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3.3 On seeing objects single (undated)

272131/6/I/23, 1r That we see objects at first single when our eyes 28are properly directed

301 If there was a duplicity to the eye, however custom and habit might 31bring us to pass immediately from this perception to the tangible unity 32signified by it; yet it is probable that by accurate attention we might be 33able to check that custom and attend to the pure objects of Sight since we 34find this in other like instances tho' difficult is not impossible. When a 35common man sees the inside of a Church he thinks the pillars appear to his 36eye of equal height and thickness. But a person accustomed to attend to 37his perceptions can easily be convinced that tho they have really the same 38tangible dimension yet their visible dimensions diminishes as they are 39more distant from him and that in perspective they must be so represented. pg 3291Now as we can by strict attention perceive the visible dimension of an 2object tho accustomed constantly to consider it onely as a sign of the 3tangible it is probable that by care and pains we might be able to attend to 4the visible Number of Objects tho commonly onely used as a Sign of the 5tangible Number. [Yet I never found] This might be particularly expected 6of painters who are accustomed to separate the visible appearance from 7the tangible that by looking first with one eye and then with both they 8might be able to perceive some duplicity in the latter case yet I think there 9is no evidence that any person ever saw an object double when his eyes 10were properly directed to it.

112 In all the Instances of double Vision we know the two Images are 12seen [at a certain ] as it were in different points of the same Sphere and 13at a certain angular Distance, [now this is as difficult] nor can I form a 14conception of two objects seen at the same time which have no distance 15nor proximity. And I apprehend every other person will find it impossible 16for him to conceive this. Now it is as difficult to account for this as for 17single vision. For it is impossible to give a reason why the mind in seeing 18an Object double with the two eyes should {naturally} place the two [1v]19images I at one certain distance rather than another. It may as well place 20them at no distance as at any particular Distance & then the Vision will 21be single. And indeed when in double Vision the two Images are seen 22at a certain Distance (as I think they always are) it seems necessarly to 23follow that by moving the axis of one of the eyes {in a certain manner} 24while the other remains fixed the distance between the images would be 25diminished and at last evanishing would give single vision.

26Dr Jurin mentions a case of a Clergyman who having been blind for 27some years with a Gutta serena was cured by a Salivation and for some 28time after saw objects double but by degrees the Images approached [and 29at last saw] and at last he saw as distinctly as before, he tells us of an 30Instance or two {one or two others} of this kind mentioned by Dr Briggs in 31his Nova Visionis Theoria. It seems very probable that in these cases the 32double vision was owing to the persons having lost the habit of directing 33their eyes properly to an object and when they recovered that faculty they 34had no more double vision

35[Dr Smith mentions] we have another Instance from Cheselden of a 36person who had one eye distorted by a blow on the head who at first saw 37objects double but by use came first to see the most familiar objects and 38at last all objects {tho the distortion continued} It were to be wished we 39had been more particularly informed of the circumstances of this case. pg 3301Perhaps {he learned to turn} when one eye was [turned] directly to the 2Object which might take some time the other was too oblique to give any 3image or gave so faint an Image that he soon acquired the habit of not 4attending to it.

5These are all the Instances I have met with that may be brought in 6support of Dr Smiths opinion that we naturally see things double but 7by use come to judge them single. And I think it appears from what is 8observed above that they do not conclude the point. He [gives a Reason] 9observes farther that there is the same difficulty in accounting for our 10hearing [objects] {sounds} single with our two ears as in seeing objects 11single with two eyes: But here I apprehend the cases differ Sounds have 12not properly any place and there is noting by which two perfectly similar 13& synchronous sounds can be distinguished. But every visible Object hath 14a visible place, which distinguisheth it from objects that have a different 15place tho ever so like to it and seen at the same time. Now the Difficulty 16lys in this why [when the eyes have a certain Direction with regard to 17an Object] every point in one retina [has a point] corresponds in such a 18manner to the point similarly situated in the other that images falling upon 19these corresponding points are seen the Same Place |

21[2r] A Phenomenon in Optics

23Let A, B be the two Eyes, a, b two objects seen by them, in the three 24following ways successively; first by interposing the Obstacle (g) so as 25that the Eye {A} sees b, but not a & the Eye B sees a but not b Secondly 26by removing all obstacles so as each eye may see both objects at once; 27Thirdly by interposing the two Obstacles e & f so that the Eye A may see 28a & not b & the Eye B may see b & not a. find from Experience that [in 29the first Case] keeping the Eyes & Objects in the unmoved all along, in 30the first Case the Objects will appear nearest to each other in the second 31case they will appear more distant and in the third case most distant of all.

