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181 minis conatu, levi quidem illo, et per instinctum et quasi sponte facto, in idem Punetum CorporiS, quod contemplatur, dirigantur; quo fit, ut imago ejus prope
EXprimatur; nam ibi loci fere semper persectissimus est ViSUS. 242. Pupilla in tonebris patula, admissa vel vividiore facta luee, Statim Contrahitur: et hoc consensui inter retinam et musculum qui pupillam regit, eamque vel arctat vel amplificat, plane tribuendum eSt, neque omnino irritationi ipsius musculi. Nam Suno Pro Sus musculo isto, si nullus Visus est, nulla fit pupillae contractio. Quin et musculi pupillarum utriusque oculi conspirant; ita ut, dirigantur in idem punctum corporis, quod contemplatur, non visi conatu hominis, illo quidem levi, et facto per instinctum et quasi sponte; quo fu, ut inrugo ejus evrimatur prope centrum, utriusque retinae ; nam visus est
fere semper ibi loci perfectissimuS.
tenebris, statim contrahitur luce admissa vel facta vividiore ; et hoc plune tribuendum est consensui inter reti Nam et musculum, qui regit pupillum, que vel arctat vel amplificat eam, neque omnino est tribuendum irritationi musculi ipsius. Nam isto musculo prorsus Sano, Si est nullus visus, nulla con
et musculι pupillarum utriusque Octili conspirant; itutowardg the fame potnt os a body which we contemplate, notindeed without an essori, although slight, and executed by instinci, and as it were, Spontaneously: in this Way thei mage of an object is expressed near the centre of oachretina ; sor in that pari vision is nearly always most perseel. 242. The pupil hicli is dilatsed in the dark, is immediatoly contracted by light being admitted or rendered more vivid: this is to bo attributed to a sympathy between theretina and the muscie, Whicli controuis the pupil, and contracts or dilates it, and not entirely to irritation of the muscioitssis. For is that muscie be in a found state, and therebe no vision, no contraction os the pupil takes place. More-over, the muscies of the pupiis of the two eyes harmonire,
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admissa ad alterum oculum luce, utriusque Simul pupillae contrahantUr. 243. Contrahitur quoque
pupilla, siquid oculo proprius quam soliti limites
distincti visus accurate in tuemur; hoc, ut videtur, Consilio, ut radii superflui et maxime obliqui, et quos vires oculi refringentes injustum braim vix potuissent colligere, excludantur. Parvidetur esse ratio, quod pu
pilla infantibus et junioribus
admodum patula, Senibus fere Contractior sit, quibus
Nondum pro certo ConStat, num aliis quoque modiS, oculus ad varias rerum quaS contemplatur distantias, Semet accommodet: hunc Vero nonnulli putarunt esse USUm processuum ciliarium; ut, luce admissa ad alterum
243. Quoque pupilla contrahitur, si intuemur quid proprius quam limites solitidistincti visus: hoc consilio, ut videtur, ut super sui et marime obliqui radii, et ut rudito quos vireS, T fringentes oculi viae potuissent colligere in justum
scum, ea cludantur. Parratio videtur esse, qu0d pupilla sit admodum patula infantibus ci junioribus, sed fere contractior Senibus, quibus oculi solent sieriplaniores. Non irem COH- stat pro certo, reum, quoque aliis modis, oculuS accommodet semet ad varias distantias rerum quas contemplatur : vero nonnulli putarunt hunc esse usum ciliarium
aud most oblique rays, and the rays whicli the refractingpowers of the eye Could Scarcely collect into a proper focus,may be excluded; there Seems to be the fame reason sorthe pupil buing very much dilated in infanis and yo ung
certain known, as to Whether the eye can accommodateitseli tu any other way to the disserent distances of objecis
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nempe ut lentem a solita Sede dimoveant, set longius a retina trahant, quo magi Srefringantur radii, PriuS- quam in eam incidant. Certissimum autem est, OCulum, vel his viribus tum musculi
qui pupillam regit, tum pro-c suum ciliarium, vel aliis minus cognitis, quoddammodo mutari, et ad varias rerum quaS contemplatur distantias, intra certos limiteS, accommodari. 244. Diu multumque disputatum est inter physiologos, mathematicos, metaphy-δicos, quo Paeto fiat, ut, ab inversa super retinam de
rectam videamus; et cur a processuum; nempe ut dimoveant lentem a solita
sede, et trahant cillam l. e. lentemo longius a retina, quo radii magιs refringantur, priusquam incidunt in eam scilicet retinum9. Autem est certissimum, Oculum mutari quodammodo vel his viribus, tum musculi qui re
tis, et accommodari ad Naritis distantius rerum quas contemplatur, intra certos limiteS.
