of the Production of Colours, 389 
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 distance 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 determined 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 sufficiently 
copious to produce a sensible effect, the fringes began to ap- 
pear ; and it was easy to estimate the proportion of their breadth 
to the apparent breadth of the hair, across the image of which 
they extended. I found that six of the brightest red fringes, 
nearly at equal distances, occupied the whole of that image. 
The breadth of the aperture was anc ^ its distance from 
the hair t 8 q of an inch: the diameter of the hair was less than 
500 
of an inch; as nearly as I could ascertain, it was 
1 
600' 
Hence, we have for the deviation of the first red fringe at 
the distance - s -- ; and, as - 8 - : _ 
I O ’ } IO I 
I I 
000 
I 
600' 
1 1 or - for* 
480000’ 43 636 1Ui 
the difference of the routes of the red light where it was most 
intense. The measure deduced from Newton's experiments is 
■3 j jo o' I thought this coincidence, with only an error of one- 
ninth of so minute a quantity, sufficiently perfect to warrant 
completely the explanation of the phenomenon, and even to 
render a repetition of the experiment unnecessary; for there 
are several circumstances which make it difficult to calculate 
much more precisely what ought to be the result of the mea- 
surement. 
3 E 
MDCCCIL 
