mo 
OPTICS. 
fimilar motion would be obferved, were we to turn our- 
felves round with our eyes ihuf, and not to open them 
till we became giddy ; for in this cafe, as the furrounding 
cbjefls could not fend their pictures to the retinas, there 
would confequently be no fpeCtra to prefent themfelves 
afterward in rotation. But whoever will make the expe¬ 
riment will find, that objects about him appear to be 
equally in motion, when he has become giddy by turning 
himfelf round,. whether this has been done with his eyes 
open or fhut. I (hall now venture to propofe my own 
opinion upon this fubje'Ct. 
“ If the eye he at reft, we judge an objeft to be in mo¬ 
tion when its picture falls in fucceeding times upon dif¬ 
ferent parts of the retina ; and, if the eye be in motion, we 
judge an object to be at reft as long as the change in the 
place of its pidfure upon the retina holds a certain cor- 
refpondence with the change of the eye’s pofition. Let 
us now fupppfe the eye to be in motion, while, from feme 
diforder in the fyftem of fenfation, we are either without 
thofe feelings which indicate the various petitions of the 
eye, or are not able to attend to them. It is evident, that, 
in fuch a ftate of things, an object at reft muft appear to 
be in motion, fince it fends in fucceeding times its picture 
to different parts of the retina. And this feems to be what 
happens in giddinefs. I was firft led to think fo from ob- 
ferving, that, during a flight fit of giddinefs I was acci¬ 
dentally feized with, a coloured fpot, occafioned by look¬ 
ing fteadily at a luminous body, and upon which I hap¬ 
pened at that moment to be making an experiment, was 
moved in a manner altogether independent of the pofi¬ 
tions I conceived myeyes to poffefs. To determine this 
point, I again produced the lpot, by looking feme time 
at the flame of a candle : then turning myfelf round till 
I became giddy, I fuddenly difcontinued this motion, and 
directed my eyes to the middle of a fheet of paper, fixed 
upon the wall of my chamber. The fpot now appeared 
upon the paper, but only for a moment; for it immedi¬ 
ately after feemed to move to one fide, and the paper to 
the other, nolwithftanding I conceived the pofition of my 
eyes to be in the mean while unchanged. To goon with 
the experiment, when the paper and fpot had proceeded 
to a certain difiance from each other, they fuddenly came 
together again ; and this leparation and conjunction were 
alternately repeated a number of times, the limits of the 
reparation gradually becominglefs, till at length the paper 
and fpot both appeared to be at reft, and the latter to be 
projected upon the middle of the former. I found alfo, 
upon repeating and varying the experiment a little, that, 
when I had turned myfelf from left to right, the paper 
moved from right to left, and the fpot confequently the 
contrary way ; but that, when I had turned from right to 
left, the paper would then move f rom left to right. Thefe 
were the appearances obferved while I flood ereCl. When 
I inclined, however, my head in fuch a manner as to bring 
the fide of my face parallel to the horizon, the fpot and 
paper would then move from each other, one upward and 
the other downward. But all thefe phenomena denion- 
ftrate, that there was a real motion in my eyes at the time 
I imagined them to be at reft ; for the apparent fituation 
of the fpot, with refpedt to the paper, could not poffibly 
have been altered, without a real change in the pofition of 
thofe organs. To have the fame thing proved in another 
way, I defired a perfon to turn quickly round, till he be¬ 
came very giddy ; then to flop himfelf, and look fteadfaftly 
at me. He did fo, and I could plainly fee, that, although 
he thought his eyes were fixed, they werein reality moving 
in their lockets, firft toward one fide and then toward the 
other.” 
Of COLOURS. 
The variety of colours, as they are prefented to us by 
the fubftances that furround us, is immenfe, and from 
them arifes the admirable beauty of the works of nature 
in the animal, in the vegetable, and in the mineral, king¬ 
dom ; or, more properly Ipeaking, in the-univerfe. 
