634 



ACCOUNT OF SOME CASES 



I employed a hmse liair ; but they were then 

 no longer visible. With a fibre of- wool, 

 on the contrary, they became very large 

 and conspicuous: and, with a single silk- 

 worni's thread, their magnitude was so much 

 increased, that two or three of them seemed 

 to occupy the whole field of view. They ap- 

 peared to extend on each side of the candle, 

 inthe same order as the colours of thin plates, 

 seen by transmitted light. It occurred to me 

 that their cause must be sought in the inter- 

 ference of two portions of light, one reflected 

 from the fibre, the other bending round its 

 opposite side, and at last coinciding nearly 

 in direction with the former portion; that, 

 accordingly as both portions deviated more 

 from a rectilinear direction, the difference of 

 the lengths oftheir paths would become gra- 

 dually greater and greater, and would conse- 

 quently produce the appearances of colour 

 •usual in such cases ; that, supposingthem to be 

 inflected at right angles, the difference would 

 amount nearly to the diameter of the fibre, 

 and that this difference must consequently be 

 smaller as the fibre became smaller ; and, the 

 number of fringes within the limits of a right 

 angle becoming smaller, that their angular 

 distances would Consequently become greater, 

 and the whole appearance would be dilated. 

 It was easy to calculate, that, for the light 

 least 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 dis- 

 tance from the fibre. 



I therefore made a rectangular hole in a 

 card, and bent its ends so as to support a hair 

 parallel to the sides of the hole : then, upon 

 applying the eye near the hole, the hair of 

 course appeared dilated by indistinct vision 



into a surface, of which the breadth was de- 

 termined by the distance of the hair and the 

 magnitude of the hole, independently of the 

 temporary aperture of the pupil. When 

 the hair approached so near to the direction 

 of the margin of a candle that the inflected 

 light was suflSciently copious to produce a 

 sensible effect, the fringes began to appear; 

 and it was easy to estimate the proportion of 

 their breadth to the apparent breadth of the 

 hair, across the image of which they ex- 

 tended. I found that six of tlie brightest red 

 fringes, nearly at equal distances, occupied 

 the whole of that image. The breadth of the 

 aperture was tIoo^, and its distance from the 

 hair —, of an inch ; the diameter of the 

 hair was less than-;-!-^ of an inch ; as nearly 

 as I could ascertain, it was ^^. Hence, we 

 have Tws^ for the deviation of the first red 

 fringe .at the distance 4^5 ; and, as t^oI 

 To^ro- '■•g'o-o •it-soooo*. or fxTTT '^r the dif- 

 ference of the routes of the red light, where 

 it was most intense. The measure deduced 

 from Newton's experiments is t^s^- I 

 thought this coincidence, with only an error 

 of one ninth of so minute a quantity, suffi- 

 ciently perfect to warrant completely the ex- 

 planation of the phenomenon, and even to 

 render a repetition of the experiment un- 

 necessary ; for there are several circum- 

 stances which make it difircult to calculate 

 much more precisely what ought to be th^ re- 

 sult of the measurement. 



When a number of fibres of the same kind, 

 for instance, those of a uniform lock of wool, 



are held near to the eye, we see an appear- 

 ance of halos surrounding a distant candle ; 



but their brilliancy, and even their existence, 



depends on the uniformity of the dimensions 



of the fibres; and they are larger as the fibres 



