VISIBLE. 



4. The fubleiife A B [Plait XX. Optus, fg.6.) of any 

 art of a circle appears of equal magnitude in all the points 

 D C E G, though one point be vallly nearer than another ; 

 and the diameter D G appears of the fame magnitude in 

 all the points of the periphery of the circle. Hence fome 

 have derived a hint for the moft commodious form of 

 theatres. 



5. If the eye be fixed in A [Jig. 7.), and the right line 

 B C be moved in fuch manner, as that the extremes of it 

 always fall on the periphery, it will always appear of the 

 fame magnitude. Hence the eye, being placed in any angle 

 of a regular polygon, the fides will appear equal. 



6. If the magnitude of an objeft direftly oppofite to the 

 eye be equal to its diftance from the eye, the whole ob^eft 

 will be taken in by the eye, but nothing more. Whence 

 tie nearer you approach an objeft, the lefo part you fee 

 of it. 



IV. The figure of vifible objefts is eftimated, chiefly, 

 from our opinion of the fituation of the feveral parts of it. 



This opmion of the fituation, &c. enables the mind to 

 apprehend an external objeft under this or that figure, 

 more juftly than any fimilitude of the images in the retina, 

 with the objedt, can ; the images being frequently elliptical, 

 oblong, &c. when the objects they exhibit to the mind are 

 circles, fquares, &c. 



The laws of vifion, with regard to the figures of vifible 

 objcfts, are : 



1 . Tliat if the centre of the pupil be exaftly againft, or 

 in the direction of a rigiit line, the line will appear as one 

 point. 



2. If tlic eye be placed in a direftion of a furface, fo that 

 only one line of the perimeter can radiate on it, it will appear 

 as a line. 



3. If a body be oppofed direftly towards the eye, fo as 

 only one plane of the furface can radiate on it, it will appear 

 as a furface. 



4. A remote arc, viewed by an eye in the fame plane, 

 will appear as a rigiit line. 



5. A fphere, viewed at a diitancf, appears a circle. 



6. Angular figures, at a dillance, appear round. 



7. If the eye look obliquely on the centre of a regular 

 figure, or a circle, the true figure will not be feen ; but the 

 circle will appear oval, &c. See Apparent Figure. 



V. The number of vifible objecls is perceived, not only 

 by one or more images formed in the fund of the eye ; but 

 alfo by fuch a pofition of thofe parts of the brain whence 

 the optic nerves fprlng, as the mind has been ufed to, in 

 attending to a certain place ; and that either Cngle or 

 manifold. 



Accordingly, when either of the eyes, with the contiguous 

 part of the brain, arc forced out of their juft parallelifm, 

 with the other, v.gr. by prefTing it with the finger, &c. all 

 tilings appear double ; but when they are in the requiCte 

 parallehfm, though there be two images in the fund of the 

 two eyes, yet tlie objeft will appear fiiigle. Again, one 

 thing may appear double, c." even manifold, not only with 

 both eyes, but even with only one of them open ; by reafon 

 the common concourfe of the cones of rays relleiled from 

 the objeft to the eye, citiier falls fhort of the retina, or goes 

 much beyond it. 



VI. Motion and reft are feen when the images of objedls 

 rcprcfented in the eye, and propagated to the brain, are 

 eiliier moved, or a» reft ; and the mind perceives thefu images 

 cither moving or at reft, by comparing the moved image to 

 another, with refpcu^ to which it chaugea place ; or by the 

 fituation of the eye to the objcA being continually changed. 

 So that motioa is only perceivcdj by perceiving the im.iges 



to be in dififerent places and fituations ; nor are thefe 



changes perceived unlefs effefted in time. So that to per- 

 ceive motion, a fenJible time is required. But relt is per- 

 ceived by the vifual faculty, from the reception of the image 

 in the fiime place of the retina, and the faqie fituation for 

 fome feiifible time. 



Hence the reafon, why bodies moving exceedingly fall 

 appear at reft ; thus, a live coal, fwung briflcly round, ap- 

 pears a continual circle of fire ; the motion not being cotn- 

 menfurate with vifible time, but much fwifter than the fame ; 

 fo that, in the time the foul requires to judge of any chaiig* 

 of fituation of the image on the retina, or that it is moved 

 from this place to that, the thing itfelf performs its whole 

 circuit, and is in its own place again. 



Laws of vifion, with regard to the motion of vifibles, are : 



1. That if two objefts unequally diftant from the eye 

 move from it with equal velocity, the more remote one will 

 appear the flower ; or, if their celerities are proportionable 

 to their diflances, they will appear to move equally fwift. 



2. If two objefts, unequally diftant from the eye, move 

 with unequal velocities in the fame direftion, their apparent 

 velocities arc in a ratio compounded of llie direA ratio of 

 their true velocities, and the reciprocal one of their diftances 

 from the eye. 



3. A vifible objeft, moving with any velocity, appears 

 to be at reft, if the fpace deferibed in the interval of one 

 fecond be imperceptible at the diftance of the eye. Hence 

 it is, that a near objeft, moving very flowly, as the index of 

 a clock ; or a remote one very fwiftly, as a planet ; feem at 

 reft. 



4. An objeft moving with any degree of velocity, will 

 appear to reft, if the fpace it runs over in a fecond of time 

 be to its diftance from the eye, as i to 1400 : nay, in faft, 

 if it be as I to 1 300. 



5. The eye proceeding ftratght from one place to another, 

 a lateral objeft, not too far off, either on the right or 

 left, will feem to move the contrary way : the eye, in this 

 cafe, being fenfible of its motion, diftant objefts will feem 

 to move the fame way, and with the fame velocity. 



6. If the eye and the objeA move both the lame way, 

 only the eye much fwifter than the objeft, that laft will 

 appear to go backwards. 



7. If two or more objefts move with the fame velocity, 

 and a third remain at reft, the moveables will appear fixed, 

 and the quiefcent in motion the contrary way. Thus, 

 clouds moving very fvvdftly, their parts feem to prefcrve 

 their fituation, and the moon to move the contrary way. 



8. If the eye be moved with a great velocity, lateral 

 objefts at reft appear to move the contrary way. Thus, 

 to a perfon fitting in a coach, riding briikly througli a 

 wood, the trees feem to retire the contrary way ; and to 

 people in a ftiip, &c. the ftiorts feem to recede. 



9. An objeft moving very iwiftly is not feen, unlefs it 

 be very luminous. Thus, a cannon-ball is not feen, if it is 

 viewed tranlverfely ; but if it be viewed according to the 

 line it defcribcs, it may be feen, becaufe its pidure con- 

 tinues long on the fame place of the retina, which, therefore, 

 receives a more fenfible inipreflioii from the objetl. 



10. A live coal fwung briikly round in a circle, appears 

 a continued circle of fire, becauie the impreflions made on 

 the retina of light being of a vibratory, and conicquently of 

 a lafting nature, do not prefently perifh, but continue till 

 the coal performs its whole circuit, and returns again to its 

 former place. 



u. If a perfon turns fwiftly round, without changing 

 his place, all ubjeds about him will ieem to move round in 

 a circle the contrary way ; aad this deception coiitiuuti not 



only 



