Opticks (1721, 3rd Edition) — part 14 of 18

Nor is it material whether the Hair be en- compafTed with Air, or with any other pellucid Subitance. For I wetted a poU(h'd Plate of Glafs, and laid the Hair in the Water upon the Glafs, and then laying another polifh'd Plate of Glafs upon it, fo that the Water might fill up the fpace between the GlafTes , I held them in the aforefaid beam of Light, fo that the Light might pafs through them perpendicularly, an4 the Shadow of the Hair was at the fame di- ftances as big as before. The Shadows of Scratches made in polifli'd Plates of Glafs were alfo much broader than they ought to be, and the Veins in polifli'd Plates of Glafs did alfo caft the hke broad Shadows. And therefore the great breadth of thefe Shadows proceeds from fome other caufe than the Refradion of the Air.

Let the Circle X [in Fig. i.] reprefent the middle of the Hair; A D G, B E H, C' F I, three Rays pafTing by one fide of the Hair at feveral diilances ; KNQ, LOR, MPS, three other Rays pading by the other fide of the Hair at the like diftances ; D, E, F, and N, O, P, the -places where the Rays are bent in their paf- fage by the Hair ; G, H, I and Q, R, S, the places where the Rays fall on a Paper G Q ; I S the breadth of the Shadow of the Hair call: on the Paper, and T I, V S, two Rays pafling to the Points I and S without bending when the Hair is taken away. And it's manifefl that all the Light between thefe two Rays T I and V S is bent in pafling by the Hair, and turned afide from the Shadow I S, becaufe if any part

of

[ 295 ]

of this Light were not bent it would fall on the Paper within the Shadow, and there illuminate the Paper, contrary to experience. And be- caufe when the Paper is at a great dillance from the Hair, the Shadow is broad, and therefore the Rays TI and V S are at a great dillance from one another it follows that the Hair afts upon the Rays of Light at a good diftance in their palling by it. But the aftion is llrongeit on the Rays which pafs by at leall diftances, and grows weaker and weaker accordingly as the Rays pafs by at diltances greater and great- er, as is reprefented in the Scheme : For thence it comes to pafs, that the Shadow of the Hair is much broader in proportion to the dillance of the Paper from the Hair, when the Paper is nearer the Hair, than when it is at a great di- ftance from it.

Obf. 1. The Shadows of all Bodies (Metals, Stones, Glafs, Wood, Horn, Ice, ^c.J in this Light were border'd with three parallel Fringes or Bands of colour'd Light, whereof that which was contiguous to the Shadow was broadeil and molt luminous, and that which was remo- teft from it was narroweft, and fo faint, as not eafily to be vifible. It was difficult to diltinguiOi the Colours unlefs when the Light fell very ob- liquely upon a fmooth Paper, or fome other fmooth white Body, fo as to make them appear much broader than they would otherwile do. And then the Colours were plainly vifible in this Order : The firlt or innermoit Fringe was violet and deep blue next the Shadow, and then light blue, green and yellow in the middle, and

U 4 red

[ 25^ ]

red without. The fecond Fringe was almofl contiguous to the firft, and the third to the fe- cond, and both were blue within and yellow and red without, but their Colours were very faint, efpecially thofe of the third. The Co- lours therefore proceeded in this order from the Shadow; violet, indigo, pale blue, green yellow, red ; blue, yellow, red ; pale blue, pale yellow and red. The Shadows made by Scratches and Bubbles in poUfli'd Plates of Glafs were border'd with the like Fringes of colour'd Light. And if Plates of Looking-glafs floop'd oif near the edges with a Diamond- cut, be held in the fame beam of Light, the Light which paiTes through the parallel Planes of the Glafs will be border'd with the like Fringes of Colours where thofe Planes meet with the Diamond-cut, and by this means there will fometimes appear four or five Fringes of Colours. Let A B, C D [in Fig. 2.] reprefent the parallel Planes of a Look- ing glafs, and B D the Plane of the Diamond- cut, making at B a very obtufe Angle with the Plane A B. And let all the Light between the Rays E NI and FB M pafb diredly througli the parallel Planes of the Glafs, and fall upon the Paper between I and M, and all the Light be- tween the Rays G O and H D be refrafied by ^ the oblique Plane of the Diamond-cut BD, and fall upon the Paper between K and L; and the Light which pa&s diredtly through the parallel Planes of the Glafs, and falls upon the Paper between I and M, will be border'd with three pr more Fringes at M.

