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OPTICS. 
other fide of the board, where it will appear inverted and 
magnified ; becaufe, what is in fadt perceived, is the (hadow 
of the pin upon the retina ; and the Jight that is Hopped 
by the upper part of the pin coming from the lower part 
of the enlightened board, and that which is Hopped by 
the lower part coming from the upper part of the board, 
thelhadow iniift neceffarily be inverted with refpedt to the 
objedt. This is nothing more than Mr. Grey’s experi¬ 
ment, in which he faw an inverted image of the pin, and 
which we have already noticed. 
All the varieties that can occur with refpedt to the vifi- 
ble motion of objedts, are thus fuccindtly Hummed up by 
Dr. Porterfield under eleven heads. 
x. An objedt moving very fwiftly is not feen, unlefs it 
be very luminous. Thus, a cannon-ball is not feen, if it is 
viewed tranfverfely; but, if it be viewed according to 
the line it defcribes, it may be feen, becaufe its picture 
continues long on the fame place of the retina, which 
therefore receives a more fenlible imprefiion from the 
objedt. 
2. A live coal fwung brifkly round in a circle appears 
a continued circle of fire, becaufe the impreflions made on 
the retina by light, being of a vibrating, and confeq uently 
of a lading, nature, do not prefently perilh, but continue 
till the coal performs its whole circuit, and returns again 
to its former place. 
3. If two objedts, unequally difiant from the eye, move 
with equal velocity, the more remote one will appear the 
flower ; or, if their celerities be proportional to their dif- 
tances, they will appear equally fwift. 
4. If two objects, unequally diflant from the eye, move 
with unequal velocities in the fame direction, their appa¬ 
rent velocities are in a ratio compounded of the diredt ra¬ 
tio of their true velocities and the reciprocal one of their 
diflances from the eye. 
5. A vifible objedt moving with any velocity appears 
to be at refl, if the fpace delcribed in the interval of one 
fecond be imperceptible at the diltance of the eye. Ilence 
it is, that a near-objedt moving very (lowly, as the index 
of a clock, or a remote one very fwiftly, as a planet, l'eems 
to be at refl. 
6. An objedt moving with any degree of velocity will 
appear at refl, if the fpace it runs over in a fecond of time 
be to its diftance from the eye as 1 to 1400. 
7. The eye proceeding ftraight from one place to ano¬ 
ther, a lateral objedf, not too far off, whether on the right 
or left, will feem to move the contrary way. 
■ 8. The eye proceeding flraight from one place to another, 
and being fenfible of its motion, diflant objedts will feem 
to move the fame way, and with the fame velocity. Thus, 
to a perfon running eaflwards, the moon on his right hand 
appears to move the fame way, and with equal fwiftuefs ; 
for, on account-of its diflance, its image continues fixed 
upon the fame place of the retina, from whence we imagine 
that the objedt moves along with the eye. 
9. If the eye and the objedt move both the fame way, 
only the eye much fwifter than the objedt, the laft will 
appear to go backwards. 
10. If two or more objedts move with the fame velocity, 
and a third remain at reft, the movable ones will appear 
fixed, and the quiefeent one in motion the contrary way. 
Thus, when clouds move very fwiftly, their parts feem to 
preferve their fituation, and the moon to move the con¬ 
trary way. 
11. If the eye be moved with great velocity, lateral ob¬ 
jedts at reft appear to move the contrary way. Thus, to a 
perfon fitting in a coach, and riding briikly through a 
wood, the trees feem to retire the contrary way; and to 
people in a (hip, &c. the (bores feem to recede. 
At the conclufion of thefe obfervations, Dr. Porterfield 
endeavours to explain another phenomenon of motion, 
which, though common and well known, had not been 
explained in a fatisfadtory manner. It is this : If a per¬ 
fon turns fwiftly round, without changing his place, all 
Vol, XVII. No. 1200. 
