On the Refraction of Light. 351 
images transmitted to the retina through a puncture made by the 
point of a fine needle in a piece of sheet lead, do not appear so dis- 
tinct or luminous as when viewed by the naked eye; but seem to be 
enveloped in a hazy brownness, whose intensity is inversely as the 
width of the aperture. The solar spectrum viewed through this 
puncture loses much of its brilliancy, and the nice distinction be- 
tween the. colors can no longer be observed. Whether the small 
aperture be angular or slightly elliptic, it will, when applied close to 
the eye, invariably present the figure of a perfect geometrical circle. 
If the aperture be an incision of a line in length, it will assume the 
figure of an oblong, rounded at. its two extremities. The size of 
these circles does not depend entirely on their proximity to the eye, 
on the contraction and dilatation of the pupil. 
When a number of punctures are made in the lead, at the distance 
of one sixteenth of an inch from one another, and gradually approach- 
ed to the eye, the opaque interval between them diminishes as their 
diameters increase, till at length it is obliterated, and the whole ap- 
_ pears diaphanous and filled with perfect circles cutting one another. 
But the most paradoxical appearance attending this experiment is, 
that the circumferences of these cutting circles, which receive at 
least doubly as much light as their centres, are distinctly dark, ’ 
their centres are illuminated. This anomaly is somewhat analogous 
to that discovered: by Grimaldi, whilst studying the inflection of light: 
viz. “that a body actually illuminated may seagne more ont by 
adding a new light to that which it already receives.” — 
When a number of long parallel apertures, about halfa line apart, 
are cut with the point of a penknife through a alae ate of lead, and ap- 
Plied close to the eye, no opaque interval will be found. A similar 
Phenomenon occurs when a small body, as a wire or necdle, is grad- 
ually approximated to the eye. Thus, if we close one eye and raise 
@needle, by degrees, from the page of a book, towards the other, 
the needle will appear as it approaches the eye, to lose imperceptibly 
its density:and opacity, till at last it will become diaphanous, letters be 
ing visible through it, and nothing of it remaining but a diffused shadow 
or penumbra, ‘This illusory transparency may be observed to occur 
in cylinders of even a line in diameter. It.is.moreover to be re- 
marked that the shadow (if so we may tern? it,) into which the body 
‘ppears to resolve itself, is imbued with | the color of the body. If 
Wwe look at the page of a book through a thick wire painted black, 
the letters within the shadow will seem dark and denigrated ; but if 
We look through a white wire at a black surface, it will appear whitish. 
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