I fhall conclude the account of this cafe by adding, that all the other fquinting people I have had occafion to attend to, have had one eye much lefs perfe&t than the other, according to the obferva tions of M. BUFFON and Dr. REID. These patients, where the difcafed eye is not too bad, are certainly curable, by covering the beft eye many hours in a day; as by a more frequent ufe of the weak eye, it not only acquires a habit of turning to the objects which the patient wifhes to fee, but gains at the fame time a more diftinét vifion; and the better eye at the fame time feems to lofe fomewhat in both thefe refpects, which alfo facilitates the cure.

This evinces the abfurdity of the practice of prohibiting thofe who have weak eyes from ufing them; fince the eye, as well as every other part of the body, acquires ftrength from that degree of exercife which is not accompanied with pain or fatigue and I am induced to be lieve, that the moft general caufe of fquinting in children originates from the custom of covering the weak eye, which has been difeafed by any accidental caufe, before the habit of obferving objects with both eyes was perfectly eftablifhed.

The facility with which mafter SANDFORD received the images of oblique objects on the infenfible part of the retina of one eye, whilit he viewed them with the other, induced me to obferve the fize of this infenfible fpot, and to endeavour to afcertain the cause of it.

There was formerly a difpute among philofophers, whether the choroid coat of the eye, or the retina, was the immediate organ of

vifion, which has lately been re、 vived, in fome meafure, in Dr. PRIESTLEY's valuable hiftory of Light and Colours; and it was then thought by one party in this dif pute, that the defect of the choroid coat, where the optic nerve enters the eye, was the caufe of this want of vifion in that part.

But the following obfervation fhews, beyond a doubt, the fallacy of this fuppofition: the diameter of the optic nerve, at its entrance into the eye, is about one-fixth of an inch, and the perforation of the choroid coat, through which it pafles, mult of neceflity be of the fame diameter: now the dark spot, which is feen in objects oppofed to the center of the optic nerve, if it was occafioned by the deficiency of the choroid coat, fhould, at nine inches diftance from the eye, be fifty-four times the diameter of this aperture, or nine inches in diameter; whereas I find by experiment, that a paper of one inch in diameter, could not be totally concealed, at nine inches diftance from my eye; and M. LE CAT, by accurate oblcavations, found that the infenfible part of his eye was but between the thirtieth, and fortieth part of an inch in diameter. This experiment is fo eafily made, that it can be attended with no fallacy; and at the fame time that it fhews that the infenible spot, where the optic nerve enters the eye, is not owing to the deficiency of the choroid coat, intirely fubverts the opinion of the choroid coat being the organ of vifion; for vifion exifts where the choroid coat is not.

Nor is the infenfibility of the center of the optic nerve, owing to the ingreis of the arteries along

with it into the eye: for a large branch of this artery runs along the bottom of the eye, where vifion is most diftinct, and because all this artery is covered with the expanfe of the retina on the external fide of it. Mr. SAVAGE made an experiment for another purpofe, which however fhews, that the optic artery, where it is branched under or through the retina, does not much difturb the power of vifion. It is this: if you look on a white wall on a luminous day, with the fun fhining on the wall only by its reflected light, you will difcern the parts of the wall become darker and lighter at every pulfation of the optic artery. This darker and lighter appearance is like net-work, and not uniform like the wall itself; but the whole, though rather darker while the diaftole of the artery compreffes the retina, is yet diftinctly vifible.

The following circumftance feems to give rife to the infenfibility of the central part of the optic nerve at its ingrefs into the eye, which I have obferved in feveral calves' eyes. The point of a pair of fciffars was introduced behind the ciliary circle, and the whole of the cornea, aqueous humour, iris, and cryftalline, being removed, the retina was beautifully feen through the vitreous humour fomewhat magnified. On expofing this to the funfhine, and infpecting it with nicety, a white filament, about the tenth of an inch in length, arifing from the center of the optic nerve was feen afcending ftraight upwards into the vitreous humour, like a thin white worm. The use of this may be to fupply the vitreous humour or cry

ftalline with, nourishment, whether it be a nerve or an empty bloodveffel; but this is certain, that its rifing fo high above the furface of the retina muft render it incapable of vifion: whence there is juft reafon to conclude, that this conformation must be the true caufe of the infenfibility of this part of the eye.

