VISION. 



VISION, Visio, the aft of feeing, or perceiving exter- 

 nal objeAs by the organ of fight. 



Vifioi) is well defined to be a fenfation, by which, from a 

 certain motion of the optic nerve, made in the bottom of 

 the eye by the rays of light emitted or reflefted from 

 objefts, and hence conveyed to the common fenfory in the 

 brain, the mind perceives the luminous objeft, its quantity, 

 quality, figure, &c. 



The phenomena of vifion, the caufes of it, and the 

 manner in which it is effefted, make one of the greateft 

 and mod important articles in the whole fyllcm of natural 

 knowledge. Indeed, a great part of the phyfical, mathe- 

 matical, and anatomical difcoveries and improvements of the 

 moderns, terminate here, and only tend to fet the bufincfs 

 of vifion in a clearer light. 



Hitherto refer what fir Ifaac Newton and others have 

 difcovered of the nature of light and colours ; the laws of 

 infleftion, refleftion, and refradtion of the rays, the ftrutlure 

 of the eye, particularly the retina and optic nerves, &c. 



It is not necetfary we fiiould here give a minute detail of 

 the procefs of vifion from its firft principles ; the greateft 

 part is already delivered under the refpeftive articles. The 

 eye, the organ of vifion, we have defcribed under the article 

 Eye ; and its feveral parts, tunics, humours, &c. under 

 their proper heads, Cornea, Crystalline, &c. 



The immediate and principal organ of vifion, viz. the 

 retina, according to fome, and the choroides, according to 

 others, are alfo diftinftly confidered ; as alfo the ftrufture 

 of the optic nerve, which conveys the impreffion to the 

 brain ; and the texture and difpofition of the brain itfelf, 

 which receives them, and reprefents them to the foul. See 

 Retina, Choroides, Oi'tic Nerves, Brain, Sensory, &c. 



By means of this arrangement of the various refradling 

 fubftances, many peculiar advantages are procured. The 

 furface of the cornea only, if it had been more convex, 

 could not liave collefted the lateral rays of a direft pencil 

 to a perfeft focus, without a different curvature near its 

 edges ; and then the oblique pencils would have been fub- 

 jefked to greater aberration, nor could they have been made 

 to converge to any focus on the retina. A fecond refrac- 

 tion performs both thcfe offices much more completely, and 

 has alfo the advantage of admitting a greater quantity of 

 liglit. If alfo the furfaces of the cryftalline lens, thus 

 intcrpofed, had been abrupt, there would have been a re- 

 fleftion at each, and an apparent hazinefs would have inter- 

 fered with the diftinft view of every luminous objeft ; but 

 this inconvenience is avoided by the gradual incrcafe of den- 

 fity m approaching the centre, which alfo makes the cryftal- 

 line equivalent to a much more refraftive fubftance of equal 

 magnitude ; while, at the fame time, the fmaller denfity of 

 the lateral parts prevents the ufual aberration of fpherical 

 furfaces, occafioned by the too great rcfrattion of the 

 lateral rays of dircft pencils, and caufes alfo the focus of 

 each oblique pencil to fall either accurately, or very nearly, 

 on the concave iurface of the retina, throughout its extent. 



Again, the nature of light, which is the medium or 

 vehicle by which objefts are carried to tlie eye, is laid down 

 at large under the articles Light and Colours ; and the 

 chief properties thereof concerned in vifion, under Reflec- 

 tion, Refraction, &c. ; and alfo many of its circum- 

 ftances under Ray, Medium, &c. What remains for this 

 article, therefore, is only to give a general idea of the whole 

 procefs, in which all tlio ieveral parts are concerned. 



Vision, iJiJcnnt Opinions or Sterns of. The Platonifts 

 and Stoics held vifion to be cffecled by the emilTion of rays 

 out of the eyes ; conceiving that there was a fort of light 

 thus darted out ; wliicb, with the light of the cxtcrn^J air, 



taking, as it were, hold of the objeds, rendered tiiem 

 vifible ; and thas returning back again to the eye, altered 

 and new modified by the contaft of the objeft, made an 

 imprefiion on the pupil, which gav^ the fenfation of the 

 objeft. 



The reafons by which they maintain their opinions are 

 derived, i. From the brightnefs and luftre of the eye. 

 2. From our feeing a remote cloud, without feeing one with 

 which we are encompafied (the rays being fuppofed too 

 briflc and penetrating to be flopped by the near cloud, but 

 growing languid at a greater dittance, are returned to the 

 eye). 3. From our not feeing an objeft laid on the pupil. 

 4. From the eye's being weary with feeing; i.e. by emit- 

 ting great quantities of rays. And laftly, from animals 

 which fee in the night, as cats, lions, moles, owls, and forae 

 men. 



Our own countryman, Roger Bacon, diftinguifhed as he 

 was in a variety of refpedls, does not hciitate to aflent to 

 the opinion that vifual rays proceed from the eye ; giving 

 this reafon for it, that every thing in nature is qualified to 

 difcharge its proper funftions by its own powers, in the 

 fame manner as the fun, and other celefl ial bodies. Opus 

 Majus, p. 289. 



The Epicureans held vifion to be performed by the ema- 

 nation of corporeal fpecies, or images from objefts ; or a 

 fort of atomical efiluvia continually flying off from the inti- 

 mate parts of objefts to the eye. 



Their chief reafons are, i. That the objefts muft necef- 

 farily be united to the vifive faculty ; and fince it is not 

 united by itfelf, it muft be fo by fome fpecies that reprefents 

 it, and that is continually flowing from bodies. 2. That 

 it frequently happens, that old men fee remote objefts better 

 than near ones ; the diftancc making the fpecies thinner, and 

 more commenfurate to the debility of their organ. 



The Peripatetics hold, with Epicurus, that vifion is per- 

 formed by the reception of fpecies ; but they differ from 

 him in the circumftances : for they will have the fpecies 

 (which they call intcntionales) to be incorporeal. 



It is true, Ariflotle's dodlrine of vifion, delivered in liis 

 chapter " De Afpeflu," amounts to no more than this ; 

 that objefls muft move fome intermediate body, that by this 

 they may move the organ of fight. To which he adds, in 

 another place, that when we perceive bodies, it is their 

 fpecies, not their matter, that we perceive ; as a feal makes 

 an impreffion on wax, without the wax's retaining any thing 

 of the feal. 



But this vague and obfcure account the Peripatetics have 

 thought fit to improve. Accordingly, what their mafter 

 called fpecies, the difciples undcrllanding of real proper 

 fpecies, afl'ert, that every vifible objeft cxpreffes a perfeft 

 image of itfelf, in the air contiguous to it ; and this image 

 another, fomewhat lefs in the next air ; and the third, an- 

 other, &c. till the laft image arrives at the cryftaUine, 

 which they hold for the chief organ of fight, or that which 

 immediately moves the foul. Thefe images they call inten- 

 tional fpecies. 



The modern philofophers, as the Cartcfians and New- 

 tonians, give a better account of vifion. They all agree, 

 tliat it is performed by rays of light reflefted from the fe- 

 veral points of olijefts received in at the pupil, refrafted 

 and colleded in their paffage, through the coats and hu- 

 mours, to the retina ; and thus ftriking, or making an im- 

 preffion, on fo many points thereof; which nnprclTion is 

 conveyed, by the corrcfpondent capillaments of the optic 

 nerve, to the brain, &c. 



Baptilla Porta's experiments with the camera obfcura, 

 abwit the middle of llit i6lh century, convinced htm, that 



rifion 