244. Est disputatum diisque multum inter physiolo-g0s, mathematic0s, cet m trephysicos, qu0 pucis sui, ut
super retinam; et cur una whicli it contemptates ; but some have supposed that thisis the use of the ciliary proceSSes: namely, that they remove the lens hom iis usual siluation, and puli it further homthe retina, so that the rays may be more refracted bo rethey tali upon it. Nevertheless, it is very certain, that theeye is altered in some way, either by these p0Wers, by th0se both of the muscle which goveriis the pupil, and of the Ciliary prooesses. or by others tess known, and that it is accommodated to the different distances of objects whicli it Contemptates, within certain limits. 244. It has long been a subject of much dispute amongst physiologi sis, mathematicians, and metaphysicians, how it Occurs, that we See an object erect, hom an iuverted imagepainted upon the retina, and why one object only is seen 31 2
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184 duplicu imagino, una nimirum in utroque oculo EX-PreSSa, una tantum res spectetur.
utiles vetant medicum in hanc arenam descendere, ubi tot magni nominis viri non sine laude Semei EXe CUerunt. Scire tamen juvat, ad hoc tandem ventum esse, Simplicem de hac relatam esse a Natura legem, Scilicet, ut omne visibile Spectetur secundum directionem rectae lineae a puncto retinoe, Super quod depicta fuerit ejus imago, Per Centrum oculi tranSeuntis. Nondum prolata Sunt eXperimenta neqUe argumenta, quae ostendant dictam directionem, mathematica nCCH- ratione, eSSe Veram: Sed neque error ullus, vel levis- res tantum spectetur, a
duplice immine, nimirum cub9 unu erpressa in utTO-que oculo.
245. Ipsae res parum et tiles vetant medicum de cendere in hunc arenam, ubi tot viri magni nominis e ercuerunt semet non sine laude. Tamen juvat Scire, tandem ventum esse ad hoc, Scilicet, ut omne visibile Spectetur secundum directionem rectae lineae a puncto retinae, super quod immo
ejus fuerit depicta, transeuntis per centrum oculi. Nondum everimenta Neque argumenta Sunt prolata, quae ostendant dictam directionem esse veram, muthfmatica accuratione; sed neque ullus, neque levissimus error
est detectus in huc re. Igitur oportet cui concluda-
245. Such subjecis being in them gelves os litile use, sorbid the medicat man entering into such a field of disputo, Where so many men of great repute have exercised them selves with no stight degree os honour. It is, however, ustilat to know, that it is at last concluded, that every visiblething is seen in the direction os a straight line stom thep0int of the retina, upon which was palnted the image of the objeci passing through the centre of the eye. Nei ther experimentS, nor arguments, have yet been brought lar ardio prove that the said direction is true, with mathematical accuracy; but there is not any, even the StighteSt error,
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simus, in hac re detectus est. oportet igitur ex jam
institutis experimentis Concludamus, directionem, qUa punctum visibile spectatur, esse vel hanc, vel huic quam proximam. Homo igitur sic Constitutus, hujusmodi nimirum videndi lege lata, si imago erecta suiSset, rem ipsam inversam vidisset; et pari ratione, quae inVersae depinguntur, eaSdem ereetas videt. 246. Nec verum eSt, quod primo intuitu simplicius et clarius videretur, rem Speetari secundum verum quem habet situm et directionem; vel secundum directionem radiorum lucis, aut in corneam aut in retinam incidentium : facili enim eXperimento spectari potest CO
mus.eae eaeperimentis jam institutis, directionem, qua directioneo visibile punctum spectatur esse vel hanc, vel quam pr0zimam huic. Igitur homo sic constitutus,
nimirum lege lata videndi hujusmodi, vidisset, ipsam rem inversam, si imago tuisset erecta; et par ratione, videt, easdem erectaS, quoe pinguntur inversae. 246. Nec est verum quod videretur primo intuitu simplicius et clarius, rem Spectari secundum verum situm et directionem quem habet: vel secundum directionem radiorum lucis, incidentium aut in corneam aut in retinum ἰ enim facili everimento omne corpus potest whieli has beon detected, on this hea d. Theresere We must conclude hom the experiments atready performed, that thedirection in whicli a visibie potnt is seen, is either of this kind, or very nearly allied to it. Man thus constituted, thatis, according to the law of vision atready laid down, would have seen the object itself inverted, is the image had been erect ; and for the fame reason he Sees those objecis erect whicli are palnted in an inverted direction. 246. It is not true, although at firsi Sight it might appearmore clear and simple, that the object is seen accordingio iis true siluation and direction; or according to the direction of the rays os light strihing either u pon the cornea,
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vel pluribus locis eodem tempore; et radii lucis a
puncto quovis dimanantes, vel quum primum in oculum incidunt, vel POStquam, a variis humoribus refraeti, ad fundum oculi perVeniunt, tot diversas directiones habent, ut si visus inde penderet, Omne visibile, variis eodem tempore directionibus spectatum, multiplex et
247. Quaestio de simplicst visu a duplice imagine, P
rum aCCUrate primo PrOPO- sita, postea ad majorem ac- Curationem redacta est: neque Prosecto Verum eSt, visum in homine sanissimo Semper eSse Simplicem.
