, which they seem to consider an inconvenient manner of advertising their Age upon their Nose-not reflecting that they are worn by many persons who have not seen half their years, but who being Short Sighted, are obliged to walk about in Spectacles, or forego the sight of "the Human Face Divine."
However, they suppose that Spectacles are such unequivocal evidence of Age and Infirmity- that they desire to dispense with exhibiting them as long as possible-therefore, they purchase "A READING GLASS," and habitually put it up to One and the same Eye, leaving the other involuntarily to wander ;-after a few years, the sight of the Idle Eye becomes of a
different focus to that which has been employed with the Glass and is often irreparably impaired.
"These puerile propensities, give rise to a variety of artifices, by which each individual endeavours to hide from himself and others, what no artifice can conceal, and which every one can discover, in all but himself;- these endeavours often contribute to hasten the Evils they are meant to conceal. Opticians have daily experience of the truth of this Observation, and in no instance more so than in the preference given by many to Reading-Glasses, (under whatever pretext it may be covered) merely because they think that the decay of their Sight, and their advances in age, are less conspicuous by using a Reading-Glass, than Spectacles: but the Eyes in endeavouring to See with a Reading-Glass are considerably strained, and in a short time much fatigued: and there is another objection to the use of Reading-Glasses, which arises from the unsteadiness of the hand, and the motion of the head, which occasion a perpetual motion of the Glasses, for the Eye endeavours to conform itself to each change, and this tender organ is