VISIBLE. 



guments are deduced from pofitive evidence of the change 

 of form of the cryllalline, furnilhed by the particular ef- 

 fefts of refraftion and aberration which are obfervable in 

 the different ftates of the eye ; effefts which furnifh a direcl 

 proof that the figure of the lens muft vary ; its furfaces, 

 which are nearly fpherical m the quiefcent form of the lens, 

 affuraing a different determinable curvature when it is called 

 into exertion. The objeftions which have been made to this 

 cooclufion are founded only on the appearance of a flight 

 alteration of focal length in an eye from which the cryftal- 

 linc had been extrafted ; but the h£t is neither fufficiently 

 afcertained, nor was the apparent change at all confiderable : 

 and even if it were proved that an eye without the lens is 

 capable of a certain fmall alteration, it would by no means 

 follow that it could undergo a change five times or ten times 

 as great. 



The motion of the optical axes ferves likewife, as we 

 have already obferved, to afTiil us in judging of the diftance 

 of objedts. Thefe axes, or the direftions of the rayi falling 

 on the points of moft perfeft vifion, naturally meet at a great 

 diftance ; that is, they are nearly parallel to each other ; and 

 in looking at a nearer objeft, we make them converge to- 

 wards it, wherever it may be fituated, by means of the external 

 mufcles of the eye ; while in perfeft eyes the refraftive powers 

 are altered, at the fame time, by an involuntary fympathy, fo 

 as to form a diftina image of an objeA at a given diftance. 

 This correfpondence of the fituation of the axes with the 

 focal length is in moft cafes unalterable ; but fome have 

 perhaps a power of deranging it in a flight degree, and in 

 others the adjuftment is imperfe£l : but the eyes feem to 

 be in moft perfons infeparably connefted together with re- 

 fpeft to the changes that their refraftive powers undergo, 

 altliough it fometimes happens that thofe powers are ori- 

 ginally very diff'erent in tlie oppofite eyes. 



Thefe motions enable us to judge pretty accurately, 

 within certain limits, of the diftance of an objeft ; and be- 

 yond thefe Lmlts, the degree of diftinftnefs or confuCon of 

 the image ftill continues to aflift the judgment. We efti- 

 niatc diftances much lefs accurately with one eye than with 

 both, fince we are deprived of the aflillance ufually afforded 

 by the relative fituation of the optical axes ; thus we feldom 

 fucceed at once in attempting to pafs a finger or a hooked 

 rod fideways through a ring, with one eye fliut. Our idea 

 of diftance is ufually regulated by a knowledge of the real 

 magnitude of an objett, while we obferve its angular magni- 

 tude ; and on the other hand, a knowledge of the real or 

 imaginary diftance of the objeft often diredls inir judgment 

 of its atlual magnitude. Tlie quantity of light intercepted 

 by the air interpofed, and the inteniity of the blue tint which 

 it occafions, are alfo elements of our involuntary calculation : 

 hence, in a mift, the obfcurity increafes the apparent dif- 

 tance, and confequently the fuppofed magnitude of an un- 

 known obje6\. We naturally obferve, in eftimating a dif- 

 tance, the number and extent of the intervening objedts ; fo 

 that a diftant church in a woody and hilly country ap- 

 pears more remote than if it were fituated in a plain ; and 

 for 3 fimilar reafon, the apparent diftance of an objeil feen 

 at fea, is fmaller than its true diftance. Young's Courfe of 

 Leftures on Natural Philofophy, &c. vol. i. 



Accordingly, in judging of tlie diftance of a vifible objeft, 

 we muft take into our account the angle which the objeft 

 makes, with the diftincl or confjfed reprefentation of the 

 objeft ; and the brilknefs or feeblenefs, or the rarity or 

 fpiflitude of the rays. 



