COLOUR. 
nations, and' gives an account of feveral perfons whofe vi- 
fion is fimilar to his own. Thefe perlons are, his own 
brother, Mr. Harris, of Maryport, in Cumberland, (who 
had three other brothers with the fame peculiarity in their 
vifion,) two of the author’s pupils, &c. In endeavour¬ 
ing to aflign the caufe of this peculiarity in his vifion, the 
author conjedlures that one of the humours of his eye 
mull be a. tranlparenr, but coloured, medium, fo con- 
fiituted as to abforb red and green rays principally. He 
fufpedts that it is the vitreous humour which is coloured, 
and that the colour is l'ome modification of blue. Thofe 
who have attended to the theory of colours, will ealily 
perceive how far this hypothefis may be fufficient to ac¬ 
count for the phenomena above related. 
Accidental Colours, a name given to a very cu¬ 
rious optical phenomenon, which was firlt noticed by the 
Comte de Buffon. That philofopher wrote a (hort pa¬ 
per on it, which was publiftied in the Memoirs of the 
Academy of Sciences, for 1743. If a perfon look fted- 
fattly and for a coniiderable time at a finall red fquare 
painted upon white paper, he will at laft obferve a kind 
of green-coloured border furround the red fquare. If he 
now turns his eyes to forne other part of the paper, he 
will fee an imaginary fquare of a delicate green bordering 
on blue, and correfponding exadily, in point of fize, with 
the red fquare. This imaginary fquare continues vifible 
for fome time, and indeed does not difappear till the eye 
has viewed fuccefiively a number of new objefts. It is to 
this imaginary Iquare that the improper name of acci¬ 
dental colour has been given. If the fmall fquare be 
yellow, the imaginary fquare or accidental colour of green 
is blue; the accidental colour of green is red ; of blue, 
yellow; or white, black; and on the contrary, that of 
black is white. - 
The firft perfon who gave a fatisfaftory explanation of 
thefe phenomena was ProfefforScherffer, of Vienna,whofe 
differtation, tranflated by M. Bernouilli, has been pub- 
lilhed in the twenty-fixth volume of the Journal de Phyfi- 
que. In order to underdand thefe phenomena, we diould re¬ 
collect that light coniiits of ieven rays, namely, red, orange, 
yellow, green, blue, indigo, violet; that whitenefs con- 
lids in a mixture of all thefe rays ; and that thole bodies 
which refleft but very little light are black. Thofe bo¬ 
dies that are of any particular colour, refleCt a much 
greater quantity of the rays which conditute that parti¬ 
cular colo'ur than of any other rays. Thus red bodies re¬ 
flect mod red rays ; green bodies, mod green, and fo on 
Let us alfo recolleCt, that when two impreflions are made 
at the lame time upon any of our organs of fenfation, one 
of which is drong, and the other weak, we only perceive 
the former. Thus if we examine by the prifm the rays 
redeCted by a red rofe, we lhall find that they are of four 
kinds, namely, red, yellow, green, and blue. In this 
cafe, the impreflion made by the red rays makes that 
made by the others quite infenfible. For the fame reafon, 
when a perfon goes from broad day-light into an ill-light¬ 
ed room, it appears to him at fird perfectly dark, the pre¬ 
ceding drong impreffion rendering him for fome time in¬ 
capable of feeling the weake'impreffion. 
With the afliltance of thefe two remarks, it will not be 
difficult to explain the phenomena of accidental colours. 
When a perfon conliders attentively for iome time a w'hite 
fquare lying on any black fubdance (paper for indance), 
is is evident that the part of the retina on which the white 
fquare is painted, receives a dronger impreffion than any 
other part; atleadthegreatednumberofraysdrike upon it. 
A weaker impreffion, therefore, will aCt on it with much 
lefs force than upon the red of the retina. Confequently, 
when the eye is turned from the white fquare to fome 
other part of the black paper, a fquare is perceived of the 
fame lize with the white iquare, and much blacker than 
any other part of the paper; this is evidently in confe- 
quence of the weaker impreffion made by the rays reflect¬ 
ed by the black paper upon that part of the eye previoully 
fatigued by the copious reflection from the white fquare. 
