VISION. 



retina ; that in this pr.rt there is a dillinft and vivid collec- 

 tion of all the rays received in at the pupil ; and tliat as each 

 ray carries its proper colour along with it, there are as many 

 points painted in the fund of the eye as there were points 

 vifible in the objeft. Thus is tliere a fpecies, or pifture, 

 on the retina, exaftly like the object : all the difference be- 

 tween them is, that a body is here rcprefented by a furface, 

 a furface frequently by a line, and a line by a point ; that 

 the image is inverted, the rigbt-iiand anfwering to the left of 

 the objeft, &c. and that it is exceedingly fmall ; and ftill 

 the more fo, as the objeft is more remote. 



What we have fhewn, under other articles, of the nature 

 of light and colours, readily accounts for this painting of 

 the objeft on the retina. The matter of faft is proved by 

 an eafy experiment, long iince tried by Des Cartes, thus : 

 the windows of a chamber being (liut, and light only ad- 

 mitted at one little aperture ; to that aperture apply the eye 

 of fome animal newly killed, having firll dexteroufly pulled 

 off the membranes that cover the bottom of the vitreous 

 humour, viz. the hind part of the fclerotica, choroides, and 

 even part of the retina ; then will the images of all the 

 objects, without doors, be feen diftinftly painted on any 

 white body, as on an egg-(hell, that the eye is laid upon. 

 And the fame thing is better fhewn by an artificial eye, or 

 a camera obfcura. 



The images of objefts, then, are reprefented on the retina; 

 which is only an expanfion of the fine capillaments of the 

 optic nerve, and from which the optic nerve is continued 

 into the brain. Now, any motion or vibration, impreffed on 

 one extreme of the nerve, will be propagated to the other : 

 hence the impulfe of the feveral rays, fent from the fevcral 

 points of the objeft, will be propagated as they are on tlie 

 retina, [i.e. in their proper colours, &c. or in particular 

 vibrations, or manners of preflurc, corrcfponding thereto, ) 

 to the place where thofe capillaments arc interwoven into 

 tlie fubftance of the brain. And thus is vifion brought to 

 the common cafe of fer.fation. 



For fucli, we know, is the law of the union between the 

 foul and body, that certain perceptions of the firfl do 

 necelTarily follow certain motions of the laft ; but the 

 different parts of the objeA do feparately move different 

 parts of the fund of the eye ; and thofe motions are pro- 

 pagated to the fenfory : it follows, therefore, that there 

 muft arife fo many diftinft fenfations at the fame time. See 

 Sensation. 



Hence, I. We eafily conceive, that the perception, or 

 image, in the mind, muft be tlie clearer, and more vivid, the 

 more rays the eye receives from the objtel ; and confe- 

 quemily, the largenefs of the pupil will have fome fhare in 

 the clearnefs of vifion. 



2. Confidering only one radiant point of an objeft, we 

 may fay, tliat that point would move the fenfe more weakly, 

 or be feen moreobfcurely, as it is more remote ; becaufe the 

 rays coming from any point, like all qualities propagated in 

 orbem, are always diverging ;, and therefore the more remote, 

 the fewer of them will be received in at the pupil. But the 

 pupil dilating itfelf more, as the objeft is more remote, takes 

 in more rays tlian it would otherwife do. 



3. The diftindtnefb of vifion is fomcwhat concerned in the 

 fize of the image exhibited in the fund of the eye. For 

 there Ihould be, at leall, as many extremes of capillaments, 

 or fibres of the optic nerve, in the fpacc that image poflcfTes, 

 as there ar" particles in the objcft that fend rays into the 

 pupil ; otherwife every particle will not move its feparatc 

 capillami-iit ; and if the rays from two points fall on the 

 fame capillament, it will be the fame as if only one point had 

 fallen there ; fince the fame capillament cannot be differently 



Vol. XXXVII. 



moved at tlic fame time. And hence it is, tliat the images 

 of very remote objedls being very fraall, they appear con- 

 fufcd, feveral points of the image affefting each capillament; 

 and hence, alfo, if tlic objeft be of different colours, feveral 

 particles affcdling tlie fame capillament at the fame time, 

 «mly the brightelt and moit lucid will be perceived. Thus, 

 a field, furnifhed with a good number of white flowers, 

 among a much greater quantity of green grafs, &c. at a 

 dift.ince, appears all white. See DifiinlH Vision, infra. 