Simplex vel duplex OSt, prout oculi varia ratione in
spectari, Dcct nou suo, vel pluribus locis eodem tempore ; et radii lucis dimanantes a quovis punct9, habent tot diversas directiones, vel quum primum incidunt in oculum, vel postquam perveniunt ad fundum ocuti, refracti a variis humoribus, ut, Si visus inde penderet, Omne visibile, spectatum furiis directionibus eodem tempore, videretur multipleae
247. Quaestio de simplice visu a duplice immine, primo
propositu parum accurate, est postea redacta ud majorem accurationem; ncque profecto est verum, υisum esse Semper simplicem iusanissimo homine. Visus)est simpleae vel dupleae, prout oculi fuerint conversi varia
ratione in rem qtiam Uec- Seen Out Os iis place, or in many places at the fame time ;and the rays of light emanating hom any potnt p0SSeSS Somany directions, either at the time that they first fati uponthe eye, Or after they arrive at the bottom of the eye, having been refracted by the various humours, stat, is vision depended upon this, every visibio object seen in disserent directions at the Same time, would appear multiplied and
247. The question concerning simple vision hom a doubleimage, at first explained Mith litue accuracy, has since been reduced to a greater degree os precision ; nor, indeed, is it triae, that sight is a tinnys single in a persectly healthy perSon. Sight is ei ther single or double, according as the eyeS are
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versi fusi int. Si axes binorum Oculorum in idem punctum directi fuerint, duplex imago ejus puncti, una nimirum in centro utriusque retinoe expressa, Simplicem visum efficit. Porro, reSquotquot pari ab oculo distantia ad utrumque latus talis puncti sitae sunt, Simplices simul spectantur: quarum Scilicet imagines Super retinum utramque de- Pictae, similem cum centro ejus comparatum situm habent. Centra igitur bina-Γum retinarum, et puncta
centrum situm habent, SVPra, nempe, Vel infra, vel dextrorSum, vel SiniStro 'Um, conSentire dicuntur, vel simplicem visum dare. Ites vero propius Vel remotius ab oculo positae, quam tumus. Si ares binorum Oculorum fuerint conversi in
idem punctum, duplex imago ejus, nimirum una cimago9
visum. Porrs, quotquot res sunt sitae pari distantiu oculo ad utrumque lutus tulis puncti, simul spectantur simplices ; scilicet cillures quarum imagines depictae super utrumque reti-Nam, habent similem situm
comparatum cum centroebus. Igitur centra binarum retinarrem, et puncta
in iis, quae habent similem situm quod iad centrum, Nempe Supra, vel infru, vel
dextrorsum vel sinistror Sum, dicuntur consentere vel dure simplicem visum. Vero res positae propius, vel remotius oculo, quam punctum, turned in a disserent manner toWards the Object we areviewing. Is the axes os both eyes are directed toWards the Same potnt, the doubie image of it, stat is, the one expreSSed in the centre of each retina, produces single viSion. More-over, est the objecis siluated at an equat distance se in theeye, on eaoli fide os such potui, are at the Same time seen single ; that is to say, the images of the objecis patiited oneach retina, have the samu situation relatively to iis centre. Theres ore, the centres of the two retinae, and the potnis in them Whicli possess the fame siluation in relation to the centre, Daniely above, or below, or to the right or test, are Sald to Correspond, or produce single vision. But objecis placed nearer, or surther froni the eye, than the pollit to
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punctum in quod aXes Oeulorum conveniunt, Simul cum illo spectari posSunt, earumque duplex visuS ESt. Facile ostenditur imagines
earum rerum partes retinae utriusque OCCUPare, quae
dissimiliter quod ad centrum ponuntur: nempe, in dextra parte alterius Oculi, in sinistra alterius, depingi. Partes igitur retinarum, dissimiliter quod ad centra pinSitae, non Consentiunt; hoc est, duplicem ejusdem rei, CHUS Super utramque imago depicta fuerit, visum efficiunt. 248. Varietates quaedam, seu vitia leviora hujus Sen-SUS, quae ViX morbosa habentur, Saepe ObSerVantur.