So

--*•— 'S;; *■», .■

[ 297 ]

So by looking on the Sun through a Feather or black Riband held clofe to the Eye, feveral Rain-bows will appear; the Shadows which the Fibres or Threds call on the Tunica Retina be- ing border'd with the like Fringes of Colours.

Obf.i. When the Hair was twelve Feet di- flant from this Hole, and its Shadow fell obli- quely upon a flat white Scale of Inches and parts of an Inch placed half a Foot beyond it, and alfo when the Shadow fell perpendicularly upon the fame Scale placed nine Feet beyond it ; I meafured the breadth of the Shadow and Fringes as accurately I could, and found them in parts of an Inch as follows.

The

[ 298 ]

.

At the d'lflance of

half a Foot

mne Feet

The breadth of the Shadow

X

■ST

The breadth between the middles of thebrigheft Light of the innermoft Fringes on either fide the Shadow

TVor^

7

The breadth between the Middies of the brighteft Light of the middle- moft Fringes on either fide the Sha- dow

I

^3t

'4 ~7

The breadth between theMiddles of the brighteft Light of the outmoft Fringes on either fide the Shadow

I I

tV

The diftance between the Middles of the brighteft Light of the firft and fecond Fringes

TTT

The diftance between the Middles of the brighteft Light of the fe- cond and third Fringes

1

The breadth of the luminous parr (green, white, yellow and red) of the firft Fringe

1

1 t

The breadth of the darker Space be- ° tween the firft and fecond Fringes

1

I

■?T

I

The breadth of the luminous parr of the fecond Fringe

T"?"?

The breadth of the darker Space be tween the fecond and third Fringes

1

Thef^

[ 299 ]

Thefe Meafures I took by letting the Shadow of the Hair at half a Foot diltance fall fo ob- liquely on the Scale as to appear twelve times broader than when it fell perpendicularly on it at the fame dillance, and fetting down in this Table the twelfth part of the Meafures I then took.

Obf 4. When the Shadow and Fringes were cad obliquely upon a fmooth white Body, and that Body was removed farther and farther from the Hair, the firit Fringe began to appear and look brighter than the rell of the Light at the diitance of lefs than a quarter of an Inch from the Hair, and the dark Line or Shadow between that and thefecond Fringe began to appear at a lefs diftance from the Hair than that of the third part of an Inch. The fecond Fringe began to appear at a diitance from the Hair of lefs than half an Inch, and the Shadow between that and the third Fringe at a diftance lefs thaa an Inch, and the third Fringe at a diitance lefs than three Inches. At greater diitances they became much more fenfible , but kept very nearly the fame proportion of their breadths and intervals which they had at their firit ap- pearing. For the diitance between the middle of the firit and the middle of the fecond Fringe, was to the diitance between the middle of the fecond and middle of thp third Fringe, as three to two, or ten to feven. And the lalt of thefe two diitances was equal to the breadth of the bright Light or luminous part of the firit Fringe. And this breadth was to the breadth of the bright Light of the fecond Fringe ^s feven to

four,

[ 300 ]

four, and to the dark Interval of the firft and fecond Fringe as three to two, and lo the Hke dark Interval between the fecond and third as two to one. For the breadths of the Fringes feem'd to be in the progreffion of the Numbers I, -v/-!-, v^l, and their Intervals to be in the fame progrelFion with them ; that is, the Frin- ges and their Intervals together to be in the continual progreffion of the Numbers i,v^4, v^-f, ■v^^j ^Tj or thereabouts. And thefe Proporti- ons held the fame very nearly at all diftances from the Hair ; the dark Intervals of the Fringes being as broad in proportion to the breadth of the Fringes at their hrit' appearance as after- wards at great diftances from the Hair, though not fo dark and didinft.

