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out veins or specks comparatively, and bears an excellent polish: Crown-glass is too dark and seedy for the purpose; French or British Plate is more difficult to polish well, and Spectacle-glasses made therewith are generally Grey; that is, the fine grinding from the Emery is not sufficiently polished off.
Much has been said to persuade the Public that each Spectacle-glass ought to be ground and finished singly by itself, else its defects of variety of foci, incorrect representation of objects, and false colouring, must be considerable. But every one acquainted with Glass Grinding, is well convinced, and will be ready to assert, that a Glass of so small a diameter as an inch and a half, cannot possibly be worked so steadily, nor so true, by the hand singly, nor polished off so fair and regular, as when several are worked together in a Block: this block may probably hold four dozen of glasses, which are worked, ground, and polished together in a tool of an exact radius to produce the focal length desired. The firmness of a Block of Glasses secures you against the possibility of forming any irregular sphericity, and gives it a decided advantage.
Some years since, Spectacles were brought from Holland, the glasses of which had been heated sufficiently to receive a Concave or Convex form, by being pressed in iron Pincers or Moulds of various curvatures, without any other process-it is easy to conceive that each Glass must have had an inequality of foci, extremely injurious to the sight of the unfortunate user, yet these vile glasses were almost universal throughout France and Germany.
In England it is no uncommon thing to find the Spectacle-glasses of an itinerant optician, ground on one side only, while the other side remains as it was originally cut out from the plate, without being worked at all, and, moreover, full of veins, &c. which refracting the light irregularly-distort the object and distress and greatly injure the Eye, and are as detrimental to the Eyes as the former; but the article is sold cheap, which is too often the most tempting recommendation to the million: but as a pair of the very best Glasses, which are warranted free from all imperfections, may be purchased for only two shillings-who would be so mad as to run the risk of forfeiting the fee simple of his precious Sight-for
four-and-twenty pence! for a Groat a Year!! for the Glasses seldom require changing oftener than once in half-a-dozen years, and sometimes not in a dozen!!!
ON THE VARIOUS DEGREES OF THE PERFECTION OF THE EYE AND EAR.
GOOD, and well-educated Eyes are as much delighted with the Harmony of Colours fine Ears are with the Harmony of Sounds and a cultivated Eye is as much distressed by ill-according Colours as a cultivated Ear is offended by discordant Sounds. I well remember that excellent Musician, Dr. Arnold, telling me when I was studying composition with him, that when he first began to learn counterpoint, his Ears were so excruciated by the Chord of the 2nd of the Key - i. e. the Sharp 6th-that he used to call it "the Gritty Chord."
It has been recorded that the Eyes of some persons have been gifted with such Penetrating power (as Dr. Herschell termed what I call
Illuminating power) that they could perceive the Moons of Jupiter. See G. ADAMS on Vision, 8vo. 1789, p. 64.
I have heard of, but never met with such Visual organs. Common Eyes can scarcely perceive them with a good Achromatic Opera Glass magnifying 4 times.
Father Castel invented an Ocular Harpsichord- which was strung with coloured tapes instead of wires, and being placed in a dark room, when the keys were touched, the transparent tapes which corresponded with them became visible. C. published a pamphlet describing this curious machine, which was translated into English, and I once had a copy of it.
I have met with some very sensitive Earsand have known several extraordinary Musicians who have been able, if a handful of the keys of a Harpsichord were put down, so as to produce the most irrelative combinations to name each half-note without a mistake. When I mentioned this to that excellent Organ Player, MR. CHARLES WESLEY, he said, "At the age of twenty, I could do it myself-but I can't now." He was then in his 55th year.
The delicately discriminating power of the
Ear is more the gift of Nature than extreme sensibility of the Eye, which latter is I believe always in a considerable degree the result of cultivation.- MISS CUBITT, of the Theatre Royal, Drury Lane, when only six years old, surprised me very much, by the high degree she then was gifted with the former faculty; so was MR. WATSON, of the Theatre Royal, Covent Garden: which they still retain.
MR. T. COOKE, the Singer and Composer to Drury Lane Theatre, whom no one will contradict me when I style the most extraordinary Musician of the present Age, when I put down the following Notes on a Piano-Forte, - told me at once, "I think, Sir, that you have Beef in one hand and Cabbage in the other.