The Massachusetts spy, and Worcester County advertiser. [volume] ([Worcester, Mass.]) 1825-1831, 1830-03-03

intensity OF light is necessary to distinct
vision, but the degree varies with the pre-
vous state of the organ. A person passing
from the bright day into shaded room, for
time may fancy himself in total darkness !;
and to persons sitting in the room and be-
coming accustomed to the less light so as
to see well with it, he will appear to be al
most blind. The dawn of morning after
the darkness of night appears much bright
er than an equal degree of light in the even
ing. When, as the night falls, our lamps
or candles are first introduced, the glare is
often for time offensive and the same
feeling is still stronger on opening, in the
morning, bedroom window shutters or close
drawn curtains. After the repose of night.
the sensibilty of the eye is such that the
globules of blood in the capillary vessels of
the retina produce the impression on it of
little globes of light crossing among each
other as the tortuous vessels do. To pris-
oner after long confinement in dark dun
geon, the light of the sun is almost insup-
portable. And dungeon, which to COM
mon eyes is utterly dark, still to its long
held inmate has ceased to be so. There
are various instances in the records of the
barbarous ages, of prisoners confined for
years IN utter darkness, who at last could
see and make companions of the mice which
trequented their cells. The darkness of
total eclipse after bright sunshine appears
much more deep than that of midnight, be-
cause ofthe contrast. The long polar night
of months ceases to appear very dark to the
polar inhabitants. If an eye be directed
for time to black wafer laid on sheet
of white paper, and afterwards to another
part OF the sheet, portion of the size of the
wafer will appear brilliantly illuminated :;
for the ordinary degree of light from it ap-
pears intense to the part of the eye lately
receiving almost none. An eye directed
long and intensely upon any minute object
as VIII 90 -~;l-- matches a sneck in tho dis
tant horizon, supposed to be ship, or when
hunter on the brown heath keeps his eve
liked on some game nearly of the color of
the heath, or when an astronomer gazes
long at little star-has the sensibilty of
It's centre at last exhausted, and ceases to
perceive the object; but on directing the
axis OF the eye little to one side of the OBJ
Ject, so that an image may be formed only
near the centre, the object may he again
perceived, and the centre in the mean time
enjoying repose, will recover its power.
But the most extraordinary fact connect
ed with the sensibilty of the retina is, that
if part of it be strongly exercised by look
ing for time at an object of any bright
color, on then turning the eye away or al
together shutting it, an impression or spect-
rum will remain of the same form as the OBJ
ject lately contemplated, but of a perfectly
different color. Thus, if an eve be direct
ed for time to red wafer laid on white
paper, and be then shut or turned to anoth-
nff ha koo nj br,,- hk
wafer will be seen, and USce UerSd, green
wafer will produce a red spectrum, an or
ange wafer will similarly produce blue
spectrum, yellow one a violet spectrum,
&C.. and cluster of waters will produce
similar cluster of opposite colors. If the
hand be then held over the eyelids to dark
en- the eyes and prevent entirely the APP
proach of light, the spectrum of the bright
parts will be luminous, surrounded by dark
ground, and when the hand is again remov-
ed the contrary will be true. Again, if the
eye be in degree fatigued by looking at
the setting sun, or even at window with
bright sky beyond it, or at any very bright
object, on then shutting it, the lately con-
templated forms will be perceived, first of
one vivid color, and then of another, until
perhaps all the primary colors have passed
I in view. These extraordinary facts prove
| that the sensation of light and color, al
| though excitable by light, is also producible
without it. This truth gave occasion to
Darwin's ingenious theory, that the sensa-
ton of any particular color, of red for in
stance, is dependent upon certain state OF
contraction of the minute fibres of the reti-
ma, as the sensation of particular tone de-
ponds on a certain frequency of vibration OF
some part of the Garland that the fbres,
when fatigued in that condition, seek relief
when at liberty, by throwing themselves in
to an opposite state,-as man whose back
is fatigued by bending forward, relieves
himself not by merely standing erect, but
by bending the spine ackwards-which
new condition, whether produced by light
or any other cause, gives the sensation Of
green. He applied his explanation similar
ly to all other cases of color. 1t is remark
able that the colors which thus appear op
posite to each other in kind are those which
when the solar spectrum produced by a
prism, is painted round wheel or circle
are opposite to each other in place.
| .4raott's Physics.
GAsTRoEoMIc JOKE. A lon of mutton wal
on the table, and gentleman took the carving
knife in hand. " Shall cut il saddle-wise I
said he. "You had better cut it bride wise,
said his friend -" for then we shall stand
better chance Of getting BIT in our mouths -