Obf. f. The Sun fhining into my darken'd Chamber through a Hole a quarter of an Inch broad ; T placed at the difhnce of two or three Feet from the Hole a Sheet of Paflboard, which was black'd all over on both fides, and in the middle of it had a Hole about three quarters of an Inch fquare for the Light to pafs through. And behind the Hole I falten'd to the Pali- board with Pitch the Blade of a Iharp Knife, to intercept fome part of the Light which palfed through the Hole. The Planes of the Paft- board and Blade of the Knife were parallel to one another, and perpendicular to the Rays. And when they were fo placed that none of the Sun's Light fell on the Paitboard, but all of it paffed through the Hole to the Knife, and there part of it fell upon the Blade of the Knife> and part of it paffed by its edge : I let this part of

the

[ 30I ]

the Light which paiTed by, fall on a white Pa* per two or three Feet beyond the Knife, and there faw two Itreams of faint Light fhoot out both ways from the beam of Light into the Iha- dow like the Tails of Comets. But becaufe the Sun's direft Light by its brightnefs upon the Paper obfcured thefe faint ftreams, fo that* I could fcarce fee them, I made a little hole in the midlt of the Paper fo,r that Light to pafs through and fall on a black Cloth behind it ; and then I faw the two Itreams plainly. They were like one another, and pretty nearly equal in length and breadth, and quantity of Light. Their Light at that end next the Sun's dired Light was pretty llrong for the fpace of about a quarter of an Inch, or half an Inch, and in all its progrefs from that diredl Light decreafed gradually till it became infenfible. The whole length of either of thefe Itreams meafured up- on the Paper at the didance of three Feet from the Knife was about fix or eight Inches.; fo that it fubtended an Angle at the edge of the Knife of about I o or IX, or at molt 14 Degrees. Yet fometimes I thought I faw it ilioot three or four Degrees farther, but with a Light fo very faint that I could fcarce perceive it, and fufpeded it might (in fome meafure at leaftj arife from feme other caufe than the two lireams did. For placing my Eye in that Light beyond the end of that Itream which was behind the Knife, and looking towards the Knife, I could fee a hne of Light upon its edge, and that not only when my Eye was in the Hne of the Streams,, but al- fo when it was without that line either towards ^ the

C 302 ]

the point of the Knife, or towards the handle. This Une of Light appeared contiguous to the edge of the Knife, and was narrower than the Light of the innermofl Fringe, and narroweft when my Eye was farthelt from the direft Light, and therefore feem*d to pafs between the Light of that Fringe and the edge of the Knife, and that which palfed neareft the edge to the moft bent, though not all of it.

Obf, 6. I placed another Knife by this, fo that their edges might be parallel and look to- wards one another, and that the beam of Light might fall upon both the Knives, and fome part of it pafs between their edges. And when the diflance of their edges was about the 400th part of an Inch the fh-eam parted in the mid- dle, and left a Shadow between the two parts. This Shadow was fo black and dark that all the Light which pafTed between the Knives feem*d to be bent, and turn'd afide to the one hand or to the other. And as the Knives ilill ap- proached one another the Shadow grew broad- er, and the Streams fliorter at their inward' ends which were next the Shadow, until upon the contaft of the Knives the whole Light va- nilh'd leaving its place to the Shadow.

And hence I gather that the Light which is lead bent, and goes to the inward ends of the Streams, paiTes by the edges of the Knives at the greateil diftance, and this diilance when the Shadow begins to appear between the Streams is about the 800 part of an Inch. And the Light which pafTes by the edges of the Knives at diftances Itill lefs and lefs is more and

more

[ 303 ]

more bent, and goes to thofe parts of the Streams which are farther and farther from the direft Light, becaufe when the Knives ap- proach one another till they touch, thofe parts of the Streams vanilh lalt which are fartheft froin the dired Light.

Obfj. In the fifth Obfervation the Fringes did hot appear, but by reafon of the breadth of the hole in the Window became fo broad as to run into one another, and by joining, to make one continued Light in the beginning of the Streams. But in the fixth, as the Knives approached one another, a little before the Shadow appear'd between the two flreams, the Fringes began to appear on the inner ends of the Streams on either fide of the dired Light, three on one fide made by the edge of one Knife, and three on the other fide made by the edge of the o- ther Knife. They were diltinfteit when the Knives were placed at the greateft diflance from the hole in the Window, and flill became more diflinft by making the hole lefs, infomuch that I could fometimes fee a faint Lineament of a fourth Fringe beyond the three above men- tioned. And as the Knives continually ap- proach'd one another, the Fringes grew di- ftinfter and larger until they vanifla'd. The outmoft Fringe vanifh'd firfl, and the middle- moil next, and the innermoli: laft. And after they were all vanifli'd, and the hne of Light which was in the middle between them was grown very broad, enlarging it felf on both fides into the Streams of Light defcribed in the fifth Obfervation, the above mention'd Shadow be- gan

C 304 ]

^an to appear in the middle of this line, and divide it along the middle into two lines of Light, and increafed until the whole Light va- nifh'd. This enlargement of the Fringes was fo great that the Rays which go to the inner- moit Fringe feem'd to be bent above twenty times more when this Fringe was ready to va- nifh, than when one of the Knives was taken away.