539 
objects about will feem to move round'in a circle the 
contrary way; and this deception continues not only 
while the perfon himfelf moves round, but, which is more 
furprifing, it alfo continues for fome time after he ceafes 
to move, when the eye, as well as the object, is at abfo- 
lute reft. 
The reafon why objedts appear to move round the con¬ 
trary way, when the eye turns round, is notfo difficult to 
explain : for though, properly fpeaking, motion is notfeen, 
as not being in itfelf the immediate objedt of fight; yet, by 
the fight, we eafily know when the image changes its place 
on the retina, and thence conclude that either the objedt, 
the eye, or both, are moved. But, by the fight alone, we 
can never determine how far this motion belongs to the 
objedt, how far to the eye, or how far to both. If we 
imagine the eye at reft, we aferibe the whole motion to 
the objedt, though it be truly at reft. If we imagine the 
object at reft, we aferibe the whole motion to the eye, 
though it belongs entirely to the objedt; and, when the 
eye is in motion, though we are fenfible of its motion, yet, 
if we do not imagine that it moves fo fwiftly as it really 
does, we aferibe only a part of the motion to the eye, and 
the reft of it we aferibe fo the objedt, though it be actually 
at relt. This laft, he fays, is what happens in the prefent 
cafe, when the eye turns round ; for, though we are fenfi- 
■ ble of the motion of the eye, yet we do not apprehend 
that it moves fo faff as it really does ; arid therefore the 
bodies about appear to move the contrary way, as is agree¬ 
able to experience. 
But the great difficulty Hill remains, viz. Why, after the 
eye ceafes to move, objects fhould, for fome time, Hill ap¬ 
pear to continue in motion, though their pidtures on the 
retina be really at reft, and do not at all change their 
place. This, he imagined, proceeds from a miftake we 
are in with refpedt to the eye, which, though it be abfo- 
lutely at reft, we neverthelefs conceive as moving the con¬ 
trary way to that in which it moved before ; from which 
miftake, with refpedt to the motion of the eye, the objedts 
at reft will appear to move the fame way which the eye is 
imagined to move ; and, confequently, will feem to con¬ 
tinue their motion for fome time after the eye is at reft. 
This is ingenious, but perhaps not juft. An account of 
this matter, which feems to us more fatisfadtory, has been 
lately given to the public by Dr. Wells.. 
“Some of the older writers upon optics (fays this in¬ 
genious philofopber) imagined the vifive fpirits to be con¬ 
tained in the head, as water is in a veil'd ; which therefore, 
when once put in motion by the rotation of our bodies, 
mult continue in it for fome time after this has ceafed ; 
and to this real circular movement of the vifive fpirits, 
while the body is at relt, they attributed the apparent mo¬ 
tion of objedts in giddinel's. Dechales law the vveaknefs 
of this hypothefis; and conjedtured, that the phenome¬ 
non might be owing to a real movement of the eyes ; but 
produced no fact in proof of his opinion. Dr. Porterfield, 
on the contrary, fuppofed the difficulty of explaining it 
to conlift in (bowing, why objedts at reft appear in morion 
to an eye which is alfo at reft. The folution heoffered of 
this reprefentation of the phenomenon is not only ex¬ 
tremely ingenious, but is, I believe, the only probable one 
which can be given. It does not apply, however, to the 
fadt which truly exifts; for I (hall immediately (how, that 
the eye is nut at reft, as he imagined. The laft author I 
know of who has touched upon this fubjedt is Dr. Darwin. 
His words are, ‘ When any one turns round rapidly on 
one foot till he becomes dizzy, and falls upon the. ground, 
the fpedtra of the ambient objedts continue to prefent 
themfelves in rotation, or appear to librate, and he feems 
to behold them for fome time in motion.’ I do not in¬ 
deed pretend to undefftand his opinion fully; but this 
much feems clear, that, iffuch an apparent motion of the 
furrounding objedts depends in any way upon their fpec- 
tra, or the illufive reprefentations of thole objects, occa- 
fioned by their former impreflions upon the retinas, no 
7 L funilar 
