opinion feems upon the whole to be nearer to the truth *.

That the eye cannot fee both near and remote objects diftinctly at the fame time, may be eafily proved. Let a tree, a house, or fome other object be upwards of 50 feet from you; fhut one eye, and whilst you are looking with a single eye at the tree, &c. hold a pin, a pencil, or fome other object in the fame direction at about a foot distance from the eye; and it will be found that whilft you fee the pin diftinctly, the tree will appear indiftinct; but if you adjust your eye so as to fee the tree diftinctly, then the pin will appear indiftinct.

The eyes of fome perfons are more capable of adjustment than thofe of others. In old perfons the humours grow thicker, and the parts lefs pliable; hence their eyes are lefs capable of adjustment than in young perfons.

The eyes of fome perfons can be adjufted for diftant objects better than for near objects, and vice verfa. When the eye is defective, and by its fize or other conformation, parallel rays form their foci before they arrive at the retina, then the person can fee very near objects only. Such perfons are faid

For farther information on this fubject, the reader may perufe Priestley's Hiftory of Vifion, Light, and Colours; Olbers's de oculi mutationibus internis; and Young's Paper in the Philofophical Tranfactions, vol. for 1801. Art. II.

to

to be near-fighted, or they are called myopes. When the eye is flatter than ordinary, then the foci of rays from pretty near objects are formed beyond the retina. Perfons with fuch eyes are called prefbyta; they can adjust their eyes for objects beyond a certain distance only. The latter is generally the cafe with old perfons; but the eyes of old perfons fometimes are incapable of adjustment both for very near and for very distant objects. This comes from a rigidity or want of pliability in the parts *...

Thofe imperfections may in great measure be remedied by the use of proper glaffes or spectacles; for fince in near-fighted perfons the rays of light converge to a focus too foon, viz. before they come to the retina, concave lenfes, which diminish the convergency, must remove the imperfection. And for those who can fee diftant objects only with tolerable distinction, viz. in whose eyes the rays do not converge foon enough, convex lenfes, which increase the convergency, muft remove the imperfection.

When the defect comes from rigidity, as in fome old perfons, then thofe perfons require concave glaffes for viewing diftant objects, and convex glaffes

* Those defects are frequently brought on or increased by habit, as by the conftant cuftom of viewing objects either from too near or from too great a distance; as also by the ufe of improper glaffes.

for

for viewing near objects; for their eyes want both adjustments*.

The

*The effential and extenfive ufe of fpectacles, which affords comfort to fo great a number of individuals, who would otherwife be a burden to themfelves and to fociety, is an inftance of the great usefulness of the science of optics.

No pains have been fpared to render fpectacles as perfect as poffible, and a variety of contrivances have been from time to time offered to the public. Spectacles have been made with two lenfes for each eye; alfo the lenses have been made plano-convex or plano-concave, or of other fhapes; but upon the whole, fingle lenfes, either double concave, or double convex, of clear glafs, well polished and regularly formed, are the best.

When the eyes of perfons firft begin to be affected by

age, the opticians furnish them with fpectacle lenfes, of about 40 inches focus, which glaffes are therefore called number 1ft, or glaffes of the first fight; viz. for the fight when it firft begins to be impaired by age. But I find confiderable difference between the focal diftances of fpectacles, N° 1. made by different opticians. When the focal length is about 16 inches, the lenfes are called N° 2. About twelve inches is the focal length of N° 3. Ten inches is what they call N° 4. Nine inches is that of N° 5. Eight inches is the focal length of N° 6. Seven inches is the focal length of number 7. Six inches is the focal length of N° 8.; and fometimes they make fpectacles of a focus horter ftill. Concave fpectacles are alfo named by fimilar numbers.

In choosing spectacles, actual trial is the best guide; but care must be had to use spectacles that do not magnify more

than

The capability of adjuftment is greater or lefs in different eyes, and it is frequently different in the two eyes of the very fame perfon; but in all eyes there is a limit, within which vifion is not distinct. This is called the limit of diftin&t vifion; and with fome perfons it is as fhort as one inch, whilft in others it exceeds 20 inches; but in common it will be found to lie between fix and 10 inches.

All the retina, as far as it is extended, is capable of receiving the moft perfect image of objects. There is, however, a fingle fpot where no vifion takes place; and this fpot, which is about a 40th of an inch in diameter, lies exactly upon the infertion of the optic nerve; fo that we cannot perceive the image of any cbject that falls upon this spot at the hind part of the eye, provided the other eye be fhut. The exiftence of this (which we may call

than is juft fufficient either for reading, or for other neceffary purposes.

When a variety of fpectacles cannot actually be tried, the defect of the fight may be expreffed by mentioning the distance from which the perfon can read, or other peculiarities, from which the neceffary glaffes may be determined pretty nearly. An inftrument for meafuring the exact limits of diftin&t vifion was fome years ago contrived by Dr. Potterfield, who named it an Optometer (fee his Work on the Eye, vol. I.) and an improved one for the fame purpose was lately contrived by Dr. Thomas Young. See his Paper on the Mechanism of the Eye, in the Philofophical Tranfactions, vol. for 1801.

infenfible)

infenfible) fpot is most convincingly proved by the following eafy experiment.

Let three pieces of paper of different shapes, A, B, C, fig. 1c, Plate XXI. be faftened on a wall, at the distance of about two feet from one another, and let a perfon, keeping one of his eyes fhut, place himself nearly oppofite to the middle paper B, and beginning pretty near to it, let him retire gradually backwards, whilft the open eye is turned obliquely towards the outfide paper, viz. that paper which is next to the eye that is fhut; he will find a fituation (which generally is at the distance of about 10 feet from the papers) where the middle paper will entirely disappear, while the outermoft papers continue visible. In that fituation the image of the middle paper falls exactly upon the insertion of the optic nerve.

This obfervation has been often adduced as the foundation of an argument to prove, that the feat of vision is not exactly at the retina, and that either the choroides or fome other part of the eye receives the impreffion of light, &c.: but as nothing positive is known with refpect to this fubject, viz. of the manner in which the perception of objects is conveyed to the fenforium; and having fhewn that a picture of the objects, &c. is actually painted on the retina, which is going as far as we can in tracing the action of light; I fhall not detain my reader with long and unprofitable difquifitions relative to it.

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