And from this and the former Obfervation compared, I gather, that the Light of the firft- Fringe pafled by the edge of the Knife at a di- fiance greater than the Sooth part of an Inch, and the Light of the fecond Fringe pafTed by the edge of the Knife at a greater diitance than the Li^hc of the firft Fringe did, and that of the thnd at a greater diftance than that of the fecond, and that of the Streams of Light de- fcribed in the fifth and fixth Obfervations paf- fed by the edges of the Knives at lefs dillances than that of any of the Fringes.

Obf. 8. I caufed the edges of two Knives to be ground truly Itrait, and pricking their points into a Board fo that their edges might look towards one another, and meeting near their points contain a rectilinear Angle, 1 fallen 'd their Handles together with Pitch to make this Angle invariable. The diflance of the edges of the Knives from one another at the diflance of four Inches from the Angular Point, where the edges of the Knives met, was the eighth part of an Inch, and therefore the Angle contain'd by the edges, was about i Degree s4- The Knives thus fix'd together I placed in a beam

©f

[ 305 3

of the Sun's Light, let into my darken'd Cham- ber through a hole the 4xd part of an Inch wide at the dittance of lo or 15- i'eet from the hole, and let the Light which palled between their edges fall very obliquely upon a fmooth white Ruler at the diilance of half an Inch, or an Inch from the Knives, and there faw the Fringes made by the two edges of the Knives run along the edges of the Shadows of the Knives in lines parallel to thofe edges without growing tenfibly broader, till they met in An- gles equal to the Angle contained by the edges of the Knives, and where they met and joined they ended without. crolTing one another. But if the Ruler was held at a much greater di- ftance from the Knives, the Fringes where they were farther from the place of their meeting, were a httle narrower, and became fomething broader and broader as they approach'd nearer and nearer- to one another, and after they met they crofs'd one another, and then became much broader than before.

Whence I gather that the diitances at which the Fringes pafs by the Knives are not increa- fed nor alter'd by the approach of the Knives, but the Angles in which the Rays are there bent are much increafed by that approach ; and that the Knife which is neareft any Ray determines which way the Ray fhall be bent, and the other Knife increafes the bent.

Obf.^. When the Rays fell very obliquely upon the Ruler at the diilance of the third part of an Inch from the Knives, the dark line be- tween the firit and fecond Fringe of the Sha-

X dow

[ 30^ ]

dow of one Knife, and the dark line between the firft and fecond Fringe of the Shadow of the other Knife met with one another, at the diilance of the fifth part of an Inch from the end of the Light which pafTed between the Knives at the concourfe of their edges. And therefore the diilance of the edges of the Knives at the meeting of thefe dark hnes was the i6oth part of an Inch. For as four Inches to the eighth part of an inch, fo is any length of the edges of the Knives meafured from the point of their concourfe to the diflance of the edges* of the Knives at the end of that length, and fo is the fifth part of an Inch to the i6oth part. So then the dark lines above mention'd meet in the middle of the Light which pafTes be- tween the Knives where they are diftant the i6oth part of an Inch, and the one half of that Light pafTes by the edge of one Knife at a di- ilance not greater than the 310th part of an Inch, and falling upon the Paper makes the Fringes of the Shadow of that Knife, and the other half palTes by the edge of the other Knife, at a diflance not greater than the 3 loth part of an Inch, and falling upon the Paper makes the Fringes of the Shadow of the other Knife. But if the Paper be held at a diflance from the Knives greater than the third part of an Inch, the dark lines above mention'd meet at a great- er diflance than the fifth part of an Inch from the end of the Light which palTed between the Knives at the concourfe of their edges ; and therefore the Light which falls upon the Paper where thofe dark lines meet palTes between the

8 • Knives

i

[ 307 ]

Knives where their edges are diftant above the 1 6oth part of an Inch.

