iy4 O" '^' Prgperttf! tf Light.— 'Colours Jram Specula^ 



In the image whers thefe meafures were taken, there were feven other elllpflc fpots, 3 

 little lefs and nearly equal ; all the others were much fmaller and more confufed. 



Obfervation 6. On viewing the furface of the fpeculum attentively in thit place whence 

 the rays formed the oblong and firft mentioned elliptic fpots, I faw a dark but very thin long 

 fcratch, and a dark dent, fimilar in (hape to the dark fpaces- on the image i the dark fpot 

 meafurcd lefs than -^\-^ oi an inch ; which mukcs its whole fur£ice to the whole polifheJ 

 furface, as i to 342251 fuppofing the former circular or nearly fo. All thefe meafures will 

 be found to agree very well, for their fmallnefs and delicacy: thus, the ratio laft mentioned 

 is nearly the fame which we obtain by comparing the image and the fpot: the like may be 

 faid of the two fpots mentioned in the table, i. e. their axes are proportional. I now could 

 produce what fpots I pleafed, by gently fcratching the fpeculum, or by making lines, dots, 

 &c. with ink, and allowing it to dry; for thefe laft formed convex fibres, which produced 

 coloured fringes as well as the concavities, agreeably to what was deduced a priori. 



Obfervation 7. The whole appearajice \yhich I have been defcribing bore fuch a clofe 

 and complete refemblance to the fringes raa<le round the (badows of bodies, that the identity 

 of the caufe in both cafes could not be doubted, ^n order, however, to fliew it ftill further, 

 I meafured the breadths of two contiguous fringes in feveral different fets : the meafure- 

 ments agreed very well, and gave the breadth of the firft fringe .0056, and of the fecond 

 .0034 ; or of the firft .0066, and of the fecond ^0034. The ratio of the breadths by the firft 

 is 28 to 17 ; by the fecond, 30 to 17; of which the medium is 29 to 17 : and this is precifely 

 the ratio of the two innermoft fringes made by a "hair, according to Sir Ifaac Newton's mea- 

 JTurement ; the firft being, according to him, ~o of an inch j the fecond, ^^ of an inch *. 

 Farther, the two innermoft rings made by plates have their diameters (not breadths) in the 

 r.atio of I fi- to 2 if, and the diflance between the middle ©f the innermoft fringes (made by 

 a hair), on either ilde the ftiadow, is to the fame diftance in the fecond fringes as 3-^ to ^j- ^ 

 therefore, the diameters of the two firft rings made by the fpecks in the fpeculum, are as 

 ^6?- to ttVt; which ratio differs exceedingly little from that of i fi to 2 |, the ratio of the 

 diameters of rings made by plates, either thofe called by Newton thick, or thofe which he 

 names thin : for fuppofe this difference nothing, %\X ^|| = i fi X -jt-Vt > and the differ- 

 ence between thefe twoproduiils (now ftated equal) is not much above ^ in reality. 



Obfervation 8. The laft thing worth mentioning in thefe phenomena was this : I viewed, 

 the fringes through a prifm, holding the refracting angle upwards, and the axis parallel to 

 that of the dark fpace ; then moving it ti'l. the objefls ceafed defcending, I faw in that 

 pofture the fringes much more diftindt and numerous; for I could now fee five with eafe, and 

 feveral more lefs diftinftly.. This led me to try more minutely the truth of the 5th propofi- 

 tion, with refpeft to the nitmber of the fringes fiirroundlng the ihadows of bodies in diredl 

 light. Having produced a bright fet of thefe by a blackened pin -^ of an inch in diameter, 

 I viewed them through a well made prifm, whof® refradling angle was only 30°, and heli 

 this angle upwarils, when the fringes were on the fide of the fhadow oppofite to me. 1 

 then moved the prifm round on its axis ; arid when it was in the pofture between the afcend 

 and defcent of the objefts, Iwas much pfeafed to fee five fringes- plainly, and a great num- 

 ber beyondj decreafing in fize and brightnefs till they became too fmall and confufed for 



, . * Of tics, book iii. obC. y -^ Book ii, parts i and 4, 



.9 fiS>»t, 



