792 COL 
refulting colour are all contiguous. For it is evident, 
that the fum of (lx arches, reprefenting thefe fix colours, 
will be divided into two equal parts by the line which paffes 
through the centre of the circle and their common centre 
of gravity; and that if the fame line be produced till it 
reaches the circumference of the circle on the other fide, 
it will alio divide the arch reprefenting the feventh or 
omitted colour into two equal parts. Let us fuppofe, for 
inftance, that the violet is omitted, and that we want to 
know the colour refulting from the mixture of the other 
fix colours, we have only to bil’eft the arch reprefenting 
the violet, and from the point of fefrion to draw a dia¬ 
meter to the circle, the arch of the circle oppofite to the 
violet through which the diameter paffes, will indicate 
the colour of the mixture. The arch reprefenting the 
violet being 8o°, let us take the half of it, which is 40 0 , 
and let us add to it 45 0 for the red, 27 0 for the orange, 
and 48° for the yellow, we fhall have 160 0 , which wants 
20 0 of half the circumference of the circle. It now we 
add the 6o° for the green, the fum total will be 220°, 
confiderably more than half the circumference: conl'e- 
quently the common centre of gravity is neareft the green 
arch ; but it falls xo° nearer the yellow than the ftraight 
line which joins the centre of the circle and the centre of 
gravity of the green arch. Hence we fee that the refult¬ 
ing colour will be green, but that it will have a fhade of 
yellow. It is evident then that the accidental colour of 
violet mult be green with a fhade of yellow : and this is 
a6tu.dly the cafe, as any one may prove by making the 
experiment. 
Suppof'e, now, we wifh to know the accidental colour 
of green, or, which is the fame thing, the colour refult¬ 
ing from the mixture of all the primitive rays except the 
green. The green arch is 6o°, the half of which is 30 0 ; 
if to this we add 6o° for the blue arch, and 40 0 for the 
indigo arch, we fhall have 130 0 , or 50 0 lefs than a femi- 
circle. If to this we add the violet arch, which is 8o°, 
we fhall have 30 0 more than the femicircle ; confequently 
the common centre of gravity falls neareft the violet, and 
it is io° nearer the red arch than is the centre of gravity 
of the violet arch. Hence we know that the accidental 
colour of green will be violet or purple, with a fhade of 
red ; and experiment confirms the fail. 
Buffon obferved, that the accidental colour of blue 
was reddifh and pale. Let us fee whether we fhall obtain 
the fame refult from our method. Let us fuppofe that 
Buffon employed a light blue. In that cafe, if to 30, the 
half of the blue arch, we add 60 for the green, 48 for the 
yellow, and 27 for the orange, we fhall have 163°, or 
15 0 lefs than half the circumference of the circle : con- 
fequentiy the common centre will fall neareft the red 
arch, but within 1 5 0 of the orange. The accidental co¬ 
lour muft therefore be red, with a fhade of orange; or, 
which is the fame thing, it muft be a pale red. In the 
fame manner we may difcover, that the accidental colour 
of indigo is yellow, inclining a good deal to orange ; and 
that the accidental colour of indigo and blue together is 
orange, with a ftrong fhade of red. Both of which cor- 
refpond accurately with experiment. It would be eafy 
to indicate, in the fame manner, the accidental colour of 
any primitive colour, if what ha's been faid were not fuf- 
ficient to explain the caufe of accidental colours, and to 
fhow that their phenomena correfpond exactly, both with 
the Newtonian theory of optics, and with what we know 
to be the laws of our fenfations in other particulars. 