791 
For the very fame reafon, if, after looking for a fufficient 
time at a white fquare lying on a black ground, we turn 
- our eyes upon a iheet of white paper, we perceive a very- 
well defined black Iquare. In this cafe the part of the 
retina already fatigued is not fo fenfible to the rays re¬ 
flected by the white paper as the other parts of it which 
have not been fatigued. The reafon then that black is 
the accidental colour of white is diffidently evident; On 
the contrary, when we look a fufficient time at a black 
fquare lying upon a white ground, if we turn our eyes to 
any other part of the white paper, or even upon black 
paper, we fhall perceive a fmall iquare anfwering to the 
black fquare, and much brighter than any other part of 
the paper; evidently becaule that part of the retina on 
which the black fquare was painted being lefs fatigued, 
is more fufceptible of impreflions than any other part of 
the eye. Thus we fee why the accidental colour of black 
is white, and why that of white on the contrary is black. 
Thefe fads, indeed, have been long known, and they have 
been generally explained in this manner. 
When a perfon has looked for a fufficient time at a 
red fquare placed on a Iheet of white paper, and then turns 
his eyes to another part of the paper, that part of the re¬ 
tina on which the red was painted being fatigued, the 
red rays refleCVed from the white paper ceafe to make any 
fenfible impreffion on it, and confequently there will be 
feen upon the white paper a fquare fimilar to the red 
fquare, and the colour of which is that which would re- 
fult from the mixture of all the rays of light except the 
red. In general, therefore, the accidental colour is the 
colour which refults from the mixture of all the rays of 
light, thofe rays excepted which are the fame with the 
primitive colour. Now, in order to difcover thefe acci¬ 
dental colours, let us recolleft the manner which Newton 
employed to determine the colour which refults from the 
mixture of leveral others, the fpecies and quantity of 
which are known. He did it by dividing the circum¬ 
ference of a circle, fo that the arches are to one another 
in the proportion of a firing fhortened by degrees, in 
order to found one after another the notes of an oifave ; 
which is nearly the proportion that the different rays oc¬ 
cupy when light is decompoied by means of the prifm. 
Or fuppofe the circumference of the circle, as ufual, di¬ 
vided into 360 degrees, the different rays, according to 
Benvenut, fiiould occupy the following arches : 
Red, 
- 
- 
- 
- 
45 °’ 
Orange, 
- 
• 
- 
- 
27. 
Yellow, 
- 
- 
- 
- 
48. 
Green, 
- 
- 
- 
- 
60. 
Blue, 
- 
- 
- 
- 
60. 
Indigo, 
- 
- 
- 
- 
40 . 
Violet, 
- 
- 
- 
- 
80. 
Let us now compare the aftion of colours on one ano¬ 
ther with that of different weights; and for that purpofe 
let us fuppofe each colour concentrated in the centre of 
gravity of its arch. In order to find the colour refult¬ 
ing from any mixture, we have only to find the common 
centre of gravity of the arches which reprefent the dif¬ 
ferent colours : the colour refulting from the mixture 
will be that of the arch to which the common centre of 
gravity approaches nearefi. And if that common centre 
of gravity is not in the ftraight line which joins the cen¬ 
tre of the circle, and the centre of gravity of the arch to 
which it is molt contiguous, the refulting colour will ap¬ 
proach more or lefs to the colour of the contiguous ar- h 
towards which the line, palling through the centre of the 
circle and the common centre of gravity of the arches, 
falls. And farther, the refulting colour wilt be more or 
lefs deep, according to the diftance of the common centre 
of gravity from the centre of the circle. 
In the cafe under confideration, namely, to determine 
the different accidental colours, the application of this 
method is remarkably eafy; becaufe only one of the ieven 
primitive colours is excluded, and confequently the fix 
colours from the mixture of which we w'ifli to know the 
refulting 
