SIG 
SIENITE, in mineralogy, one of the 
compound primitive rocks that consists es- 
sentially of crystals and grains of hornblend 
imbedded in felspar. Quartz and mica are 
occasionally found in sienite, but in very 
small quantity. It is commonly in mass, 
and is rarely either schistose or stratified. It 
sometimes contains metallic veins. 
SIEVE, or Searce, an instrument serv- 
ing to separate the fine from the coarse 
parts of povfders, liquors, and the like ; or 
to cleanse pulse from dust, light grains, &c. 
It is made of a rim of wood, the circle or 
space whereof is filled with a plexus of silk, 
tiffany, hair, linen, wire, or even thin slices 
of wood. The sieves which have large 
holes are sometimes also called riddles, 
such as the coal or lime sieve ; the garden- 
sieve, &c. When drugs are apt to evapo- 
rate, on being passed through the sieve, it is 
usual to have it covered with a lid. 
SIGESBECKIA, in botany, so named 
from John George Siegesbeck, a German, 
prefect of the Petersburg!! garden, a genus 
of the Syngenesia Polygamia Siiperflua class 
and order. Natural order of Compositae 
Oppositifoli®. Corymbifer®, Jussieu. Es- 
sential character : calyx exterior, five- 
leaved, proper, spreading ; ray halved ; 
pappus none j receptacle caffy. There are 
tliree species. 
SIGHT, sense of. The organ of sight is 
a globular body, which contains within it 
transparent substances, fitted to form on 
the back part of it a picture of the object 
of sight. An examination of the eye of an 
ox will give a pretty accurate idea of the 
general structure of the human eye. We 
see in front a horny transparent substance 
called the cornea. Next to this is a wa- 
tery fluid called the aqueous humour, in 
which the iris floats like a delicate curtain, 
with a hole in the middle called the pupil. 
Behind the iris we find a solid body with 
two convex surfaces ; this is called the 
crystalline lens. Next to this is the vi- 
treous humour, ajelly-like, transparent sub- 
stance, which fills the ball of the eye. The 
retina consists of exceedingly minute fibres 
from the optic nerve, which are spread over 
the whole of the back part of the inner 
surface of the eye. Behind the retina is a 
mucous or slimy matter, which in the hu- 
man eye is of a dark colour, and serves to 
imbibe the rays of light which pass through 
the retina, so as to prevent the confusion 
which would arise from the reflection of 
them. The retina is the immediate organ 
of sight. The rays of light, proceeding 
SIG 
from every visible point of the object of 
sight, enter the eye through the cornea, 
and pass through the pupil ; they are re- 
fracted by the three humours of the eye, 
so as to form upon the retina an exqui- 
sitely beautiful and distinct, though mi- 
nute, picture of the object. A pretty cor- 
rect idea of the formation of this picture 
may be obtained, by carefully cutting off 
the back coating of an ox’s eye, and bold- 
ing behind it a piece of paper to receive 
the picture of a luminous object. For a more 
minute account of the structure of the eye, 
see Anatomy. 
What effect is produced upon the optic 
nerve by the formation of this picture upon 
the retina is not certainly known ; it is suf- 
ficient for our present purpose, that, by 
means of the nerve, &c. the impression, 
whatever it be, is communicated to the 
mental organs, and produces in them those 
effects which, when attended with con- 
sciousness, are called sensations. See Men- 
tal Philosophy, §. 11 . 
If the sensation produced by the object 
of sight be considered unblended with the 
relics of other sensations, we find that it is 
merely what can be communicated by a 
minute picture on the retina. The sensa- 
tion of colour can be thus communicated, 
and this is the only sensation which can be 
considered as appropriate to the sight. The 
sensation of figure can be thus communi- 
cated, but only of figure in two directions, 
length and breadth ; for the picture on the 
retina can have, only those two dimensions. 
The sensation of magnitude can also be 
thus communicated, but not of real magni- 
tude ; for the visible sensation of real mag- 
nitude cannot be conveyed by a picture, 
which is almost indefinitely smaller than 
the real object. To use the illustration of 
Adam Smith : “ If you shut one eye, and 
hold immediately before the other a small 
circle of plain glass, of not more than half 
an inch in diameter, you may see through 
that circle the most extensive prospects, 
lawns, and woods, and arms of the sea, and 
distant mountains. You are apt to imagine 
that the visible-picture, which you thus see, 
is immensely great and extensive; but it 
can be no greater than the visible circle 
through which you see it. If, while you 
are looking through the circle, you could 
conceive a fairy hand and a fairy pencil 
to come between your eye and the glass, 
that pencil' might delineate upon that little 
glass the outline of all those extensive lawns, 
and woods, and arms of the sea, and distant 
