{ "id": "569583b7e975756b68bebd0770421470386744c2", "text": "III. A plain and easy Experiment to confirm Sir Isaac Newton's Doctrine of the different Refrangibility of the Rays of Light. By the same.\n\nAfter the Experimentum Crucis made by two Prisms, I shou'd not give the following Experiment, but that it is so easy to be made, that by it those who want the Apparatus (or are unwilling to be at the pains) to make the Experimentum Crucis, may at any time satisfy themselves of the Truth of the fore-mention'd Doctrine.\n\nLet the Candle \\(A\\) be set before the Bar of a Chimney Looking-Glass, such as is represented by \\(HH\\) (Fig. 25) which is a Piece of Looking-Glass Plate consisting of four Planes, seen in the Section of it \\(\\alpha f d \\beta\\), viz. \\(d \\beta\\) which is quick-silver'd behind, \\(f \\alpha\\) a Plane parallel to it, \\(f d\\) one of the Side-planes bezell'd towards \\(d \\beta\\), or inclin'd to it in an Angle of about 40 Degrees (tho' from 30 to 40 will do, but the greater the Angle the better, if it does not exceed 45°.) \\(\\alpha \\beta\\) the other Side-Plane inclin'd in the same Angle to \\(\\beta d\\).\n\nThe Rays of the Candle which come from \\(A\\) to \\(\\gamma\\) fall obliquely on the Plane \\(\\alpha \\beta\\), so that instead of going on to \\(\\alpha\\), they are by Refraction made to incline more towards the Perpendicular \\(pp\\), namely to go on in the Line \\(\\gamma c\\), and then are reflected from the Point \\(c\\) on the quick-silver'd Surface, in the Direction \\(c x\\), so as to make the Angle \\(x c d = \\gamma c \\beta\\). Now as the Rays which wou'd go to \\(x\\), if not refracted, emerge obliquely from the Plane \\(\\alpha \\beta\\), they leave the Direction \\(c x\\), and decline from the Perpendicular \\(pp\\), and, being differently refracted, open into four differently colour'd Rays; viz. \\(b R\\) a red Ray, \\(t T O\\) a Ray made\nmade up of Orange and Yellow; \\( bG \\) a Ray made up of Green and Blue or a Sea-Green, and \\( bP \\) a purple Ray.\n\nIf from the place \\( Ee \\) you look full upon the Point \\( b \\), the Spectrum or Image of the Candle at \\( b \\) will appear double, but not mix'd; that is, there will appear a Sea-green Spot and a red Spot, as it were, one upon another; but not so as to produce a mix'd or intermediate Colour. Then if the right Eye or Eye at \\( E \\) be shut, there will appear only a green Spot to the Eye at \\( e \\); if the Eye at \\( e \\) be shut, the Eye at \\( E \\) will see only a red Spot.\n\nIf you come nearer to \\( b \\), so that the Eyes at \\( e_1, e_2 \\) receive the most and the least refrangible Rays, there will be a double Spectrum, viz. a red and a purple one just touching, or upon one another: and the Phænomenon will answer as before. (Fig. 25).\n\nIf keeping both Eyes open, you direct their Axes towards \\( O \\) a Point nearer than the usual place of the compound Spectrum \\( S \\), (Fig. 26.) which Point is in a Line from the Nose \\( N \\) to the Point \\( S \\); or in other Words, if you look full at \\( O \\), or at the End of your Finger held in \\( O \\), the red and the blue (or purple Spot) will appear to be divided from each other after the manner represented at \\( pr \\) (in Fig. 27) where the Red will appear to be on the Right-hand, and the Blue on the Left.\n\nTo make plain what is meant by seeing the Spectrum and \\( r \\) whilst we look full at \\( O \\), I beg leave to explain the Distinction between looking and seeing; that I may the better shew how this Phænomenon proves that the Sensation of different Colours is caus'd by Rays differently refracted.\n\nI. Definition.\n\nThe Optic Axis is a Line which going thro' the Center of the Convexity of all the Coats and Humours of the Eye\nEye, falls upon the Middle of the Retina, as \\( \\alpha \\alpha \\) or \\( A \\alpha \\)\nFig. 28.\n\nII. Definition.\n\nTo look at any Point, is to turn both Eyes towards it in such manner, that the Optic Axes making an Angle at the said Point as \\( \\alpha \\), the Rays from \\( \\alpha \\) may have the Optic Axis for their Axis, and (by their Convergence upon the Retina after Refraction in the Eye) may paint the Image of the said Point upon the Middle of the Retina of each Eye, where the Optic Axis in each Eye falls.\n\nIII. Definition.\n\nTo see without looking, is to direct the Optic Axes to some other place than to the Point which is then seen; and in such a case, the Image of the Point seen will be projected upon a part of the Retina of each Eye, where the Optic Axis does not fall, namely either nearer to the Nose \\( N \\) as in (Fig. 26.) at the Points of the Retina mark'd \\( n \\); or farther from the Nose than the Middle of the Retina, as at \\( o \\) in Fig. 29.\n\nWhatever is seen, by being look'd at with both Eyes, always appears single, by reason of the Communication between the Middle of the Retina in one Eye, and the Middle of the Retina of the other: there being no such Communication between any other part of the Retina in one Eye, and the Correspondent part of the Retina in the other, when these correspondent parts are equally distant from the Nose.