From the theory above given, which is that ofprofelTor 
Scherffer, the following confequences may be deduced : 
1, The accidental colour of a red fquare, lying upon a 
white or a black ground, ought to be blackifh, if we caft 
our eyes Upon a red coloured furface. 2. If the l’urface 
upon which we look at a red fquare be itfelf coloured, if 
it be yellow, for inftance, the white paper upon which 
we afterwards caft our eyes will appear blue, with a green 
fquare in it correfponding to the original red fquare. And, 
in general, we ought to perceive the accidental colour of 
1 
OUR. 
the ground on which the fquare is placed, aG well as the 
fquare itfelf. 3. If while we are looking at the little 
fquare we change the fituation of the eye, fo that its 
image fhall occupy a different place on the retina, when 
we turn our eyes to the white paper we (hall fee two 
fquares, or at leaft one unlike the figure of the original 
one. 4. If the white paper on which we look be farther 
diftant than the little fquare, the imaginary fquare will 
appear confiderably larger than the true one. 5. If while 
we are looking at the little fquare, we gradually make 
the eye approach to it, without altering its fituation, the 
imaginary fquare will appear with a pale border. Thefe, 
and many other confequences that might eaffly he deduced, 
will be found to take place conftantly and accurately, if 
anyone choofes to put them to the teft of experiment j 
and therefore may be confidered as a complete confirma¬ 
tion of the theory given above of the caufe of accidental 
colours. 
There is another circumftance refpefting accidental co¬ 
lours which deferves attention. If we continue looking 
ftedfaftly at the little fquare longer than is neceflary, in 
order to perceive its accideutal colour, we -fhall at lait fee 
its border tinged with the accidental colour of the ground 
on which the fquare is lying. For inftance, if a white 
fquare be placed upon blue paper, its border becomes 
yellow; if upon red paper,- it becomes green ; and it be¬ 
comes reddifh upon green. In like manner, the border 
of a yellow fquare becomes greenifh upon a red ground, 
and that of a red fquare on a green ground becomes pur¬ 
ple. The caufe of the phenomenon feems to depend upon 
the contradlion and extenfion of the image of the fquare 
painted on the retina. We know for certain, that the 
diameter of the pupil changes during our infpefting the 
fquare ; at firft it becomes lefs, and afterwards increafes. 
And, though we cannot fee what paffes in the bottom of 
the eye, we can fcarcely doubt that fimilar movements 
are going on there, if we attend to the changes that are 
continually taking place in the bolder of the little fquare; 
fometimes it is large, fometimes fmall; atone time it dif- 
appears altogether, and the next moment makes its ap¬ 
pearance again. 
There is another phenomenon connected with acci¬ 
dental colours which, though all perfons more or lefs ex¬ 
perience and obferve, yet it is not fo eafy to explain ; 
namely, that if we look at thefe little fquares for a very 
long time, till the eye is very much fatigued, their acci¬ 
dental colours will appear even after we flnit our eyes. 
The fame thing takes place if we attempt to look at a 
very luminous objeft; as the fun, or a ftrong fire. Pro- 
feflor Scherffer thinks that this may be partly owing to 
the light which ltill paffes through the eye-lids. That 
feme light paffes through the eye-lids is evident, becaufe 
when we look towards a ftrong light with our eye-lids 
fhut, we fee diftin&ly their colour, derived from the 
blood veffels with which they are filled ; and if we pals 
our finger before our eyes, we fee the fhadow of the fin¬ 
ger though our eye-lids be fhut, provided our eyes be 
turned towards the window. But that this light is not 
fufficient to explain the phenomenon in queftion is evi¬ 
dent from this circumftance, that the fame accidental co¬ 
lours make their appearance though we go immediately 
into the darkefl place. Hence it feems more likely to arife 
from the idea, or impreflion made by the primary obje£t, 
than from any intervention of light after the eyes are fhut. 
To CO'LOUR, <v. a. [ coloro , Lat.] To mark with feme 
hue, or dye—The rays, to fpeak properly, are not co¬ 
loured: in them there is nothing elfe than a certain power 
and difpofition to ftir up a fenfation of this or that co¬ 
lour. Newton .—To palliate; toexcufe; to drefs in fpe- 
cious colours, or fair appearances.—I told him that I 
would not favour or colour in any fort his former folly. 
Raleigh .—He colours the falfehood of ./Eneas by an exprefs 
command from Jupiter to forfake the queen. Drydcn 
To make plaulibie.—We have fcarcely heard of an infur- 
re&ion that was not coloured with grievances of the higheft 
kind 
