OF THE PRODUCTION OF COLOURS. 131 



fibre of wool, on the contrary, they became very large and 



confpicuous: and, with a tingle filk- worm's thread, their 



magnitude was To much increafed, that two or three of them 



feemed to occupy the whole field of view; They appeared to 



extend on each fide of the candle, in the fame order as the 



colours of thin plates, feen by tranfmitted light. It occurred Remarks and 



tome, that their caufe muft be fought in the interference of ,nfe ^ ences » A 

 ,. ,. , „ ~ ? r, .1 portion of light 



two portions of light, one reflected upon the fibre, the other re fl e a e d from 



bending round its oppofite fide, and at laft coinciding nearly one fide nearly 

 in direction with the former portion ; that accordingly as both direaion w i t h 

 portions deviated more from a rectilinear direction, the differ- another portion 

 ence of the length of their paths would become gradually ^HengthT* 

 greater and greater, and would confequently produce the differ and pro-5 

 appearances of colour ufual in fuch cafes ; that, fuppofing duce co,our » 

 them to be inflected at right angles, the difference would 

 amount nearjy to the diameter of the fibre, and that this dif- 

 ference muft confequently be fmaller as the fibre became 

 fmaller; and, the number of fringes in a right angle be<- 

 coming fmaller, that their angular diftances would confequently 

 become greater, and the whole appearance would be dilated. 

 It was eafy to calculate, that for the light leaft inflected the 

 difference of the paths would be to the diameter of the fibre, 

 very nearly as the deviation of the ray, at any point, from the 

 rectilinear direction, to its diftance from the fibre. 



I therefore made a rectangular hole in a card, and bent its precife repeti- 

 ends fo as to fupport a hair parallel to the fides of the hole ; t5on of tne «- 

 then, upon applying the eye near the hole, the hair of courfe 

 appeared dilated by indiftinct vifion into a furface, of which 

 the breadth was determined by the diftance of the hair and the 

 magnitude of the hole, independently of the temporary aperture 

 of the pupil. When the hair approached fo near to the direc- 

 tion of the margin of a candle that the inflected light was 

 fufficiently copious to produce a fenfible effect, the fringes 

 began to appear ; and it was eafy to eltimate the proportion 

 of their breadth to the apparent breadth of the hair, acrofs 

 the image of which they extended. I found that fix of the 

 brightest red fringes, nearly at equal diftances, occupied the 

 whole of that image. The breadth of the aperture was 

 ?W3> ^nd its diitance from the hair ^ of an inch: the 

 diameter of the hair was lefs than 3-i^ °f an inc ^ 5 as nearly 

 as I could afcertain, it was -5—5. Hence we have -^^ for 



th« 