\n\nThere is indeed a Communication between the Nervous Fibres on the Right-side of the Retina of one Eye, and the nervous Fibres on the Right-side of the Retina of the other Eye, and so of those on the Left: but no single Object can be so painted in each Eye, as to have its Image on the Right or Left Part of one\none Retina that communicates with the Right or Left part of\nthe other, of the same bigness and at the same time as in the\nother; because in whatever Position the Object is, it must be\nnearer to one Eye than to the other, except it be just in a Line\nfrom the Nose betwixt the two Eyes straight forward.\n\nHence it is that if there be two Candles set before any\none, the First at the Distance of one Foot, and the Second\nat the Distance of two Feet, from the Eyes; he that looks\nat the second Candle at B will see it single, but see the first\nCandle or the Candle A double; one Appearance being\nin the Line AD γ, the other in o AE, because it paints it\nself upon oo in the Retina of each Eye, which Points are\nnot the middle Points, but farther from the Nose than the\nmiddles mm.\n\nSo if B be the first Candle, and C the second, he that\nlooks at B will see C double, because it is painted in the\nRetina at the Points nn nearer the Nose than mm; and so\nwill appear to be in the same Position as pr in Fig. 27.\n\nIf γ ρ be two Candles so disposed, Fig. 30. that by the\nInterposition of a perforated Board FF, γ can paint it self\nonly in the Eye R, and ρ in the Eye L. Upon making the\nOptic Axes meet at B and to tend towards ρ and γ, ρ and γ\nwill each paint an Image on the Middle of the Retina of\neach Eye, by crossing their Rays at B: and thus the two\nCandles will appear to be but One, or rather to be in one\nPlace, upon the account of the Communication of the\nMiddle of each Retina. But if instead of the Candles,\nρ be a piece of red Silk, and γ a piece of green Silk, the\nsame Position of the Eyes will make an Image at B, ap-\npearing like a red and green Spot together without a\nMixture of the Colours. If ρ be a red hot Iron, and γ a\nCandle of Sulphur, the Phænomenon will be more distinct.\nIf the Optic Axes be turn'd directly towards γ and ρ, as if\nthere was no Board FF in the way, there will appear two\n\nA a a a 2 Holes\nHoles in the Board, the one having the red hot Iron in it, the other the Candle.\n\nNow if, of the refracted Rays of the Candle in the first Case (Fig. 25.) those which diverge from each other, so as to fall into each Eye, cause the same Sensations respectively, as the Rays which come from a red hot Iron and those which come from a blue Candle; it is evident that the Candle in the first case affords red-making and blue-making Rays after Refraction, and that those Rays are differently refrangible; the red \\( b R \\) (Fig. 25.) the least refrangible, as declining less from the Perpendicular \\( w w \\); and the Purple as \\( b P \\) declining most from the said Perpendicular.\n\nThe same will (ceteris paribus) be found true in the intermediate Rays; and to be certain that the Experiment is as I have related it, the Planes \\( a f \\) and \\( f d \\) of the Barr may be covered with Paper.\n\nIV. An Account of what appear'd on opening the big-belly'd Woman near Haman in Shropshire, who was suppos'd to have continued many Years with Child. Communicated by Dr. Hollings M.D. from Shrewsbury.\n\nA Marry'd Woman, near Haman about Three Miles from Shrewsbury, about the 40th Year of her Age, had then first the common Reasons to believe she was with Child: at the Time of her Account she had the usual Signs of Labour, and a good Midwife, tho' mistaken, assur'd her it was so, but that the Child was so big she could not be delivered without bringing it away in pieces. She not submitting to that, her Pains were soon off, and she continued without any other Disorders Nine Months longer, when", "source": "olmocr", "added": "2026-01-12", "created": "2026-01-12", "metadata": { "Source-File": "/home/jic823/projects/def-jic823/royalsociety/pdfs/103083.pdf", "olmocr-version": "0.3.4", "pdf-total-pages": 8, "total-input-tokens": 12728, "total-output-tokens": 2557, "total-fallback-pages": 0 }, "attributes": { "pdf_page_numbers": [ [ 0, 0, 1 ], [ 0, 0, 2 ], [ 0, 0, 3 ], [ 0, 1792, 4 ], [ 1792, 3598, 5 ], [ 3598, 5274, 6 ], [ 5274, 7198, 7 ], [ 7198, 8782, 8 ] ], "primary_language": [ "en", "None", "None", "en", "en", "en", "en", "en" ], "is_rotation_valid": [ true, true, true, true, true, true, true, true ], "rotation_correction": [ 0, 0, 0, 0, 0, 0, 0, 0 ], "is_table": [ false, false, false, false, false, false, false, false ], "is_diagram": [ false, true, true, false, false, false, false, false ] }, "jstor_metadata": { "identifier": "jstor-103083", "title": "A Plain and Easy Experiment to Confirm Sir Isaac Newton's Doctrine of the Different Refrangibility of the Rays of Light. By the Same", "authors": "J. T. Desaguliers", "year": 1714, "volume": "29", "journal": "Philosophical Transactions (1683-1775)", "page_count": 8, "jstor_url": "https://www.jstor.org/stable/103083" } }