I do not affirm, that the human eye, either during infancy or in our riper years, is fimilar in conformation to that of a calf, nor have we fufficient opportunities to obferve them; but I fufpect this veffel may, after the growth of the animal, be totally obliterated; and that, in fome few inftances, the optic nerve may even in this part become fenfible to light. One inftance I am certain I have feen, as it was in a man capable of the moft patient and accurate obfervation, who, on numberlefs repeated trials, at different times, in my prefence, could never lofe fight of the fmalleit object with either of his eyes.

Supplement to the Cafe in Squinting.

IT fince occurred to me, that the unusual mode of fquinting defcribed in the above paper muft have arifen from fome original difference in the fenfibility of fome parts of the eye, which might have rendered it more eafy for master SANDFORD, when a child, to obferve objects with one eye only, and that with the eye most distant from objects prefented obliquely to him.

Two circular papers, each of four inches diameter, were stuck against the wall, their centers being exactly at eight inches distance from each other. On clofing one

eye,

eye, and viewing the central fpot of one of these papers with the eye fartheft from it, and then retreating twenty-fix inches from it, the other paper became invifible. This experiment was made on five people of various ages, from ten years old to forty; and the paper difappeared to them all at about this distance, or an inch or two more or lefs: but to mafter SANDFORD the paper difappeared at about thirteen inches diftance from the wall. These papers were afterwards removed to twelve inches, and then to four inches, interval between them; and by the niceft obfervations on repeated trials I found, that the paper, equally with one eye as with the other, uniformly difappeared to him at about half the distance it did to five others.

Another curious circumftance is, that as large a paper difappeared to him at half the diftance as it did to others at the whole diftance; and hence the infenfible part of the center of the optic nerve in his eyes is, as near as can be estimated, four times the area of the infenfible part of the eyes of other people, at the fame time that the angle made between the ingrefs of the optic nerve and the bottom of the eye is twice as great as in others.

It is eafy to conceive that, in early infancy, when any object which the child wifhed to infpect was prefented obliquely to him, that on this first indiftinct view of it, before either eye could be turned towards it, it would appear much more brilliant and diftinct to the contrary eye, than to that Deareft the object, as fo great a part of it would now fall on the large infenfible Part of that eye.

This must naturally induce him to view it with the oppofite eye, to which it already appeared more brilliant and diftinct; and this to him would be fo much eafier to accomplish, as the infenfible part of the neglected eye was great enough to receive as large a part of an object as is ufually viewed at once with accuracy, and hence would not confufe the vision of the other.

I muft beg leave to add, that by wearing the artificial nofe, he has greatly corrected the habit of viewing objects with the eye furthest from them; and has more and more acquired the voluntary power of directing both his eyes to the fame object, particularly if the object be not more than four or five feet from him; and will, 1 believe, by refolute perfeverance, intirely correct this unfightly deformity. Nothing but the curiofity and novelty of the fubject can excufe the length of this paper.

A Cure of a Mufcular Contraction by Elearicity. From the fame.

M

R. PARTINGTON, in a letter addreffed to Mr. HENLY, F.R. S. gives the following account of the condition in which he found his patient, Mifs LINGFIELD, when he firft waited on her. Her head was drawn down over her right fhoulder, the back part of it was twifted fo far round, that her face turned obliquely towards the oppofite fide, by which deformity fhe was difabled from feeing her feet, or the fteps as the came down ftairs. The ferno-mafloideus mufcle was in a state of contraction and rigidity. She had

no

no material pain on this fide of her neck; but, owing to the extreme tenfion of the teguments of the left fide, he had a pain continually, and often it was very violent, particularly in fudden changes of the weather. Her pulle was weak, quick, and irregular. She was fubject to a great irritability, had frequently a little fever, which came on of an evening, and left her before morning; her fpirits were generally exceedingly oppreffed, and at times fhe was flightly paralytic.

