K I \ 



R I N 



rec- 

 the 



certain ttated intervals, not being fatisfadtory to Dr. Hcrl- riflimus eft." The length of the thin wedge of air 

 chel, he inilituted a feries of experiments with a view of koned from the line of contact, to the beginning of 

 inveltigating the cattfe of thefe phenomena. Newton's lutcrpofed it rip of paper, is 5.2 inches, from which 

 hypothefia appears to him not eafily reconcileable with the calculation, it will have the above-mentioned thicknefs at 

 minutenefs and extreme velocity of the particles of which ~'.th of an inch from the contact ; and therefore at ' 

 thefe rays, according to the Newtonian theory, are com- , „ ■■. , /,, ;;, &c. we fliall have the thicknefs *of 

 poled. The detail of this celebrated altronomcr's experi- air between the mirror and irlafs eaual to , > 

 ments would far exceed our limits ; we mult therefore con- . , . . .',.., &c. of which the fame author fays that 



they give " craflitudines aeris in omnibus annulis lucidis 

 qua parte illi lucidiflimi funt." Hence it follows that, ac- 

 cording to the above hypothecs, the rings of the fecondary 

 i.r, which extended over a fpaceof .14 of an inch, mould 



tent ourfelves with referring for an account of them to 

 the Phil. Tranf. for 1807, vol. xcvii. pt. 2. and with 

 merely Rating the confequences which he has deduced from 

 them. 



It is evident, fays our author, that the phenomena of 



more than feven interruptions of fhape and colour 



concentric rings muff have an adequate caufe, either in the in the direction of the wedge of air. 

 very nature or motion of the rays of light, or in the modi- In order to afcertain whether fuch an effect had any ex. 



fications that are given to them by the two effential fur- iftence, Dr. Herfchel viewed the fecondary fet of rings 



faces that act upon them at the time of the formation of upon every part of the glafs-plate, by moving the convex 



the rings. Hence he infers, that the true caufe may be lens from one end of it gradually to the other • and his at- 



reduced to an alternative that may be determined : for if tention being particularly directed to the third/fourth and 



it can be rtiewn, that a difpofition of the rays of light to fifth rings, which were extremely diftinift, he faw them re. 



be alternately reflected and tranfmitted cannot account for tain their fhape and colour all the time without the fmalleit 



the phenomena which this hypothetic is to explain, a pro- alteration. 



pofal of accounting for them by modifications that may be 

 proved, even on the principles of fir Haac Newton, to have 

 an exigence, will be readily admitted. Accordingly Dr. 

 Herfchel offers fome arguments for removing an obitacle 

 to the inveitigation of the real caufe of the formation of 



The fame experiment was repeated with a piece of plate- 

 glafs inftead of the metalline mirror, in order to give room 

 for the fits of eafy tranfmiflion, if they cxilled, to exert 

 themfelvcs, but the refult was ftill the fame ; and the con- 

 itancy of the brightnefs and colours of the rings of the 

 the concentric rings ; for after the very plauiible fuppofition fecondary fet plainly proved, that the rays of light were 

 of the alternate fits, which agrees fo wonderfully well with not affected by the thicknefs of the plate of air through 

 a number of facts that have been related, it will hardly be which they paifed. 



attempted, if thefe fltould be fet afide, to afcribe fome Our author next proceeds to fhew, that alternate fits of 

 other inherent property to the rays of light, by which we eafy reflection and eafy tranfmiflion, if they exilt, do not 

 might account for them : and thus we fliall be at liberty to exert themfelves according to the various thickneffes' of thin 



turn our thoughts to a caufe, that may be found in the 

 modifications arifing from the action of the lurfaces, which 

 have been proved to be the only effential ones in the forma- 

 tion of rings. Our author proceeds to obferve, that con- 

 centric rings cannot be formed by an alternate reflection 

 and tranfmiflion of the rays of light ; for if we adopt one 

 of the molt fimple methods of obtaining a fet of concen 



plates of glafs. In proof of this he fcleded a well-polifhed 

 plate of coach-glafs 17 inches long, and about 9 broad. 

 Its thicknefs at one end was ,\,Vths, and at the other ,y„ths 

 of an inch ; fo that in its whole length it differed -s^th of 

 an inch in thicknefs; and it was regularly tapering' from 

 one end to the other. This plate, with a double convex 

 lens of 55 inches laid upon it, being placed upon a final] 



trie rings, which is that of laying a convex lens upon a metalline mirror, and properly expofed to the lh'ht. 1 



plain metalline mirror, we can in this cafe have no tranf- hibited the ufual two lets of rings. In the fecondary fet 



million of rays, nor confequently any alternate reflection which was the object of attention, 1 2 rings were counted, and 



and tranfmiflion of them. He further obferves, that alter- the central fpace between them was estimated to be about 



nate fits of eafy reflection and eafy tranfmiflion, if they 1 > r times as broad as the fpace occupied by the 12 rings on 



exift, do not exert themfelves according to the various thick- either fide ; fo that the whole fpace taken 'up might be 



nefl'es of thin plates of air. In the following experiment, reckoned equal to the breadth of 40 rin<r S of a mean iize - 



he placed a plain well-polifhed piece of glafs 5.6 inches for the 12 rings, as ufual, were- gradually contracted in 



long, and 2.3 thick, upon a plain metalline mirror of the breadth as they receded from the centre, ainl by a meafure 



fame length with the glafs, and in order to keep the mirror of the whole fpace thus occupied, it was found that thr 



and glafs at a diltance from each other, he laid between breadth of a ring of a mean fize was about the ao8tl 



them, at one end, a narrow ftrip of fuch paper as is com- part of an inch. 



monly put between prints. The thicknefs of that which According to fir I. Newton's calculation of the action 



was ufed in this cafe was the 640th part of an inch. Upon of the fits of eafy reflection and eafy tranfmiflion in thick 



the glafs was put a 39-inch double convex lens ; and hav- glafs-plates, an alternation from a reflecting to a tranfmitting 



ing expofed this combination to a proper light, two com- lit requires a difference of ,, , th part of an inch in 



plete fets of coloured rings were vifible in this arrange- thicknels ( Newton's Optics, p. 277.); and by calculation 



ment. The rays which convey the fecondary fet of rings this difference took place in the glafs-plate, that was ufed 



to the eye mult pafs through a thin wedge of air, and if at every 80th part of an inch of its whole length, The 



thefe rays are endowed with permanent fits of eafy reflec- 12 rings, as well as the central colour of the* fecondary 



tion and eafy tranfmiflion, or abforption, their exertion, fet, fliould confequently have been broken by the exertion 



according to fir I. Newton, mould be repeated at every dif- of the tits at every 80th part of an inrh ; and from th*- 



ferent thicknefs of the plate of air, which amounts to the fpace over which thefe rings extended, which was about i-j 



,.,; , ; d part of an inch, of which he fays, « base eft inch, it was found, that there mult have been more than 



. raflitudo aeris in primo annulo obfeuro radiis ad prrpen- 10 fuch uilerruptions or breaks in a fet of which the »o8th 



diculum incidentibus "xhibito, qua parte is annulus obfeu- part was plainly to be diltiHguiuVJ, Put when the gloM- 



