388 Dr. Young's Account of some Cases 
As I was observing the appearance of the fine parallel lines 
of light which are seen upon the margin of an object held 
near the eye, so as to intercept the greater part of the light of a 
distant luminous object, and which are produced by the fringes 
caused by the inflection of light already known, I observed that 
they were sometimes accompanied by coloured fringes, much 
broader and more distinct ; and I soon found, that these broader 
fringes were occasioned by the accidental interposition of a hair. 
In order to make them more distinct, I employed a horse-hair; 
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-worm's thread, their magnitude was so much 
increased, that two or three of them seemed to occupy the 
whole field of view. They appeared to extend on each side of 
the candle, in the same order as the colours of thin plates, seen 
by transmitted light. It occurred to me, that their cause must 
be sought in the interference of two portions of light, one re- 
flected from the fibre, the other bending round its opposite side, 
and at last coinciding nearly in direction with the former por- 
tion ; that, accordingly as both portions deviated more from a 
rectilinear direction, the difference of the length of their paths 
would become gradually greater and greater, and would conse- 
quently produce the appearances of colour usual in such cases ; 
that, supposing them to be inflected at right angles, the dif- 
ference 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 in a right angle 
becoming smaller, that their angular distances would conse- 
quently become greater, and the whole appearance would be 
dilated. It was easy to calculate, that for the light least inflected. 