For at another time when the two Knives were dillant eight Feet and five Inches from the little hole in the Window, made with a fmall Pin as above, the Light which fell upon the Paper where the aforefaid dark lines met, pafTed between the Knives, where the diilance between their edges was as in the following Table, when the diilance of the Paper from the Knives was alfo as follows.

T)ljiances ofthe^a^er. T)'tfiances between the

from the Knives in Inches.

edges of the Knives in milleJimaL farts oj an Inch.

And hence I gather that the Light which makes the Fringes upon the Paper is nor the fame Light at all diilances of che Paper" from the Knives, but when the -Paper is held near the Krives , the Fring;es are made bv Liirlit which pailes by the edges of the Knives ar a iefs didance, and is more bent than when rhe Paper is held ar a greater diilance from the Knives. _ '.

X 2t G^C

C 308 ]

Obf. 10. When the Fringes of the Shadows of the Knives fell perpendicularly upon a Paper at a great diftance from the Knives, they were m the form of Hyperbolas, and their Dimen- fion's were as follows. Let C A, C B [in Fig. 3.] reprefent lines drawn upon the paper parallel to the edges of the Knives, and between which all the Light would fall, if it paiTed between the edges of the Knives without inflexion; DE a right line drawn through C making the An- gles A CD, BCE, equal to 'one another, and terminating all the Light which falls upon the Paper from the point where the edges of the Knives meet; e i s, f k t, and g I v^ three hy- perbolical lines reprefenting the Terminus of the Shadow of one of the Knives, the dark line between the firll and fecond Fringes of that Shadow, and the dark line between the fecond and third Fringes of the fame Shadow ; x tp^y k q and z, Ir^ three other hyperbolical lines repre- fenting the Terminus of the Shadow of the other Knife, the dark line between the firft and fecond Fringes of that Shadow, and the dark line be- tween the fecond and third Fringes of the fame Shadow. And conceive that thefe three Hyper- bolas are like and equal to the former three, and crofs them in the points i, k and /, and that the Shadows of the Knives are terminated and diftin- guiih'd from the firil luminous Fringes by the lines e t s and x i/, until the meeting and crof- ling of the Fringes, and then thofe Hnes crofs the Fringes in the form of dark lines, termina- ting the firfl luminous Fringes within fide, and diiUnguifning them from another Light which

begins

[ 309 ]

begins to appear at /, and illuminates all the triangular fpace i/ DE j comprehended by thefe dark hues, and the right line D E. Of thefe HyperboJas one Afympote is the line D E, and the- other Afymptotes are parallel to the lines C A and C B. Let r v reprefent a line drawn any where upon the Paper parallel to the Afym- ptote DE, and let this hne crofs the right lines A C in ;^ and B C in //, and the fix dark hy- perbolical lines in/, q^r -^ s^t^v\ and by mea- luring the diitances p s^ q t\ r x', and thenoe colleding the lengths of the Ordinates ^//, fiq^ nrovms, m t^ mv, and doing this at feveral diflances of the line r v from the Afymptote D D, you may find as many points of thefe Hy- perbolas as you pleafe, and thereby know that thefe curve lines are Hyperbolas differing little from the conical Hyperbola. And by meafur- ing the lines C /, C k, C /, you may find other points of thefe Curves.

For inltance, when the Knives were diflant from the hole in the Window ten Feet, and the Paper from the Knives nine Feet^ and the An- gle contained by the edges of the Knives to which the Angle A C B is equal, was fubtend- ed' by a Chord which was to the Radius as i to 32, and the diilance of the line r v from the Afymptote DE was half an Inch: I mealured the lines p s, q t, r v^ and found them o'af , o'65', 0*98 Inches refpedively, and by adding to their halfs the line \ m n (which here was the 128th part of an Inch, or o'ooyS Inches) the Sums;//, 71 q^ nr, were o'lSiS, o'33i8, oac^j^ •inches. I meafured- alfo the .diilances of the

