Pagina-afbeeldingen
PDF
ePub

toil a Pleasure," till the pleasure became a Toil,must have a much worse Memorythan I hope you have, gentle Reader!

I dare say, that "many a time and oft," this has happened to all my Readers, with much less bewitching Playthings than a pair of Spectacles must at first be to a person who after Old Time has for several months interdicted him from the amusement of Reading, &c. by this inestimable invention, finds the full enjoyment of his precious Sight — suddenly and perfectly restored!

From one of the causes above mentioned' when persons first put on Spectacles, their Eyes are generally in a state of weakness, if not of disease: moreover, at the Age which people usually find their Eyes refuse to be employed on actual service without optical aid be allowed to them, — (which, as I have before said, happens soon after the 40th year), the Visual organs occasionally get out of tune, and participate in that general deterioration of action which every part of our Machinery then begins to suffer.

Very soon after we pass the Meridian of Life, every Sense becomes duller and weaker,

but no one fails so remarkably, as the fine faculty of Sight; and although Spectacles revive the Visual powers; - they cannot restore to them the untiring energy they possessed in early life.

Elderly persons can no more play with their Eyes, either so well, or so long, as they did when they were Young-than they can with their Legs and Arms, &c.!- to expect that they can, is about as ridiculous, as to suppose that Infirmity on Crutches, has any chance of rivalling the Champion of Pedestrians in walking 1000 Miles in 1000 Hours.

Those who consider these things, will soon cease to be very much surprised, that their Eyes tire sooner at 60 than they did at 16—just in proportion, as all their other faculties become 'sooner fatigued.

[ocr errors]

The effective state of the Eyes, like that of every other part of the Human frame, depends upon that of the Circulation, which depends upon the condition of the Stomach, and the more or less stimulating Quality, and the Quantity of the material that it is supplied with.

Several Studious Persons have told me, that their Eyes are never in good order till they

F

have had their Breakfast that then, they feel as if they were all Head for three or four hours and then, have as irresistible an inclination for Bodily Exercise, as they had previously for Intellectual occupation.

Over-exercise of the Eyes, will occasion a temporary exhaustion of them, in like manner as over-exercise of the Legs will disable a person from walking with his wonted energytill Rest restores vigour to him.

-

I have often heard people complain of their Eyes being out of humour for several days, after being exposed to the glare of the lamps at the Theatre, &c. from being fatigued by sitting up after their accustomed Hour, or from other causes which distressed their Nervous System.

When I was 45 years old, I was employed

* "The instances of Weakness of Sight which occurred in the early part of my Ophthalmic practice, were marked by great constitutional delicacy, and the individuals had most clearly brought on the Disease, either by excessive attention to fine dazzling work, or by inordinate indulgence in literary pursuits protracted frequently to late hours."-Mr. J. STEVENSON on Weakness of Sight, 8vo, 1819. p. 57.

some hours during several Nights, in looking intently through Reflecting and Achromatic Telescopes, endeavouring to ascertain the comparative Illuminating powers of various Instruments, and the effect of variously constructed Magnifiers, for shewing the Division in the Ring of Saturn, and for separating some of the faintest and closest Double Stars-my Eye became so extremely tired that the sharpness of my Sight was so sensibly impaired — that for two or three days after I hardly knew any face that was 20 feet from me, and became so much alarmed, that I mentioned it to an eminent Optician, who said, "Don't be uneasy, the same thing has happened more than once to myself; your Eye has been over-worked —give it a few days' Rest, and I dare say it will soon come round again." His prediction speedily proved true. One of the tests to which I put my Eye, and

my 5 feet Achromatic, which has a double object glass of 3 aperture, was, to ascertain with how Low, and how High a power I could see the small Star near the Pole Star: was a decidedly detached point of light with a single eye-glass, which gave the Telescope a

-it

--

power of only 28, i. e. with the 2d E. G. of a compound Astronomical Eye-piece which magnifies = 44 times and it was visible with 20 intermediate Eye-tubes, the highest of which magnifies 1386 times and is a single Convex lens of the 22d of an inch focus. - The Object Glass being of

63 Inches focus,

gave, multiplied by 22

126

126

the Magnifying Power 1386.

This Instrument is one of the chef-d'œuvres of the late Mr. PETER DOLLOND- who thus speaks of it to the Gentleman he sold it to, Mr. G. Hodgson, F.R.S., (and of whom I pur chased it,) in his Letter, which is now before me, dated Nov. 11, 1803 : It has been made 10 years, and I can say that it is one of the best I ever made, and such as I cannot expect to be able to equal."

This 5 feet Telescope literally deserves to be called Achromatic, for it shews the disk of the Moon and of Jupiter as white and as free from colour as a Reflector; to its perfect Achro

[ocr errors]
« VorigeDoorgaan »