St. Mary's beacon., 1858-09-09
SELECTED MISCELLANY.


The eve is the most ssvotletfIi organ in the
human irauEe II Is the medium by which
tile telcgraplir agencies Of :igft carry our per
Ncptiot,s n ah instant to physical objects, dis
salst millions df miles o. in the caSe Ut the
heavenly bOEies. In connccticn With optics,
nlso, The most extraordinary phenomena and
hluluie.sss occur We are all familiar, by de-
Scription at Least, with +t,e mirage Of the de-
Rcrt, where the far extending plasn assumes at
u distance the appearance Uf lake, which,
as the thirsty traveller presses "n 90 part:'kci
Cf its refreshing waters, rccedcs and disappears
before him. Accounts Of ships seen at sea
sailing in the sky, of 6uhblc ships, and of
bnIies nf troops Ssict1fed on the viewicss air,
are also NUMEROUS; aud all probably arc fanil
lar with the ,Ihcnoacir& Of hOpHiHy, by which
an opposite shore seems to bc elevated
hut of the water, or to be brUrTght nearer and
4nulntains, ships or other objects, appear to
bc ~aiscd or niagnifed in dimensions. These
appearances arc caused by peculiar states OF
xhe atmosphere, by which tlic normal direction
uf the rays Of light is changed. One Of the
most singular of these optical phenomena has
been noticed re,,cntly as occurring in the wcS-
tern suburbs OF Petersburg, jVa.) and Obser-
vahle daily from the residence of Doctor Duval
of that place. The olI Blandford church,
which is distant about three miles, can he plain
~ seen from his portico during the entire
morning; but abou: three O'clock in the after
koon, IL begins 10 disappear, assuming first the
appearance Of : shadow, and finaEy, vanishing
altogether whca objects situated behind tile
church which could not be seen before, become
distinctly visible. Among these are the bOly
aud lower branches OF large sycamore tree
growing in the church yard, view of which
Is completely intercepted by the body of the
church, but which, when the phenomenon &C;
ours, can he Plesiuly kccn from the ssrcn sward
at its root to It's tupmiSt kaf. Ills disap-
pearance Of an intervening object. and the
eiiyultsing of others to the eye which should
should be hidden behind It., is very singular,
and presents one of the most interesting cases
Of optical Illusion we have ever met with. The
regular repetition Of the occurrence is also ex-
traordinary, and would afford excellent oppoT"
tunitiSs for investigation - to the causes and
manner Of its production. valley interyenes
bctween the residence Of DIR. Duval and the
church, through which a small stream OF wa
ter passes, and the city reservoir also lies be-
meath the line Of vision. The effect produced
tapon the atmosphere by the a6ton of the sun
upon che water Intervening, causes No doubt.
ahe singular illusion; but is would I've dcsira-
ble 10 have more accurate solution of the
phenomenon.
The sources of optfSM illusion are very var-
oVs; sametiInos they reside i'm The organ Of vi
sion itself; sometimes ia the mind, but more
generally, by some circumstances which dis
turb the direct course Of the Ays Of light
Thus, If q man places himself directly before a
large cOncavc mirror, but further from it than
abe centre Of its concavity, he will see an IN
sserted image of himself IN the air between him
and the mirror Of less size than himself; and
il he hold out his hand towards the mirror HE
hand u the image will come out towards his
hand and ruircido with it, of an equal bulk
when his hand is in the ceFtro of the conca-
witty, and he will imagine that he may shake
hands with his image. II he reach his hand
further, the hand Of the image viii pass by his
hand and come between it and hes body; and
IL he move his hand to either side, the band of
the image will move towards the other, SO that
whatever way the object moves, the image will
not appear to move Che same bat the contrary
way. Yet byStandcr will see nothing of the
image, because none of the reflected fays which
form it NEE. bis eyes. Phenomena of this
description all depend upon the fact that we
only see By the rays of light reflected from an
Otljec: which meets our eyes. If these rays are
intercepted or disturbed We will ctheeA nc. see
the object. as in the case of the church at
BandfOrd, or sec it distorted as when we loOk
into crooked mirror There are some ealfas |
ordinary Optical illllsions produced also as 10
colurs occasionally; and perhaps every one has !
sometimes noticed objects rs appearing 10 them !
of 3 color which they know they are rlst! Many |
Interesting occulaf spe6tfa may be produced a: |
viii. For instance, take rcd wafer, place Il |
on 9 sheet OF white paper, and closing one eye. |
icSk steadily for some time at the centre OF the
wafer, then turn the eye TO another part OF the |
paper, and you win see a green wafer. By |
repeating tlss experiment with different colOrs,
q black wafer will produce white, orange |
blue, violet abineish green image. each variety |
OF hue presenting an image OF different, that |
Is, IS accidental q opposite color. Tile phe-
noanena ef the fdta DsSyaHG sesR in the straits |
OF MesHina have never yet lcei satisfactorily
.accoauted fog. -When the fising sim throws |
his rays at an angle II IS 5degfCes Oh the sea
jnf Riggio, aud neither wand nor rain rufFie |
the smooth surface lof the wateF iti thg laii I
whs spssaatOF On an eminsncs in the city who
ElAclA his hack to The aan, md his face % tbs
'Nea observes ssit wese upsm its surface uaihl-
berless pilassrrsi aRehes; and castles distinctly
a -