X 3 brighteil

[ 310 ]

brighteft parts of the Fringes which run be- tween/ q and s t, q r and 1 1;, and next beyond r and i;, and found them o'f? o'8, and I'l 7 Inches. Obf. II. The Sun fhining into my darkened Room through a fmall round hole made in a Plate of Lead with a flender Pin as above ; I placed at the hole a Prifm to refraft the Light, and form on the oppofite Wall the Spectrum of Colours, defcribed in the third Experiment of the firft Book. And then I found that the Shadows of all Bodies held in the coloured Light between the Prifm and the Wall, were bordered with Fringes of the Colour of that Light in which they were held. In the full red Light they were totally red without any fenfi- ble blue or violet, and in the deep blue Light they were totally blue without any fenlible red or yellow ; and fo in the green Light they were totally green, excepting a little yellow and blue, which were mix'd in the green Light of. the Prifm. And comparing the Fringes made in the feveral colour'd Lights, I found that thofe made in the red Light w"here largeil, thofe made in the violet were leail, and thofe made in the green were of a middle bignefs. Ifor the Fringes with which the Shadow of a Man's Hair were border'd, being raeaiur'd crofs the Shadow at the diilance of fix Inches from the Hair; the diilance between the middle and molt luminous part of the firft or innermofl Fringe on one fide of the Shadow, and that of the like Fringe on the other fide of the Shadow was in

the full red Light ~ of an Inch, and in the full

violet

C 3" ] .

violet Vr- And the like diftance between the middle and molt luminous parts of the fecond Fringes on either fide the Shadow was in the full red Light V^, and in the violet vV of an Inch. And thefe diftances of the Fringes. held the fame proportion at all diilances from the Hair without any fenfible variation.

So then the Rays which made thefe Fringes in the red Light palled, by the Hair at a greater diftance than thofe did which made the Hke Fringes in the violet ; and therefore the Hair in caufmg thefe Fringes a6ted alike upon the red Light or leall refrangible Rays at a greater dilbnce, and upon the violet or molt refrangi- ble Rays at a lels diftance, and by thofe actions difpoled the red Light into larger Fringes, and the violet into fmaller, and the Lights of inter- mediate Colours into Fringes of intermediate bignelTes without changing the Colour of any fort of Light.

When therefore the Hair in the firft and fe- cond of thefe Obfervations was held in the white beam of the Sun's Light, and calt a Sha- dow which was border'd with three Fringes of colour'd Light, thofe Colours arofe not from any new modifications imprefs'd upon the Rays of Light by the Hair, but only from the vari- ous inflexions whereby the feveral forts of Ray's were feparated from one another, which before feparation by the mixture of all their Colours, compofed the white beam of the Sun's Light, but whenever feparated compofe Lights of the feveral Colours which they are originally difpo- fed to exhibit. In this iithObfervation, where

X 4 the

[ 312 ]

the Colours are feparated before the Light paf- les by the Plair, the lealt refrangible Rays, which when feparated from the relt make red, were infleded at a .greater diilance from the Hair, fo as to make three red Fringes at a greater di- llance from the middle of the Shadow of the Hair ; and the moit refrangible Rays which when feparated make violet, were infleded at a lefs diilance from th^ Hair, fo as to m.ake three violet Fringes at a lefs diilance from the middle of the Shadow of the Hair. And other Rays of intermediate degrees of Refrangibility were inflected at intermediate diltances from the Hair, fo as to make Fringes of intermediate Colours at intermediate dillances from the mid- dle of the Shadow of the Hair. And in the fecond Obfervation, where all the Colours are mix d on the white Light which paffes by the Hair, thefe Colours are feparated by the vari- ous inflexions of the Rays, and the Fringes which they make appear all together, and the innermoit Fringes being contiguous make one broad Fringe compofed of all the Colours in due order, the violet lying on the infide of the Fringe next the Shadow, the red on the out- fide fartheil: from the Shadow, and the blue, green and yellow, in the middle. And, in like manner, the middlemolt Fringes of all the Co- lours lying in order, and being contiguous, make another broad F'ringe compofed of all the Colours ; and the outmoft Fringes of all the Colours lying in order, and being contiguous, make a third broad Fringe compofed of all the Colours. Thefe are the three Fringes of co-

lour'd

[ 313 ]

lour'd Light with which the Shadows of all Bo* dies are border'd in the fecond Obfervation.

When* I made the foregoing Obfervations, I defign'd to repeat moft of xhem with more care and exacSnefs, and to make fome new ones for determining the manner how the Rays of Light are bent in their palTage by Bodies for making the Fringes of Colours with the dark lines be- tween them. But I was then interrupted, and cannot now think of taking thefe things into farther confideration. And iince I have not fi- nifh'd this part of my Defign, I fliall conclude, with propofmg only fome Queries in order to a farther fearch to be made by others.

