A PROBLEM in OPTICS. 20% 
parallel before they enter the telefcope to proceed on, pa~ 
rallel, after pafling through it. The extended image which 
we fee in this experiment muft therefore be formed by 
pencils of.rays, which before they entered the eye, had 
very confiderable degrees of inclination with refpec& to 
each other. But coming from a fmall diftant flame of a 
lamp, they were nearly parallel before they paffed through 
the filk handkerchief. It was therefore the threads of filk 
which gave them fuch different direCtions. 
Before the filk is placed to the eye, parallel rays of light 
will form a fingle lucid {pot, as at A, Plate III. Figure 16. 
And this fpot will ftill be formed afterwards by fuch rays 
as pafs through the little mefhes uninfluenced by the 
threads. But fuppofe the perpendicular threads by their 
action on the rays, to bend a part of them one degree to 
the right and left, another part two degrees; there will now 
be four new images formed, two on each fide of the original 
eneat A. By a fimilar action of the horizontal threads, this. 
line of five lucid points will be divided into five other lines, 
two above and two below, making a fquare of twenty-five 
bright {pots, feparated by four perpendicular dark lines and 
four horizontal ones; and thefe lucid {pots and dark lines 
will not change their places on moving the web of filk over 
the eye parallel to any of its threads. For the point of the 
retina on which the image fhall fall is determined by the 
incidence of the rays, with refpect to the axis of the eye, be-- 
fore they enter, and not by the part of the pupil through 
which they pafs. 
In order to make my experiments with more accuracy, 
I made a fquare of parallel hairs about half an inch each 
way. And to have them nearly parallel and equidiftant, 
1 got a watchmaker to cut a very fine {crew on two pieces 
of {mall brafs wire. In the threads of thefe ferews, 106 
of which made one inch, the hairs were laid 50 or 60 in 
number. Looking through thefe hairs at a fmall opening 
in the window fhutter of a dark room, .*. of an inch wide 
and: 
