VISIBLE. 



tinftion, Tacquet pretended to demonftratc, that nothing 

 can give the idea of two parallel lines to an eye fituated at 

 one of their extremities, but two hyperbolical curves, 

 turned the contrary way ; and M. Varignon maintained, 

 that, in order to make a vifta appear of the fame width, 

 it muft be made narrower, inftead of wider, as it recedes 

 from the eye. M. Bouguer obferves, that very great dif- 

 tances, and thofe that are confiderably lefs, make nearly the 

 fame impreffion upon the eye. We, therefore, imagine 

 great diftances to be lefs than they are, and on this account 

 the ground plan of a vifta always appears to rife. The 

 vifual rays come in a determinate direftion, but as we 

 imagine they terminate fooner than they do, we neceffarily 

 conceive that the place from which they ifTued is elevated. 



Every large plane, therefore, as A B [Plate XX. Optics, 

 fig. J. ) viewed by an eye at O, will feem to he in fuch direc- 

 tion as A ^ ; and confequently lines, in order to appear truly 

 parallel, on the plane A B, muft be drawn fo as that they 

 would appear parallel on the plar.e A b, and be from thence 

 projeded to the plane A B. To determine the incHnation 

 of the apparent ground plane A i to the true ground plane 

 A B, M. Bouguer direfts us to draw upon a piece of level 

 ground two ftraight lines of a fufRcient length, maktng an 

 angle of three or four degrees with one another. Then a 

 perfon placing hinifelf within the angle, with his back 

 towards the angular point, muft walk backwards and for- 

 wards till he can fancy the lines to be parallel. In this 

 fituation, a line, drawn from the point of the angle through 

 the place of his eye, will contain the fame angle with the 

 true ground plane which this does with the apparent 

 one. 



M. Bouguer alfo (hews other more geometrical methods 

 of determining this inclination, and fays, that by thefe 

 means, he has often found it to be four or five degrees, 

 though foraetimes only two, or two and a half degrees ; 

 the determination of this angle being variable, and depend- 

 ing upon the manner in which the ground is illuminated, 

 and the intenfity of the light, the colour of the foil, the 

 conformation of the eye, and the part of the eye on which 

 the objeft is painted. 



In looking towards a rifing ground, the difference be- 

 tween the apparent ground plane and the true one, he fays, 

 will be much more confiderable, fo that they will forae- 

 times make an angle of 25 or 30 degrees. Ac. Par. :755. 

 M. 156. 



2. If the eye be placed above an horizontal plane, 

 objeAs, the more remote they are, the higher will they ap- 

 pear, till the laft be feen in a level with the eye. Whence 

 it is that the fea, to perfons ftanding alhore, feems to rife 

 higher and higher the farther they look. 



3. If any number of objefts ho placed below the eye, 

 the moft remote will appear the higheft ; if they be above 

 the eye, the moft remote will appear the loweft. 



Thus the remoter parts of a horizontal walk, or long 

 floor, will appear to afcend gradually ; whereas, the ciehng 

 of a long gallery appears to defcend. 



M. Bouguer obferves, that when a man ftands upon a 

 level plane, it does not feem to rife fenlibly, but at fome 

 diftance from him : the apparent plane, therefore, has a 

 curvature in it, the form of which is not very eafy to de- 

 termine ; fo that a man ftanding upon a level plane of in- 

 finite extent, will imagine that he ftands in the centre of a 

 bafon. The cafe is the fame with a perfon ftanding upon 

 the level of the fea. 



4. The upper parts of high objefts appear to ftoop, or 

 incline forwards ; as the front of churcne?, towers, &c. 

 And ftatuee at the tops of buildings, to appear upright, 



9 



muft incline, or bend backwards. See farther under the 

 articles of Refraction and Horizon. 



II. The mind perceives the diftance of vifible objefts, 

 from the different configurations of the eye, and the man- 

 ner in which the rays ftrike the eye, and in which the image 

 is impreffed on it. For the eye difpofes itfelf differently, 

 according to the different diftances it is to fee ; i!;z. for 

 remote objefts the pupil is dilated, and the cryftalline 

 brought nearer the retina, and the whole eye is made more 

 globular ; on the contrary, for near objefts, the pupil is 

 contrafted, the cryftalline thruft forwards, and the eye 

 lengthened. 



Philofophers are agreed, that we have a power of altering 

 the form of our eyes, fo as to make the rays of any pencil 

 to converge at different diftances from the pupil : and 

 hence we are capable of viewing objefts with almoft equal 

 diftinftnefs, though they are placed at confiderably dif- 

 ferent diftances ; but with regard to the alteration that 

 takes place in the eye, and the mechanifm by which it is 

 produced, different accounts have been given. 



It was the opinion of Kepler, that the contraftion of 

 the proceffus ciliares changes the form of the eye, and by 

 the elongation of it, places the cryftalline at a greater dif- 

 tance from the retina ; whereas Des Cartes imagined, that 

 the curvature of the cryftalline itfelf fuffers an alteration by 

 the contraftion of thofe ligaments. 



M. de la Hire maintained that, in order to view objefts 

 at different diftances, there is no alteration but in the lize 

 of the pupil, or the aperture of the eye ; and he made 

 a curious experiment, which, he thought, proved his 

 affertion. 



M. Le Roi, a member of the Royal Academy at Mont- 

 pelier, has lately attempted to defend the opinion of M. de 

 la Hire, which had long been exploded by all philofophers ; 

 and he fays, that the accommodation of the eye to the 

 view of objefts, placed at different diftances, by the con- 

 traftion or dilatation of the pupil only, does not conlift 

 in the change of the place of the cryftalline, by means 

 of the ligamenta ciliaria, the ftrength of which is inade- 

 quate to the purpofe. Befides, he obferves, that they 

 are not attached to the edge of the capfula, as has been 

 fuppofed, but that they extend a confiderable way along 

 the interior furface of it, without any clofe adherence 

 to it. He is alfo of opinion that thefe fibres are not 

 mufcular, but are only ramified veffels, which, according 

 to all appearance, he fays, anfwer no other purpofe than 

 that of fecreting an aqueous humour, to lubricate the fur- 

 face of the cryftalline. 



That nothing is requifite but the contraftion of the 

 pupil in order to view the neareft objefts with diftinftnefs, 

 . is evident, he fays, from experiment. For when an 

 objeft is placed fo near, that the eye cannot bear as great 

 a degree of contraftion as is necelfary for viewing it dif- 

 tinftly, the fame end is obtained by an artificial pupil. 

 For if a fmall hole be made in a card, the neareft objeft 

 may be viewed through it with the greateil eafe and 

 diftinftnefs. 



That the variation of the pupil is fufficient for the 

 purpofe of viewing objefts at all diftances, he alfo thought 

 he could demonftrate by experiment with an artificial 

 eye ; for when, with a large aperture, the images of 

 near objefts were confufed, and ill defined upon the re- 

 tina of this inttrument, they became very diftinft, and 

 well defined, by contrafting the aperture. Ac. Paris, 

 1755. M. p. 920. 



But the moft fatisfaftory difcuffion of this fubjcft we 

 owe to Dr..Po/terfield, who proved, by a feries of experi- 

 ments, 



