204 A PROBLEM in OPTICS. 
and three inches long, holding the hairs parallel to the 
flit, and looking toward the fky, I faw three parallel lines, 
almoft equal in brightnefs, and on each fide four or five 
others much fainter and growing more faint, coloured and 
indiftin&, the farther they were from the middle line, 
which I knew to be formed by fuch rays as pafs between 
the hairs uninfluenced by them. Thinking my apparatus 
not fo perfect as it might be, I took out the hairs and put 
in others, fomething thicker, of thefe 190 made one inch, 
and therefore the {paces between them were about the 2. 
partofaninch. The three middle lines of light were now 
not fo bright as they had been before, but the others were 
ftronger and more diftin@, and I could count fix on each 
fide of the middle line, feeming to be equally diftant from 
each other, eftimating the diftance from the centre of one 
to the centre of the next. The middle line was ftill well 
defined and colourlefs, the next two were likewife pretty 
well defined, but fomething broader, having their inner 
edges tinged with blue and their outer edges with red. 
The others were more indiftin&t, and confifted each of the 
prifmatic colours, in the fame order, which by fpreading 
more and more, feemed to touch each other at the fifth or 
fixth line, but thofe neareft the middle were feparated from 
each other by very dark lines, much broader than the 
bright lines. 
Finding the beam of light which came through the win- 
dow fhutter divided into fo many diftin& pencils, I was 
defirous of knowing the angles which they made with 
each other. For this purpofe I made ufe of a {mall prif- 
matic telefcope and micrometer, with which I was favour- 
ed by Dr. Franklin. I faftened the frame of parallel hairs 
before the obje& glafs, fo as to cover its aperture entirely. 
Then looking through the telefcope, I meafured the {pace 
between the two firft fide lines, and found the angular 
diftance between their inner edges to be 13’, 15”3 from 
the middle of one to the middle of the other 15‘, 30”, and 
from 
