301 
OPTICS, 
lance appear elevated : for the globular form 
ot the earth is not perceptible to the eye ; 
and if it was, the apparent elevation of the 
sea is far greater than the arch which a seg- 
ment ot the globe would form within any dis- 
tance tint our eyes are capable of reaching. 
l or the same reason, if a number of objects 
are placed on the same plane and at the same 
height below the eye, the more distant will 
appear taller than the others ; and if the 
same objects are placed on a similar plane 
above the eye, the more distant will appear 
the lowest. 
1 he distant parts of a long wall, for the 
same reason, appear to a person who stands 
near one end to curve, or incline towards 
him. In the same manner the high wall of a 
lofty tower seems to a spectator, placed di- 
rectly under it, to bend over him, and 
threaten him with instant destruction. If anv 
person inclined to make the experiment will 
lie down on his back in a situation of this de- 
scription, at the distance of five or six feet 
from the wall of which he contemplates the 
tremendous height, he will immediately be 
made sensible of the phenomenon. 
If the distance between two objects forms 
an insensible angle, the objects, though in re- 
ality at some distance from each other, will 
appear contiguous. This is assigned by some 
astronomers as the reason why the ring or 
belt of Saturn appears as one mass of light, 
while they contend that it is formed from a 
number of little stars or satellites ranged with- 
in a certain distance of each other. 
If the eye is carried along, as in a boat, 
without being sensible of its own motion, tiie 
objects which are stationary on each side will 
appear to move in a contrary direction. Thus 
we attribute to the sun and the other heavenly 
bodies a diurnal motion, which only affects 
the earth which we inhabit. 
If two or three objects at a considerable 
distance, and on which the eve of the spec- 
tator is fixed, mo.e with equal velocity past 
a third object which is at rest, the moving ob- 
jects will appear to be actually at rest, and 
that which is really stationary will appear in 
motion. Thus the clouds which pass over 
the face of the moon appear at rest, while the 
moon itself appears to proceed rapidly along 
in an opposite direction. This happens, be- 
cause the eye which is fixed upon the clouds 
follows their motion mechanically, and there- 
fore the moon appears to move and not the 
clouds ; as in the boat we do not perceive its 
motion, but conceive the banks are retiring 
behind us. 
If the centre of the pupil, that is, the op- 
tic axis, is directed along the surface of any 
slender object in a perfectly right line, this 
line w ill appear only a point, because, in fact, 
the extremities only are visible. 
An extended and distant arch, viewed by 
an eye which is exactly in the same line, will 
appear as a plane surface ; because all the 
parts appearing equally distant, the curvature 
will not be perceived. 
If a circle is viewed obliquely it will ap- 
pear an oval, because the diameter which is 
perpendicular ‘o the eye is shortened ; in 
other words, the rays which proceed from 
the extremities form an angle so much the 
more acute as the obliquity is greater ; on 
the contrary, the diameter which is parallel 
to the eye is apparently extended. 
Such are the general principles upon which 
vision is performed ; but the sense of sight is 
limited not only with respect to distant ob- 
jects, but with respect to those which are 
near. Every person will easdy perceive that 
if a book, or arty other object, is held too 
close to the eye, the letters or the object will 
appear very indistinct and confused. This 
distance varies with respect to different eyes. 
Very near-sighted persons can see at the dis- 
tance of one or two inches ; but Where the 
eye is in a sound state, the point of distinct 
vision varies from six to ten inches, or eight 
inches as a medium. 
To understand the reason of this, it is ne- 
cessary to remember that objects are made 
visible by cones >f diverging rays proceed- 
ing from every luminous point of an object ; 
but to have the object clearly painted on the 
retina, the rays must not enter the pupil of 
the eye too divergent. Indeed they ought to 
come in almost a parallel direction, more in 
the form of a cylinder than a cone, otherwise- 
the humours of the eye will not make them 
converge at the proper points on the retina. 
Thus, let us suppose CD (Plate HI. fig. 22) 
to be the diameter of the pupil of the eye ; () 
is then a luminous point of any object si- 
tuated at the distance of about six inches 
and OC and ODare divergent ray s proceed- 
ing from this point. Let AC and BP then 
be parallel rays. It will then be evident that 
the divergency of the rays OC and OJ) is so 
very small, that they are almost parallel when 
they arrive at the pupil ; and consequently 
the eye will be able to converge them in such 
a degree as to produce distirtet vision. 