Our feeing of objefts y^nf /;, though with two eyes, in 

 each of wliich is a feparate image, or picture ; and our 

 feeing of them ered, whereas the picture is really inverted, 

 are two great plienomena in vifion ; which we have con- 

 fidered under the article Seeing. 



For the manner of feeing and judging of the diftance and 

 magnitude of objedls, fee Visible, Magnitude, &c. 



Vision, in Optics. The laws of vifion, brought iinder 

 mathematical demonftrations, make the fubjedl of Optics, 

 ( which fee, ) taken in the greateil latitude of that word : 

 for, among the writers of mathematics, optics is generally 

 taken, in a more reflrained figniticatioii, for the doftrine of 

 ilireEt vifion ; catoptrics, for the doctrine of refleHed vifion ; 

 and dioptrics, for that of refraded vifion. 



Vision, Direft or Simple, is that performed by m.eans of 

 direct rays ; that is, of rays paiTing dircftly, or in right 

 lines, from the radiant point of the eye. Such is that 

 explained in the preceding article. Vision. 



Vision, Reflelled, is that performed by rays reflected from 

 fpecula, or mirrors. The laws of this kind of vifion, fee 

 under Rei'lection, and Mirror. 



Vision, Refrathd, is that performed by means of rays 

 refracted, or turned out of their way, by pafling through 

 mediums of difTcrent dcnfity : as air and water, and chiefly 

 through <;Iafl"cs and Iciifcs. The laws of this, fee under the 

 article Refraciion. 



Visioti, ^rch of. See Arch. 



Vision, DijTina, denotes that by which an objeA is feen 

 diftinftly. An objeCt is faid to be feen dillinCtly, when 

 its outlines appear clear and well defined, and the feveral 

 parts of it, if not too fmall. are plainly dillingi>ifhable, fo 

 that we can eafily compare them one with another, in refpeft 

 to their figure, fize, and colour. 



In order to fuch diftindt vifion, it has hitherto been com- 

 monly thought, that all the rays of a pencil, flowing from a 

 phyficnl point of an objcft, muft be exactly united in a 

 phyfical, or, at leaft, in a fenfible point of the retina. But 

 it feems certain, from the experiments mentioned by Dr. 

 Jurin, that fuch an exadt union of rays is not always necef- 

 fary to diftinct vifion. 



Hence the dodtor divides diftinft vifion into two fpeciet, 

 viz. into vifion p/rfeSly diJlinS, or perfeH vifion, and vifion 

 imperfellly diJlinS ; wliicli he calls fimply by the name of 

 dijlina vifion. The former is lliat in which the rays of each 

 pencil are colleded into a fingle phyfical, or fenfible point 

 of tiie retina ; the other fpecies is that in which thofe rays 

 occupy feme larger fpace upon the retina, yet fo as the 

 object is diftinftly perceived. 



PcrfcH vifion in a given eye, and a given difpofition of 

 that eye, depends only upon the diftance of the objcd; it 

 has no dependence upon the magnitude of the object ; b»t 

 dijlintl vifion, in a given eye, and a given difpofition of th* 

 eye, depends upon the diflancu and magnitude of the objeft 

 jointly. There appearing, therefore, a real difference be- 

 tween ptrJcH vifion, and what wc call dijUnii vifion, the 

 learned doaor has enquired very particularly into the 

 reafbn why an object may be feen diftindly without pcrfort 

 vifion, 



M m He 



