OPTICS. 
by the orange-colour; of this he fpoke very confidently, 
laying, “ Tliis is the colour of grafs; this is green.” Mr. 
H. alio (howed him a great variety of ribbons, the colours 
of which he fometimes named rightly, and fometimes as 
differently as pofiible from the true colours. It is proper 
to mention, that the experiment of the ftriped ribbon was 
made in the day-time, and in a good light. 
A more partial imperfection of vifion is noticed in the 
Phil. Tranf. for the following year, 1778. The gentleman 
himfe'lf, Mr. J. Scott, of Market Raifin, gives an account 
of his defeCts, in a letter to the Rev. Mr. Whilfon, of Tri¬ 
nity College, Cambridge. 
“ I am very willing to inform you of my inability con¬ 
cerning colours, as far as I am able, from my own common 
obfervation. It is a family-failing: my father has exaftly 
the fame impediment; my mother and one of my lifters 
were perfeCt in all colours; my other lifter and myfelfalike 
imperfeCt : my laft-mentioned filter has two fons, both 
imperfeCt; but (lie has a daughter who is very perfeCt. I 
have a fon andxlaughter, who both know all colours with¬ 
out exception ; and f'o did their mother : my mother’s own 
brother had the like impediment with me, though my 
mother, as mentioned above, knew all colours very well. 
“ Now I will inform you wha-t colours I have lead know¬ 
ledge of. I do not know any green in the world ; a pink- 
colour and a pale-blue are alike ; I do not know one from 
the other. A full red and a full green the fame ; I have 
often thought them a good match ; but yellows (light, 
dark, and middle), and all degrees of blue, except thofe 
very pale, commonly called fky, I know perfeOly well, 
and can difcern a deficiency, in any of thofe colours, to 
a particular nicety ; a full purple and deep blue fome¬ 
times baffle me. 
“I married my daughter to a genteel worthy man a few’ 
years ago : the day before the marriage he came to my 
houfe, drefled in a new fuit of clothes. I was much dif- 
pleafed that hefhould come (as Ifuppofed) in black'; and 
laid, he fliould go back to change his colour. But my 
daughter faid, it was my eyes that deceived me. He was 
a gentleman of the law, inafinerich claret-coloured drefs, 
■which is as much a black to my eyes as any black that 
ever was dyed. 
“ I have a general good fatisfa&ion in the midft of this 
my inability, can fee objebls at a diftance when I am on 
travel with an acquaintance, and can diftinguifh the fize, 
figure, or fpace, equal to molt, and, I believe, as quick, 
colour excepted. If your learned fociety can fearch out 
the caufe of this very extraordinary infirmity, and find a 
method for an amendment, you will be fo obliging to ac¬ 
quaint me. I am, &c.” 
Another inftance of a partial defeft in difcerning co¬ 
lours, has been detailed, pretty much at length, under 
the article Colour, vol. iv. p. 790. 
While fome have not been able to diftinguifh colours 
even by the organs appropriated for that pnrpofe, it has, 
on the other hand, been pretended, by certain ignorant 
perfons, that they coulcl diftinguifh colours by the touch ; 
but the teftimony of divers intelligent perfons, who have 
had the misfortune of being blind, in confe'quence of 
which their touch has, from neceffity, become very exqui- 
fite, has conftantly contradifted thofe vain aifertions. Be¬ 
sides, it wiij appear from the theory of colours, that to dif- 
criminate colours by the touch is utterly impraiSticable. 
There are indeed certain pigments of common ufein paint¬ 
ing, which, either from their roughnefs, fmoothnefs, 
unduofity, or other quality, may affeft the touch; and 
with a little practice a perfon may learn to diftinguifh the 
feel of vermilion, which looks red, from that of fap-green, 
which looks green, and fo forth; but this is not the art 
of diftinguiiliing colours by the touch; it is only the art 
of diftinguifhing certain peculiarities of furface. In faff, 
if two pigments exadfly of the fame texture (and feveral 
fuch there are), but of different colour, be prefented to 
the fingers of a man with his eyes (hut, he will pronounce 
them to be exadfly of the fame colour. 
