MAN 
MAN 
without, cannot enter the aperture in the fame ex- 
tent they poflefifed on leaving the object, fince, if 
fo, they would require the aperture to be as large 
as the objeft; and as each part, and every point of 
the object, emits the image of itfeif on every fide, 
and the rays, which form thefe images, pafs from 
all points of the objeft as from fo many centres ; 
fo none of them can pafs through the aperture but 
thofe that arrive at it in different direftions. 
Hence the fmall hole becomes the centre of the 
whole obje£l, at which the rays flowing from tlie 
lower, as well as the higher parts of the objeft, 
arrive in converging dire£lions; and confequently 
they muft crofs each other in the central point, 
and thus reprefent the pifture of the objed" on the 
oppofite wall in an inverted pofition. 
In like manner, it is eafy to conceive that all 
-objefts appear double, whatever our prefent fen- 
fations may feem to tell us to the contrary. To 
convince us of this, we have only to compare the 
fituation of any one objefb on fhutting one eye, 
and then compare the fame fituation by fhutting 
the other. If, for inftance, we fhut the right eye, 
we fhall find it hide a certain part of the room vi- 
fible, and another part of the room concealed 
which before was vifible: however, if both eyes 
be opened, the part concealed will appear to lie 
between the two extremes. But the faft is, we be- 
hold one image of the concealed obieft on the 
right, and another on the left; though, from habit, 
we fuppofe that we fee but one objeft placed be- 
tween both, our fenfe of feeling having correfted 
the objeft of fight. And thus alfo, if inftead of 
two eyes we had two hundred, we fliould at firfL 
imagine the objefts increafed in proportion, till 
one fenfe had correfted the errors of another. 
Having therefore two eyes, may be fuppofed 
from thefe premifes to be rather an inconvenience 
than a benefit, fince one eye would anfwer the 
purpofes of fight as well, and be lefs liable to illu- 
fion. But it is remarkably otherwife: two eyes 
greatly contribute, if not to diftinft, at leaf!: to ex- 
tenfive vifion. When an obje6l is placed at a 
moderate diftance, by the means of both eyes we 
comprehend a larger fhare of it than we poiTibly 
could with one; the right eye taking in a greater 
portion of it's right fide, and the left of it's cor- 
refpondent fide. Thus both eyes, in fome mea- 
i'ure, fee round the objefr. 
In either eye there is a point, which has no 
vifion whatfoever; fo that if one of them only is 
employed in feeing, there is a part of the ohjeft to 
which it is totally blind. To be convinced of 
this, we have only to try a very eafy experiment: 
if we take three black patches, and ftick them on a 
white wall, about a foot diftant from each other, 
and about the height of the obferver's eye; then 
retire fix or feven feet back, and fliut one eye; by 
trying for fome time, we fhall find, that while we 
diftinftly behold the black fpots to the right and 
left, that which is in the middle remains totally 
tinfeen. In other words, when we bring that part 
of the eye where the optic artery runs, to fall on 
the obje6l, it will then become invifible. This 
defeft, however, in either eye, is always corrected 
by both, fince the part of the objeft unfeen 
by one viiW be very diftinflly perceived by the 
other. 
Befides the former defe6ls, we can have no idea 
of diftances from the fight, without the help of the 
touch. Naturally, every objeft we fee appears to 
be within our eyes; and a child, who has as yet 
made little ufe of the fenfe of feeling, mufl fup- 
pofe that every tiling it fees makes a part of it- 
feif: fuch objefts only appear more or lefs 
bulky, as they approach or recede from it's eyes; 
fo that a fly that is near, will feem larger than an 
ox at a diftance. Experience alone can redify 
this miftake; and a long intimacy with the real 
magnitude of every objeft quickly informs us of 
the diftance at which it is feen. The Man who is 
moft remote in a file of fokliers, appears in reality- 
much lefs than the Man next to us: however, we 
do not perceive this difference, but continue to 
think liim of equal feature; for the numbers we 
have feen thus leffened by diftance, and have 
found, by reiterated experience, to be of the na- 
tural fize, inftantly correft the fenfe, and every ob- 
je6l is viewed with nearly it's natural proportion. 
But it is otherwife if we obferve objcfts in Ibch 
fituations where we have not had fufficient expe- 
rience to corre6l the errors of the eye : if, for in- 
ftance, we look at Men from the top of a high 
fteeple, in that cafe they appear very much c'lm'i- 
nifiied, as we have not had an opportunity of cor- 
reftingthe fenfe fufficiently in that fituation. 
But though a fmall degree of refleftion will 
ferve to affure us of the truth of thofe pofitions, 
it may not be amifs to corroborate them by aa 
authority which cannot be called in queftion. 
That eminent anatom iff thefelden navino; couch- 
ed a boy of thirteen for a carara6t, Vv'ho had hi- 
therto been blind, and thus at once reftored him 
to fight, curioufly marked the progrefs of his 
mind on that occafion. This youth, though till 
then incapable of feeing, yet was not totally blind, 
but could tell day from night, as perfons in fimi- 
lar fituations always may: he could alfo, with a 
ftrong light, dlftinguifh black from white, and 
either from the vivid fcarlet dye; however, he 
perceived nothing of the foim of bodies; and, 
without a bright light, not even colours them- 
felves. At firft, he was couched only in one of 
his eyes ; and when he faw for the firft time, he 
was fo totally incapable of judging of diftances, 
that he fuppofed his eyes touched every objcft 
which he faw, in the fame manner as his hands 
might be faid to feel them. The cbjefts moft 
agreeable to him were ilich as had plain furfaces 
and regular figures; though as yet he could form 
no idea of their difi'erent figures, or alllgn a rea- 
fon why one pleafed him iTaOre than another. He 
delighted moCi in green colours; but black ob- 
je6ls, as if recalling to his remembrance his for- 
mer blindnefs, he re.2;arded with horror. When 
thofe things were fnevvn him to which he had been 
formerly familiarized by the fenfe of feeling, he 
beheld them with earneftnefs, in order to remem- 
ber them a fccond time ; but as he had too many 
to recollecTc at once, he forgot the greateft num- 
ber. He exprefied great furprize on finding that 
thofe perfons and things he loved beft, were not 
the moft beautiful to the fight; and even teftified 
difpleafure that his parents were not fo handfome 
as he conceived them to be. Two months almoft 
elapfed before he could difcover that a pi6lure re- 
fembled a folid body : till then he only confi- 
dered it as a fiat furface, varioufly ihadov^ed; 
but, wlien he began to perceive that thofe kind 
of ftiadings aftually reprefented human beingjj, 
he endeavoured to exaniine by his touch, v.'he- 
ther they had not the ufual qualities of fuch bo- 
dies; and was much difippointed to find what 
he expeded a very unequai furface to be fmooth 