The eyes of feme perfons are of that unfortunate and 
unaccountable conformation, that they are not able to 
diftinguifli colours. Of this extraordinary defect in vifion 
we have the following ir.flances in the Phil. Tranf. for 
1777. The account was firft communicated from Mr. 
Huddart to Dr. Prieftley. The man’s name was Harris, by 
trade a fhoemaker, who lived at Maryportin Cumberland, 
near which place, viz. at Alionby, Mr. Huddart lived. 
Mr. H. had often heard from others that Harris could 
difeern the form and magnitude of all objeCts very dif- 
tindtly, hut could not diftinguifli colours. This report 
having excited Mr.II.’s curiollty, he converfed with him 
frequently on the fubjedf. The account he gave was this : 
That he had reafon to believe otherperfons law fomething 
in objects which he could not fee; that their language 
feemed to mark qualities with confidence and precifion, 
which he could only guefs at with helitation, and fre¬ 
quently with.error. His firft fufpicion of this arofe when 
lie was ahout four years old. Having by accident found 
in ~the ftreet a child’s flocking, he carried it to a neigh¬ 
bouring -holife to inquire for the owner : he obferved the 
people called it a red flocking, though he did not under¬ 
hand why they gave it that denomination, as he himfelf 
thought it completely deferibed by being called a /tucking. 
The circumftance however remained in his memory, and, 
with other fubfequent obfervations, led him to the know¬ 
ledge of his defedt. He obferved alfo that, when young, 
other children could difeern cherries on a tree by feme 
pretended difference of colour, though he could only dif¬ 
tinguifh them from the leaves by their difference of lize and 
fliape. He obferved alfo, that by means of this difference of 
colour they could fee the cherries at a greater diftance than 
he could, though he could fee other objects at as great a dif¬ 
tance as they; that is, where the fight was not affiited by the 
colour. Large objects he could fee as well as otherperfons; 
and even the fmaller ones, if they were not enveloped in 
other things, as in the cafe of cherries among theieaves. 
Mr. H. believes he could never do more than guefs the 
name of any colour; yet he coulddiftinguifh white from 
black, or black from any light or bright colour. Dove or 
ft raw colour he called white, and different colours he fre¬ 
quently called by the fame name ; yet he could difeern a 
difference between them when placed together. In gene¬ 
ral, colours of an equal degree of brightnefs, however they 
mightotherwifediffer, he frequently confounded together. 
Yet a ftriped ribbon he could diftinguifh from a plain one ; 
but he could not tell what the colours were with any tole¬ 
rable exadtnefs. Dark colours in general he often miftook 
for black ; but never imagined white to be a dark colour, 
nor a dark to be a white colour. 
He had two brothers in the fame circumftances as to 
fight; and two other brothers and lifters, who, as well as 
their parents, had nothing of this defedt. One of the firft- 
mentioned brothers is mailer of a trading veil'd belonging 
to Mary port. Mr. H. met him in December 1776, at 
Dublin, and took the opportunity of converting with him. 
He wifhed to try his capacity to diftinguifli the colours in 
a prifm ; but, not having one by him, he afked. him whe¬ 
ther he had ever feen a rainbow ? He replied, he had 
often, and could diftinguifli the different colours; meaning 
only, that it wascompoled of different colours, for hecould 
not tell what they were. Mr. H- then fhowed him a piece 
of ribbon : he immediately, without any difficulty, pro¬ 
nounced it a ftriped and not a plain ribbon. He then at¬ 
tempted to name the different (tripes ; the feveral ftripes 
of white he uniformly, and without hefitation, called 
white; the four black ftripes lie was deceived in, for three 
of them he thought brown, though they were exactly of 
the fame fhade with the other, which he properly called 
black. Pie (poke, however, with diffidence as to all thofe 
ftripes ; and it mult be owned the black was not very dif- 
tincl: the light-green lie called yellow; but he was not 
very p.ofitive: he laid, “I think this is what you call yel¬ 
low.” The middle ft ripe, which had a flight tinge of red, 
he called a fort of biue. But he was molt of all deceived 
a by 