591 
The ideas entertained by the' ancients concerning the 
nature of colours, were moftly wild and abfurd ; nor has 
the prefent theory, imperfeCt as it is, been formed without 
an innumerable variety of experiments, obfervations, and 
the concurring'i'nveftigations of a great many ingenious 
perfons. The followers of Pythagoras called colour the 
fuperficies of bodies ; Plato confidered it as a flame iffuing 
from them; Zeno called it the fifft configuration of matter; 
and Ariftotle faid it w’as that which rendered bodies ac¬ 
tually tranfparent. We need not add a formal refutation 
of thofe extravagant idea's, which were the mere offspring 
of the imagination, unfupported by experience and by 
reafon. The philofophers of thofe times paid little or no 
regard to experiments ; hence they made no difcoveries 
or improvements worthy of being recorded. A longand 
unprofitable period of nearly 2000 years elapfed, from the 
commencement of philofophical ftudies in Greece, until 
about the time of Des Cartes, when the revival of learning 
in Europe renewed with additional vigour the enquiries 
concerning the nature of light and colours. And it is 
curious to obferve by what (mall Iteps, and circuitous 
ways, any ufeful difcoveries were made. (See Prieftley’s 
Hiftory of Vifion, Light, and Colours.) Des Cartes confi¬ 
dered colour as a modification of light, and he attributed the 
difference of colour to the prevalence of the diredt or ro¬ 
tatory direction of light. Grimaldi, Defchales, and others', 
fuppofed that a certain elaftic medium of a peculiar kind 
filled the univerfe, and that the differences of colour de¬ 
pended upon the quick or flow vibrations of that medium. 
Rohault imagined that the different colours were pro¬ 
duced by the rays of light entering the eye at different 
angles with refpect to the optic axis. And Dr. Hook 
imagined that colour is caufed by the fenfation of the 
oblique or uneven pulfe of light; which being capable of 
no more than two varieties, he concluded there could be 
no more than two primary colours. 
Such were the ideas of philofophers refpefling the na¬ 
ture of colours, when fir Ifaac Newton began to examine 
the fu'ojedi in his cautious experimental manner, by which 
means, about the year 1 666, he difcovered the foundation 
of a theory of colours, which has been juftly adopted and 
admired by his contemporaries, as well as by the iucceed- 
ing generations. The progrefs of this theory, together 
with Mr. Delaval’s experiments on tranfparent bodies, 
have been detailed under the art : '2 Chromatics, vol. 
ix. p. 524-31. and for an account of that optical delufion 
called accidental colours, fee the article Colour, p. 791 
of the fame volume. 
Having referred from the former article to the prefent, 
for the mention of any recent difcoveries or experiments, 
it is proper here to obferve, that it is a very general oni- 
nion at prefent, that the primitive colours of light are 
not (even, but three only ; namely, red, yellow, and blue; 
becaufe it has been afcertained, that painters can produce 
all the other colours, by mixing either all thofe three 
colours together, or two of them in equal proportion. 
A recent writer of eminence in the philofophical world 
(M. C. A. Prieur) has ftarted another theory. Hethinks 
that the primitive colours are three in number; but he 
fuppofes them to be the red, the green, and the violet ; 
and that the other colours of the fpeftrum are formed 
from a mixture of the'fe 5 that is, the yellow from the 
red and green, the blue from the green and the violet. 
See. Anna/es ds Ckimie, Sept. 1806. 
The following new and ingenious theory of 1 ip-ht and 
colours appeared in the Monthly Magazine for jail. 1814. 
The article is figned with the author’s name, Jofepii 
Reader, M.D. at Cork ; we (hall therefore give it entire. 
“ Sir Ifaac Newton took his ideas of blacknefs or dark- 
rsefs from Des Cartes, who obferved, that black fuffo- 
cates or extinguifhes the rays that fall upon it, whereas 
white reflects them. Mr. Boyle, taking up this opinion, 
fays, ‘Many learned men fuppofed that fnow affebts the 
eyes, not by a borrowed light, but by a native one; but, 
having placed a quantity of fnow in a room from which 
all 