^tery i. Do not Bodies a6l upon Light at a dittance, and by their a6Hon bend its Rays, and is not this adion (caterts fartbus) Itrongeft at the lead diltance ?

^. 1. Do not the Rays which differ in Re- frangibility differ alfo in Flexibility, and are they not by tl"keir different Inflexions feparated from one another, fo as after feparation to make' the Colours in the three Fringes above defcri- bed ? And after what manner are they infleded to make thofe Fringes ?

^i. 3. Are not the Rays of Light in pafling by the edges and fides of Bodies, bent feveral times backwards and forwards, with a motion like that of an Eel ? And do not the three Frin- ges of colour'd Light above mention'd, arife from three fuch bendings,?

^//. 4. Do not the Rays of Light which fall upon Bodies, and are refleded or refrackd, be- gin

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gin to bend before they arrive at the Bodies; and are they not reflected*, refradted and in- flefted by one and the fame Principle, admg varioufly in various Qircumftances ?

^^. S' Do not Bodies and Light ad mutually upon one another, that is to fay, Bodies upon Light in emitting, refleding, refrading and in- ileding it, and Light upon Bodies for heating them, and putting their parts into a vibrating motion wherein heat confiits ?

^. 6, Do not black Bodies conceive heat more eafily from Light than thofe of other Co- lours do by reafon that the Light falling on them is not refleded outwards, but enters the Bodies and is often refleded and refraded within them, until it be ilifled and loft ?

^. 7. Is not the ftrength and vigour of the adion between Light and fulphureous Bodies obferved above, one reafon why fulphureous Bodies take fire' more readily, and burn more vehemently, than other Bodies do ?

^. 8. Do not all fix'd Bodies when heated beyond a certain degree, emit Light and fliine, and is not this EmilTion perform'd by the vi- brating Motions of their parts ? And do not all Bodies which abound with terreftrial parts, and efpecialiy with fulphureous ones, emit Light as often as thofe parts are fufficiently agitated ; whether that agitation be made by Heat, or by Fridion, or Percuflion, or Putrefadion, or by any vital Motion, or any other Caufe ? As for inftance ; Sea Water in a raging Storm ; Quick- filver agitated in vacuo ; the Back of a Cat, or Neck of a Horfe obliquely ftruck or rubbed in

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a dark place; Wood, Fleili and Fifii while they putrefy; Vapours ariling from putrefy 'd Wa- ters, uiually call'd Ignes Fatut ; Stacks of moid Hay or Corn growing hot by fermentation; Glow-worms and the Eyes of fome Animals by vital Motions ; the vulgar Thofphorus agitated by the attrition of any Body, or by the acid Particles of the Air; Ambar and fome Dia- monds by ilriking, prefling or rubbing them; Scrapings of Steel {Iruck off with a Fhnt ; Iron hammer'd very nimbly till it become fo hot as to kindle Sulphur thrown upon it ; the Axle- trees of Chariots taking fire by the rapid rota- tion of the Wheels; and fome Liquors mix'd with one another whofe Particles come toge- ther with an Impetus, ?.s Oil of Vitriol diitilled from ^irs weight of Niere, 'and then mix'd with twice.* its weight of Oil of Annifeeds. So alfo a Globe ofGlafs about 8 or lo Inches in diameter, being put into a Frame where it may be fwift- ly turn'd round its Axis, will in turning ihine where it rubs againft the palm of ones Hand apply 'd to it : And if at the fame time a piece of white Paper or a white Cloth, or the end of ones Finger be held at the diitance of about a quarter of an Inch or half an Inch from that part of the Glafs where it is m.oll in motion, the eledtick Vapour which is excited by the friction of the Glafs againlt the Hand, will by dafhing againit the white Paper, Cloth or Fin- ger , be put into fuch an agitation as to emit Light, and make the white Paper, Cloth or Fin- ger, appear lucid Hke a Glow-worm; jmd in ruiliing out of the Glafs 'will fometimes pufii

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againfl the Finger Co as to be felt. And the fame things have been found by^ rubbing- a long and large Cylinder of Glafs or Ambar with a Pa- per held in ones hand, and continuing the fri- iSion till the Glafs grew warm.

^//. 9. Is not Fire a Body heated fo hot as to emit Light copioufly ? For what elfe is a red hot h'on than Fire ? And what elfe is a burning Coal than red hot Wood ?