202 A PROBLEM 1n OPTICS. 
The Anfwer, by Mr. RITTENHOUSE. 
DEAR SIR, 
HE experiment you mention, with a filk handker- 
chief and the diftant flame of a lamp, is much more 
curious than one would at firft imagine. For the object 
we fee is not the web of the handkerchief magnified, but 
fomething very different, as appears from the following 
confiderations. 41ft. A diflin& image of any objec, placed 
clofe to the eye, cannot be formed by parallel rays, or 
fuch as iffue from a diftant luminous point: for all fuch 
rays, pafling through the pupil, will be collected at the 
bottom of the eye, and there form an image of the lumi- 
nous point. The threads of the handkerchief would only 
intercept part of the rays, and render the image lefs bril- 
liant. 2dly. If the crofs bars we fee were images of the 
filk threads, they muft pafs over the retina, whilft the 
threads are made to pafs over the pupil; but this, as you 
obferve, does not happen; for they continue ftationary. 
3dly. If the image on the retina was a picture of the ob- 
ject before the eye, it muft be fine or coarfe, according to 
the texture of the handkerchief. But it does not change 
with changing the filk, nor does it change on removing 
it farther from the eye. And the number of apparent 
threads remains the fame, whether 10, 20, or 30 of the 
filk threads pafs acrofs the pupil at the fame time. The 
image we fee muft therefore be formed in fome different 
manner; and this can be no other than by means of the 
inflection of light in pafling near the furfaces of bodies, as 
defcribed by NewTon. 
It is well known in optics that different images of the 
different points of objets without the eye are formed on 
the retina by pencils of rays, which, before they fall on 
the eye, are inclined to each other in fenfible angles. And 
the great ufe of telefcopes is to encreafe thefe angles, re- 
gularly, in a certain ratio; fuffering fuch rays as were 
parallel 