To this it IS owing, I. That objefts which appear ob- 

 fcure, or confufed, are judged to be more rcniote ; a princi- 

 ple which the painters ak to make fome of their figurts 



4 



appear farther diftant than others on the fame plane. Thu«, 

 fuppofmg the eye to be accommodated to a given diftance, 

 objefts at all other diftances may be reprefented with a cer- 

 tain indiilinftnefs of outline, which would accompany the 

 images of the objefts themfelves on the retina : and this 

 indiftindnefs is fo generally neceffary, that its abfence 

 has the difagreeable effeift called hardr.efs. The apparent 

 magnitude of the fubjefts of our defign, and the relative 

 fituations of the intervening objedfs, may be fo imitated 

 by the rules of geometrical perfpeftive as to agree per- 

 feftly with nature, and we may ftill further improve the re- 

 prefentation of diftance by attending to the art of aerial 

 perfpeClive, which confifts in a due obfervation of the lofs 

 of light, and the blucifh tinge, occafloned by the interpofi- 

 tion of a greater or lefs depth of air between us and the 

 different parts of, the fcenery. 



We cannot indeed fo arrange the pifture, that either the 

 focal length of the eye, or the pofition of the optical axes, 

 may be fuch as would be required by the aftual objefts : 

 but we may place the pifture at fuch a diftance, that neither 

 of thefe cntcrions can have much power in detefting the 

 fallacy ; or, by the interpofition of a large lens, we may 

 produce nearly the fame effefts iu the rays of light, as if 

 t-hey proceeded from a pifture at any required diftance. la 

 the panorama, which has lately been exJiibited in many parts 

 of Europe, the effefts of natural fcenery are very clofely 

 imitated : the deception is favoured by the abfence of all 

 other vifible objefts, and by the faintnefs of the light, which 

 affifts in concealing the defefts of the reprefentation, and for 

 which the eye is ufually prepared, by being long detained in 

 the dark winding paflages which lead to the place of exhibi- 

 tion. Young, ub'ifupra. Si;e Apparent Magnitude. 



2. To this it is likewife owing, that rooms, whofe wallc 

 are whitened, appear the fmaller ; that fields covered with 

 fnow, or white flowers, fhew lefs than when clothed with grafs ; 

 that mountains covered with fnow, in the night-lime, appear 

 the nearer ; and that opaque bodies appear the more remote 

 in the twilight. 



III. The magnitude or quantity of vifible objefts is 

 known chiefly by the angle comprehended between two 

 rays drawn from the two extremes of the objeft to the cen- 

 tre of the eye. An objeft appears to be as large as the 

 angle it fubtends ; or bodies feen under a greater angle ap- 

 pear greater ; and thofe under a lefs, lefs, &c. Hence the 

 fame thing appears now bigger, and now lefs, as it is lefs or 

 more diftant from the eye. This we call the Apparent Mag- 

 NITUDK ; which fee. 



Now, to judge of the real magnitude of an objeft, we 

 confider the diftance ; for, fince a near and remote objeft 

 may appear under equal angles, the diftance muft neceffarily 

 be eftimated ; that if it be great, and the optic angle fmall, 

 the remote objeft may be judged great ; and vice -verfa. 



The magnitude of vifible objefts is brought under certain 

 laws, demontlrated by the mathematicians ; as, 



1. That the apparent magnitudes of a remote objeft are 

 as the diftances reciprocally ; or rather, in a fomewhat lefs 

 ratio. 



2. That the co-tangents of half the apparent magnitudes 

 of the fame objedts, are as the diftances j hence the apparent 

 magnitude and diilance being given, we have a method of 

 determining the true magnitude ; the canon is this. As the 

 whole fine is to the tangent of half the apparent magnitude, 

 fo is the given diftance to half the real magnitude. The 

 fame canon, inverted, will, from the diftance and magnitude 

 given, determine the apparent one. 



3. Objefts feen under the fame angle, have their mag- 

 nitudes proportional to their diftances. 



4. The 