32Explained. 1 When both Objects are seen by both Eyes their apparent 33Distance will be measured by the Angle bAa or bBa. 2 When the obstacle 34g is interposed the apparent distance of the Objects will be measured by 35the Angle bDa and when the obstacles e & f are interposed the apparent 36distance of the Objects will be measured by the angle bCa.

37Cor⟨1⟩ When Ba & Ab are parallel the Objects a & b tho they are as 38far asunder as the Eyes are may be seen coinciding. This I find holds in 39Experience.

pg 331

16Cor 2 This phenomenon {in Cor 1} Confirms what I have advanced 17of the Cause of Single Vision viz. that by an Original law of our Nature 18pictures on the corresponding parts of the Retina are Seen in the same 19place. It seems also to Confute Porterfields Theory. For when Ab & Ba are 20parallel the Objects ought to be seen at an Infinite distance by his Theory 21The laws of our Nature may be supposed more constant and invariable 22than those that are founded upon habit & Custom. What Porterfield has 23laid down as a law of Nature is transgressed in innumerable instances. 24Indeed in every instance where it does not fall in with what I have laid 25down as a law of Nature. What I have laid down as a law of Nature does 26not appear to be transgressed in any one Instance.

27Is it not probable that in hares & other Animals whose eyes are averse 28the two eyes have opposite hemispheres so that two objects in the Axes 29of the two eyes appear diametrically opposite, but if we suppose the hare 30to have human eyes these two opposite objects would seem to coincide. |

[2v] 313 If Custom could bring us to see Objects single which in the same 32circumstances were seen [Single] {double}, it might be expected that we 33might by practice acquire the Habit of seeing Objects single when their 34Images do not fall upon correspondent parts of the Retina. We are every 35day of our lives receiving images in this way, when we know they belong 36to one Object those that have made innumerable Experiments to examine 37the Circumstances of double Vision, find that this double Appearance 38never vanishes by Custom.

39Prop If the Rays coming from a Point of the Object do not meet in pg 3321a point of the Retina but {on a point} either before or behind it they 2will be dissipated upon the Retina through a Space similar to the Pupil 3& therefore in the human Eye Circular, which we may call the Circle 4of Dissipation. And the Di⟨a⟩meter of the Circle of Dissipation is to the 5Diameter of the Pupil as the distance of the Focus from the Circle of 6Dissipation to its Distance from the Pupil

7Cor 1. The Circle of Dissipation is least cæteris paribus when the 8pupil is least & by admitting Rays onely through a small hole in an opaque 9plain before the pupil the Radius of Dissipation may be diminished at 10pleasure

11Cor 2 The Confusion of an object will be greater or less as the Ratio of 12the Diameter of Dissipation to the Diameter of the Image is greater or less

13Cor 3 Hence a larger object will appear less confused (cæteris 14paribus) than a small one in the Ratio of the Diameters inversely this is 15the Reason why a large print may be read distinctly at a great Variety 16of Distances. But the limits of Distinct Vision in a small print are much 17more confined. Take a Book that has three different Sizes of Print; hold 18it so near the Eye that the Smallest begins to be confused; & the middle 19& largest print will still appear distinct; bring it nearer till the Middle 20Size begin to appear confused & the largest will still appear distinct & 21the smallest very confused. We ought therefore to distinguish between 22perfect Vision and distinct Vision calling that perfect Vision where the 23Rays from one point of the Object do meet in one point of the Retina & 24calling that Distinct Vision where the Radius of Dissipation bears but a 25small proportion to the Diameter of the Image

26Prop. Let r be the Radius of the true Image of a Circular Object in the 27Eye, p the Radius of [the circle of] Dissipation. And suppose the Object to 28be bright white upon a black ground. And first let r > p I say the Image 29upon the Retina will consist of a Circle as bright as the Object whose 30Radius is r - p & annular Penumbra surrounding the Circle whose breadth 31is 2p & whose brightness gradually diminishes from the inner side till it 32evanishes qu⟨i⟩te at the outter side of the Annulus. (MS ends)

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