She dated the origin of her diforder at fomething more than two years from that period. She was fuddenly seized, going out of a warm room into the cold air, with a pain upon the back of her head, which admitted of fmall abatement for fome months, contracting gradually the muscles to the melancholy deformity we then beheld; and notwithstanding every prudent means had been used to fubdue it, and the ftrictly adhered to every article prefcribed to her by the faculty, he was fenfible of little variation fince, and that rather on the unfavourable fide.

I urged her to make a trial of Electricity. She was willing while fhe was in London to try the experiment; and, though the weather was remarkably tempeftuous, fhe came to me the first tolerable day, and was electrified the first time February 18, 1777

I fet her in an infulated chair, and, connecting it by a chain to the prime conductor of a large electrical machine, I drew ftrong fparks from the parts affected for about four minutes, which brought on a very profufe perfpiration (a circumftance he had been unac

cuftomed to) which feemed to relax the maftoideus muscle to a confiderable degree; but, as the fparks gave her a good deal of pain, I defifted from drawing them, and only fubjected her a few minutes longer to the admiffion of the fluid, which paffed off without interruption from the pores of her fkin and adjacent parts. The next time she came to me was the 24th of the fame month: as fhe had been in the afternoon of the first day's experiment a good deal difordered, I changed the mode of conducting, and fet her in a common dining chair, while I dropped, for five minutes, by the means of a large difcharging rod with a glafs handle, very ftrong fparks upon the mastoideus mufcle, from its double origin at the fernum and clavicula to its infertion at the back of the head. She bore this better than before, and the fame good effect followed in a greater degree, and without any of the fubfequent inconveniences. I faw her the third time on the 27th: fhe affured me the had escaped her feverish fymptond's on an evening, and that her spirits were raised by the profpect of getting well; that, fince the last time I electrified her, the had more freedom in the motion of her head than fhe had ever experienced fince the firft attack of her diforder. I perfifted in electrifying her after the fame manner, March 3d, 5th, 6th, 7th, and 9th; from each time the gained fome advantage, and her feverish tendency and nervous irritability went off entirely.

The weather now fetting in very unfavourable, and fearful of lofing the advantages we had happily reaped from our early efforts, I requested

requested the favour of you, as her next-door neighbour, to electrify her every evening while fhe was in town, and the might, if any alteration took place, fee me occafionally. Fortunately for her, you accepted the propofal, and to your judgment and caution in the conduct of it for the next fortnight (three evenings only excepted) you brought about the happy event; and have received her teftimony of gratitude for relieving her from a condition, under which life could not be defirable, to a comfortable affociation with her family and friends.

I am, &c.

THE method I purfued was, to place the lady upon a stool with glafs legs, and to draw ftrong fparks, for at least ten minutes, from the mufcles on both fides of her neck. Befides this, I generally gave her two fhocks from a bottle containing 15 fquare inches of coated furface fully charged, through her neck and one of her arms, croffing the neck in different directions. This treatment fhe fubmitted to with a proper refolution; and it gave me fincere pleafure to find it attended with the defired fuccefs.

particularly with refpect to the means by which the inhabitants of those northern climates are enabled not only to protect themselves from fuffering by its inclemency, but to turn it to their advantage, and even to enjoy amufements peculiar to it. Which will juftify the obfervation of our excellent philofophical poet, who fays,

"What happier natures fhrink at with affright,

"The hard inhabitant contends is right." ESSAY ON MAN.

If I could have communicated any experiments which might have. helped to discover the natural caufe of freezing, and have ferved either to confirm fome of the several theories already given of this phænomenon, or to establish a new one; I fhould have thought my remarks more worthy your Lordship's attention, and have long fince put the loofe notes, which have lain by me, on this fubject, into fome form. But though I made many experiments in freezing feveral fubftances, I cannot pretend to have found any thing new or fingular to remark as to the nature of conge lation. Yet ftill, I hope it may afford your Lordship fome amufement to obferve the fagacity and addrefs which the human mind exerts in the application of the proper means of felf-prefervation: and the confideration of the wonderful provifion which the wisdom and goodness of divine Providence has fuited to the peculiar wants of his creatures, I am fure your Lordfhip will always look upon as the moft interefting part of natural hiftory.

It is neceflary for me to premife, that in the courfe of thefe obferva