Myopes dicuntur, qui viSum in qu0d cpunctum θ ares
sunt specturi simul cum illo, que visus earum est duplex
utriusque retinae, quoe pυ-nuntur dissimiliter quod ad centrum: nempe, depingi in derissi parte alterius oculi, in sinistra parte9 alterius. Igitur partes Te-tiuarum, positae dissimiliter
quod ad centra, non conmutiunt : hoc est, essciunt δε-plicem visum ejusdem rei,
248. Quaedam varietates, seu leviora vitia hujus sensus, quae viae habentur morbosa,
Saepe Observantur. Illio diacuntur Myopes,'- qui habent
whicli the axes of the eyes are directed, may be Seen simultaneously, and the vision resulting hom them is single. Itis eastly shown that the images of those objecis occupy paris of each retina, whicli are siluated disserently in regard to iis Centre ; stat is to say, they are palnted on the right sideos one eye, and on the len Os the other. Therelare, theparis of the retina, siluaten dissimilarly in respect to iis Centre, do not correspond : they produce doubie vision os the objeci, the image of which was palnted upon them. 248. Certa in stight disorders of this sense, Whicli arescareely to be considered diseases, are osten observed. Those
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nil clare et accurate vident, nisi proxime ad Oculum admoveantur. Hoc fit propter radios lucis nimis refractOS, in focum nimis cito collec- tos, et iterum a foco dimananteS, Priusquam ad reti-Nam PerVeniant; quo fit ut indistinctam super eam imu-ginem depingant. Causa hujus vitii frequentissima est, nimia totius oculi vel humoris ejus cujusdam Con-VeXitas, veluti corneae nimis Prominentis, &c. quae radios lucis per oculum transeuntes nimis refringit. InCommodum, junioribus familiare, dum SeneSeunt, aliquando, ut fertur, sublevatur: quod tamen minime perpetuum est. Quum primo levius hujusmodi vitium observatUr, brevem visum, scilicet, qui vident nil accurate et clare, nisi admoveantur proaeime ad
radios lucis nimis refractos, collectos nimis cito in jocum, et iterum dimanantes a foco, priusquam perveniant ad retinam ; quo si ut depingant
indistinctam imaginem super eam. Frequentissima causa hujus vitii est, nimia converitas totius oculi, vel cujusdam humoris ejus, veluti corneae nimis prominentis, si c. quae nimis refringit radios lucis transeuntes per
oculum. Hoco incommodum, fumiliare junioribus,
aliquando sublevatur, ut fertur, dum senescunt: quod tamen est minime perpetuum. Quum levius vitium hujusmodi primo observatur, ps- persons are called myopes, or near-sighted, who see nothingaccurately or Clearly, unless it be brought close to the eye. I his occurs froni the rays of light being tuo much refracted, . tOO Soon collected into a focus, and again diverging homiliat lacus, besore they reach the retina; hence it is, thatthey paint an indistinet image upon it. A most common cause of this disorder is too great a convexity of the wholueye, or os either of iis humours ; as the cornea being too Prominent, and too powerintly refracting the ra3s of light in their passage through the eye. This inconvenience, commonto Foung people, is salii Sometimes to be alleviated in old
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consuetudine res remotas contemplandi, Oculosque a valde minutis et vicinis rebus cohibendi, quodammodo vinci, non SeCUS RUCOntrario usu idem vitium acquiri, potest: nimirum, quia oculus quodammodo ad rerum, quaS Contemplatur, distantias Sese ReeΟ-modat 243 . Vitrum con-CRVUm, quod radios plus facit dimanare priusquam in
oculum incidant, simplicissimum est, et certissimum Myopibus auxilium. 249. Presbyopes Voca tur, qui Visum longum habent, vicinarum rerum Confusum, remotiorum satis
distinctum. Contrarias hoc vitium ae alterum agnoscit Causas; oculum imprimis test quodammodo vinci consuetudine contemplandi res remotas, que cohibendi oculos a valde minutis et vicinis
trario tisti : Nimirum, quis oculus quodammodo accommodat sese iad distantias r Tum, quas contemplatur
243). Concavum vitrum, quod facit radios plus d a- Nare priusquam incidant in Oculum, est simplicissimum et certissimum aurilium MyopibuS.
Presbyopes, qui habent longum visum, confusum via sumὰ vicinarum rerum,