[aon Ts 
N° XXIV. 
An Optical Problem, propofed by Mr. HoPKinson, 
and folved by Mr. RITTENHOUSE. 
Philadelphia, March 16th, 1785. 
DEAR SIR, 
Read Feb. FE TAKE the liberty of requefting your attenti- 
hg 2 on to the following problem in apéics.. It is L 
Believe entirely new, and the folution will afford amufe- 
ment to you and inftruCtion to me. 
Setting at my door one evening laft f{ummer, I took a 
filk handkerchief out of my pocket, and ftretching a por- 
tion of it tight between my two hands, I held it up be- 
fore my face and viewed, through the handkerchief, one 
of the ftreet lamps which was about one hundred yards 
diftant; expecting to fee the threads of the handkerchief. 
much magnified. Agreeably to my expectation I obferv- 
ed the filk threads magnified to the fize of very coarfe 
wires; but was much furprifed to find that, although I- 
moved the handkerchief to the right and left before my 
eyes, the dark bars did not feem to move at all, but re- 
mained permanent before the eye. If the dark bars were 
occafioned by the interpofition of the magnified threads 
between the eye and the flame of the lamp, I fhould have 
fuppofed that they would move and fucceed each other, as 
the threads were made to move and pafs in fucceflion be- 
fore the eye; but the fact was otherwile. 
To account for this phenomenon exceeds my {kill in 
optics. You will be fo good as to try the experiment, 
and if you find the cafe truly ftated, as I doubt not you 
will, I fhall be much obliged by a folution on philofophi- 
cal principles, I am fir, with great fincerity, 
Your moft effectionate friend, 
And very humble fervant, 
F. HOPKINSON.. 
