A P P A 
CD be changed from the direct pofition to the oblique one 
CV, its apparent magnitude would then be only the angle 
C Ed, inftead of the angle CED. 
A i' B 
C & V 
If the eye E be placed between two parallels AB, CD, 
thefe parallels will appear to converge or come nearer and 
nearer to each other the farther they are continued out, 
and at lafl they will appear to coincide in that point where 
the fight terminates, which will happen when the optic- 
angle BED becomes equal to about one minute of a de¬ 
gree, the (mailed angle under which an objeCt is vifihle. 
Alio the apparent magnitudes of the fame objeCt FG or 
BD, feen at different distances, that is the angles FKG, 
BED, are in a lefs ratio than the reciprocal ratio of the 
diftances, or the diftance increafes.in a greater ratio than 
the anglp or apparent magnitude diminifhes. But when 
the objeCt is very remote 1 , or the optic angle is very fmall, 
as one degree or thereabouts, the angle then varies nearly 
as the difiance reciprocally. But, although the optic an¬ 
gle be the ufilial or fienfible meafure of the apparent mag¬ 
nitude of an objeCt, yet habit, and the frequent experi¬ 
ence of looking at diftant objects, by which we know that 
they are larger titan they appear, lias fo far prevailed up¬ 
on the imagination and judgment, as to caufe this alio to 
have Come (hare in our eftimation of apparent magnitudes; 
fo that thefe will be judged to be more than in the ratio 
of the optic angles. 
The apparent magnitude of the fame object, at the fame 
diftance, is different to different perfons, and different ani¬ 
mals, and even to the fame perfon, when viewed in diffe¬ 
rent lights, all which may be occalioned by the different 
magnitudes of the eye, cauling the optic angle to differ as 
that is greater or lefs : and fince, in the fame perfon, the 
more light there comes from an objeCt, the lefs is the pu¬ 
pil of the eye, looking at that object; therefore the optic 
angle will alfo be lefs, and confequently the apparent mag¬ 
nitude of the objeCt. Every one muff have experienced 
the truth of this, by looking at another perfon in a room, 
and afterwards abroad in the funfhine, when he always 
appears fmaller than in a room where the light is lefs. So 
alfo, objeCts up in the air, having more light coming 
from them than when they are upon the ground, or near 
it, may appear lefs in the former cafe than in the latter; 
as the ball of the crofs on the top of St. Paul’s church, 
■which is fix feet in diameter, appeal s lefs than an object 
of the fame diameter feen at the fame diffance below, near 
the ground. And this may be the chief reafon why the 
fun and moon appear fo much larger when feen in the ho¬ 
rizon, where their beams are weak, than when they are 
railed higher, and their light is more bright and glaring. 
Again, if the eye be placed in a rare medium, and view 
an object through a denfer, as glafs or water, having plane 
furfaces; the objeCt will appear larger than it is: and 
contrariwife, fmaller. And hence it is ■that fifties, and 
other objefts, feen in the water, by an eye in the air, al¬ 
ways appear larger than in the air. In like manner, an 
object will appear larger when viewed through a globe of 
glafs or water, or any convex fpherical fegments of thefe ; 
and, on the contrary, it will appear fmaller when viewed 
through a concave of glafs or water. 
Apparent Distance, is that diftance which we judge 
an objeCt to be from us, when feen afar off". This is com¬ 
monly very different from the true diftance ; becaufe we are 
apt to think that all very remote objects, whdfe parts can¬ 
not well be diftinguiflied, and which have no other vifible ob¬ 
jects near them, are at the fame diftance from us; though 
perhaps they may be thoufands or millions of miles off; 
•as in the cafe of the fun and moon. The apparent diltan- 
V01,; L Ng,p. 
R E N T. St, 
ces of objects are alfo greatly altered by the refraCtion of 
the medium through which they are feen. 
Apparent Figure, is the figure or (hape which an 
objeCt appears under when viewed at a diftance; and is 
often very different from the true figure. For a ftraight 
line, viewed at a diftance, may appear but as a point; a 
furface, as a line; and a folid, as a furface. Alfo thefe 
may appear of different magnitudes, and the furface and 
folid of different figures, according to their fituation with 
refpeCt to the eye: thus, the arch of a circle may appear 
a ftraight line; a fquare, a trapezium, or even a triangle; 
a circle, an elliplls; angular magnitudes, round; and a 
Sphere, a circle. Alfo all objects have a tendency to.round- 
nefs and fmoothnefs, or appear lefs angular, as their dif¬ 
tance is greater: for, as the diftance is increafed, the 
fmaller angles and afperities firft difappear, by fubtending 
a lefs angle than one minute ; after thefe, the next larger 
difappear, for the fame reafon; and fo on continually, as 
the diftance is more and more increafed ; the objeCt feem- 
ing (till more and more round and fimooth. So, a triangle, 
or fquare, at a great diftance, (hews only as a round (peck ; 
and the edge of the moon appears round to the eye, not- 
withftanding the hills and valleys on her furface. And 
hence it is alfo, that near objects, as a range of lamps, or 
fuch like, feen at a great diftance, appear to be contigu¬ 
ous, and to form one uniform continued magnitude, by 
the intervals between them difappearing, from the fmall- 
nefs of the angles fubtended by them. 
Apparent Motion, is either that motion which we 
perceive in a diftant body that moves, the eye at the fame 
time being either in motion or at reft; or that motion 
which an objeCt at reft feems to have, while the eye itfelf 
only is in motion. The motions of bodies at a great dif¬ 
tance, though really moving equally, or palling over equal 
fpaces in equal times, may appear to be very unequal and 
irregular to the eye, which can only judge of them by the 
mutation of the angle at the eye. And motions, to be 
equally vifible, or appear equal, liiuft be direCtly propor¬ 
tional to the diftances of the objeCts moving. Again, 
very fwift motions, as thofe of the luminaries, may not 
appear to be any motions at all, but like that of the hour- 
hand of a clock, on account of tire great diftance of the 
objeCt'S: and this will always happen, when the fpacc ac¬ 
tually paffed over in one fecond of time is lefs than about 
the 14000th part of its diftance from the eye; for the 
hour-hand of a clock, and the ftars about the earth, move 
at the .rate of fifteen feconds of a degree in one fecond of 
time, which is only the 13751 part of the radius or diftance 
from the eye. On the other hand, it is poftible for the 
motion of a body to be fo fwift, as not to appear any mo¬ 
tion at all; as when through the whole fpace.it deferibes 
there conllantly appears a continued furface or (olid as it 
were generated by the motion of the.objeCt, as when any 
thing is whirled very fwiftly round, describing a ring, &c. 
Alfo, the more oblique the eye is to 
■the line which a diftant body moves 
in, the more will the apparent motion 
differ from the true one. So, if a bo¬ 
dy revolve with an equable motion in 
the circumference of thecircle ABCD, 
&c. and the eye be at E in the plane 
of the circle ; as the body moves from 
A to B and C, it feems to move (lower 
and (lower along the line ALK, till 
when the body arrives at C, it appears 
at reft at K ; then when it really moves 
from C by D to F, it appears to move 
quicker and quicker from K by L to 
A, where its motion is quickeft of.all; 
after this it appears to move (lower 
and flower from A to N while the body 
moves from F to H : there, becoming 
ftationary again, it appears to return from N to A in the 
ftraight line, while it really moves from H by I to A in 
the circle. And-thus it appeai>.to. move in the line KN 
9 X by- 