If, on the contrary, the point O was nearer 
to the pupil, or if the pupil was larger, they 
would fall more diverging upon the eye, and 
the image ot 'he object would be formed at a 
point behind the retina, so as to be very im- 
perfect and confused. Hence w e may easily 
perceive the use of a single lens of a short 
focus, or high magnifying power, such as is 
employed in the single microscope. It ren- 
ders these divergent rays less divergent ; and 
consequently, assists the eye in making them 
converge to that point which is necessary to 
distinct vision. 
From the principles laid down it may ea- 
sily be understood why very minute objects 
are imperceptible to the naked eye. If those 
objects could, consistently with distinct vi- 
sion, be brought near to the eye, they would 
be perceived as well as by the aid oV a mi- 
croscope: hence some very near-sighted per- 
sons may be said to have microscopic eyes; 
but at six or eight inches (the limit of distinct 
vision) these objects subtend too small an an- 
gle to be perceptible. ' Opticians say that the 
eye -is not capable of perceiving any object 
which subtends an angle of less than half a 
minute of a degree. The image on the re- 
tina is in this case less than the part of 
an inch, and the object itself at six inches dis- 
tance less than the part of an inch 
broad. AU smaller objects are invisible. 
All very distant objects, upon the same 
principles, appear indistinct ; for their images 
on the retina are so extremely small, that 
the distinction of parts is not perceptible. 
Thus if a man, of six feet stature, is viewed 
at the distance of a mile, his image on the re- 
tina will not be more than the thousandth 
part of an inch in length. We cannot be 
surprised, therefore, if the eye can discern 
nothing of his features, or the minuter pari# 
of his body. 
Distant objects, however, appear not only 
indistinct but obscure • and this last effect is 
from a deficiency of light, very many of the 
rays being intercepted in ‘their passage 
through the air. Hence the difference in the 
appearance of such objects in a dark and 
cloudy day, w hen the air is impregnated with 
vapours, from that which they assume when 
the sun shines full and strong upon them. 
With a single glass the defects in sight, 
with respect to many objects, either too near, 
or at too great a distance, for the persons la- 
bouring under them, are remedied ; but there 
are cases where the object is so far distant, or 
so minute, that, though its outline may reach 
the eye, its parts must still, even with the aid 
ot a single lens, be indistinctly perceived. 
The art of man has discovered a remedy, in a 
great degree, for this imperfection ; and by 
means of a combination of glasses lias open- 
ed a wide field for iiis researches into the 
wonders of nature: he can now trace the 
limbs of an insect invisible to the naked eye; 
or he can make the celestial objects appear 
to him as if their distance had been on a sud- 
den diminished by many million - of miles. 
Ordeal in tmmcv s . — From w hat has been 
i-d ted concerning vision, the principle of the 
single microscope will be easily understood. 
Since the eye cannot have a distinct percep- 
tion ot any obje< t at a nearer distance than 
six or eight inches, and since there are many 
objects wh ch at that distance must be wholly 
imperceptible, or at be t appear as points, 
an instrument which can render them visible, 
is a very desirable attainment. 
It has been sufficiently explained that ob- 
jects appear larger or smaller m proportion 
to the angle under which they are seen. Since 
therefore the rays by w hich small objects are 
rendered visible by the microscope, must 
come from the extreme points of that object, 
it is manifest that though tb-- apparent mag- 
nitude is increased by the interposition of the 
lens, its real magnitude remains the same, 
i he lens enables us to view it at a shorter 
distance ; it will therefore appear exactly as 
much larger in diameter through the lens', as 
its distance from the glass is less than the 
nearest distance of distinct vision with the 
naked eye. 
Let A (Plate III. fig. 1) be then a point 
of an object not visible to the eye at a less 
distance than AB, because the rays are too 
divergent for distinct vision. Mow if the 
same object is placed in the focus C of the 
lens D, the rays which proceed from it will 
be rendered parallel by passing the lens ; and 
therefore the object is rendered distinctly vi- 
sible to trie eye at E. It will then of course 
appear as much larger through the lens than 
to the naked eye, as CD is less than AB. 
it the object AB is in the one focus of the 
lens DE, and the eve in the other focus F 
(fig. 2), as much of the object will be visible 
as is equal to the diameter of the lens ; for 
the rays AD and BE proceed through the 
extremities of the lens, and are united at the 
focus F, and render the extreme parts of the 
object visible. Hence a maxim in optics, 
“ that when an object is placed in one focus 
ot a lens, and the eye in the other, the object 
appears just twice as large as it would to the 
