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This is so absolutely true, that not only One Person cannot choose for Another -but One Eye has often very little notion what Glass will be best even for its own Brother, so extremely does the Left Eye occasionally differ from the Right.

No faculty of man varies more in its nature, or is susceptible of so high a degree of Improvement and Refinement by Art, as the Sense of Sight. The highest degrees of its discriminating power are acquired slowly and imperceptibly.

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From organic imperfection and neglect of cultivation, many People pass through Life who (it may be said, comparatively,) never Seei. e. whose Eyes never have the faculty of accurately appreciating either Form-or Colour:to very few indeed is it given to perfectly perceive and portray both.

There cannot be a more evident proof of the

"Every person acquainted with Optics and the nature of vision, knows that what is generally comprehended under the term Seeing, is a complex operation; an art acquired by degrees, in which judgment and imagination are concerned together with simple perception." - CRISP on Vision, 8vo. 1796, p. 106.

general defect in people's Sight — than the general acceptance of capricious and unreasonable Fashions, which appear to be prevalent, in proportion that they are in direct opposition to all the principles of good taste, and which to a fine Eye, are frequently frightful, and absolutely painful to behold.

From the different modes of colouring of different Artists, I suppose that the Eyes of no two Painters feel exactly the same impression of colours, and objects, appear of

different Colours accordingly as they are illuminated with different Lights.

"The Mole's dim curtain, and the Lynx's beam,"

РОРЕ.

do not differ more extremely than does the Sight of different persons- and of the same persons at different Ages.

The peculiar conformation of the Eyes, differs quite as much in different persons, as the construction of their Noses; it is just as impossible to guess exactly what Spectacles will best suit the Sight of another Person, as it is to tell what Tunes are most delightful to or what Tastes are most delicious

his Ear,
to his Tongue.

Nothing can be more erroneous than the common notion, that there is an invariable Rule, that a certain form of Glass is calculated for a certain Age. No Rule has more exceptions: but this Vulgar Error has been productive of great and irremediable Injury to the Eyes of Thousands!

Hence, the grand desideratum, is to instruct people how to choose wisely for Themselves which, I hope, even those who are totally unacquainted with the subject may do with the most perfect ease and accuracy, by the help of this Treatise, which is not published for the purpose of recommending some particular Oculistor of persuading the reader that the simplest of all Optical instruments, spherical Spectacle Glasses, can only be purchased of One Optician:- is not a collection of crude conceits, but is a plain impartial statement of all the Facts I have accumulated during my consideration of the subject for 30 years past in which I have

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Nought extenuated, nor set down aught in malice."

My Motto, friendly Reader, is

"TRUTH, TEMPERED BY GOOD HUMOUR."

"Men must be taught as if you taught them not,
And things unknown propos'd as things forgot.
Without Good-Breeding, Truth is disapprov'd;
That only, makes superior sense belov'd."

Essay on Criticism.

I am sure

I do not think it is my Business it is not my Pleasure, to register the various pretended improvements in Spectacles which have from time to time been proposed to the Public, — such as the Sympathetic Pebbles —

which " as the Sight alters, they will alter also to the Sight, by which one pair will last the wearer for Life," &c. &c. &c.!!! This would be irksome to the Writer, and useless to the Reader. I shall only mention the most

remarkable.

In 1758, Mr. B. Martin published a pamphlet recommending what he called

" VISUAL GLASSES,"

the use of which, he assures us, would be productive of peculiar advantages," — they might be but, by Mr. Adam's account, not to the gentlefolk who bought them.

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The desire men have to increase their

Business, and extend their Fame, has, in many instances, been the origin of alterations and

inventions, injurious to Science and detrimental to the Public.

"To this we may, with propriety, impute the invention of Visual Spectacles. But the good sense of the world, which always, in the long run, justly appreciates the value of every invention, now leaves Visual Spectacles to the neglect they merit."- G. ADAM'S Essay on Vision, 8vo. 1792, p. 113.

I beg to be excused saying more, than that I do not think that any of the Spectacle Glasses which have come before my Eye, have any superiority over the Common Spherical Lenses now in general use.

In instituting Experiments for ascertaining the distinctness and brightness of Spectacle Glasses-be extremely careful, that the Glass is not only of exactly the same quality, but also of exactly the same Focal length-or any attempt at comparison will be useless.

If one of the Glasses be only of a very little shorter focus than the other-objects will always make a very different, and sometimes a much stronger impression on the Eye.

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