TREATISE osr the SCIENCE of 
OPTICS. 
INTRODUCTION. 
Optics, in its more extenfive acceptation, is a mixed 
'mathematical fcience, which explains the manner by 
which vifion is performed in the eye; treats of fight in 
the general; gives the reafons of the feveral modifica¬ 
tions or alterations which the rays of light undergo in 
the eye ; and fiiows why objects appear fometimes greater, 
fometimes fmaller, fometimes more diftinft, fometimes 
more confufed, fometimes nearer, and fometimes more 
remote. In thisextenfive fignification, it is confideredby 
fir Ifaac Newton, in his admirable work called Optics. 
The knowledge of the ancients in this department of 
fcience was very imperfect. It was the opinion of Pytha¬ 
goras, that vifion is caufed by particles continually fly¬ 
ing from the furfaces of bodies, and entering the pupil of 
the eye; whereas Empedocles and Plato fuppofed, that 
the caufe of vifion is fomething emitted from the eye, 
which, meeting with fomething elfe that proceeds from the 
objeft, is thereby reflefted back again. But, though the 
Platonic philofophers were miftaken as to the progrefs of 
vifion, they were acquainted with two very important 
and fundamental principles of this fcience: viz. thatlight, 
from whatfoever it proceeds, is propagated in right lines, 
differing in this refpeft from found ; and, that when it is 
reflefted from the furfaces of poliflied bodies, the angle 
of incidence is equal to the angle of refleftion. 
Ariftotle, amidft the variety of other objects that en¬ 
gaged his attention, did not overlook the fcience of op¬ 
tics, though many of his obfervations were fanciful and 
erroneous. Thus we find, that he maintained, in oppo- 
fi'tion to Empedocles, that light is incorporeal; conceiving 
that, if it were not a mere quality, but a real fubftance, 
the motion of it would not be infenlible, in palling from 
the eaft to the weft, though it might efcape our notice in 
a Imaller diftance. His remarks on the rainbow, halos, 
aud other phenomena of a fimilar nature, which were 
claffed by the ancients under the denomination of meteors, 
though, in lome refpefts, blended with error, are never- 
thelefs, in others, juft and true, and have led to a more 
unexceptionable and philofophical account of the caufes 
that produce them, and the various circumftances that 
attend them. 
Epicurus, and the philofophers of his fchool, as we 
learn from Lucretius, entertained fome correft notions of 
vifion, though they were Hill far from the truth. They 
conceived vifion to be performed in confequence of cer¬ 
tain Jimulacra, or images, continually thrown off from 
the Surfaces of bodies, and entering the eye. This was 
the fubftitute in their philofophy for rays of light, and 
had at leaft the merit of reprefenting that which is the 
medium of vifion, or which forms the communication 
between the eye and external objefts, as fomething pro¬ 
ceeding from the latter. The idea of Jimulacra or J'pectra, 
flying off continually from the furfaces of bodies, and 
entering the eye, was perhaps as near an approach to the 
true theory of vifion as could be made before the ftruc- 
ture of the eye was underftood. 
In the arts connefted with optics, the ancients had 
made fome progrefs. They were fufficiently acquainted 
with the laws of refleftion to conftruft mirrors both plane 
and fpherical. They made them alfo conical; and it ap« 
Vol.XVII. No. 1196. 
pears from Plutarch, that the fire of Vefta, when extin- 
guifhed, was not permitted to be rekindled but by the 
rays of the fun, which were condenfed by a conical fpe- 
culum of copper. The mirrors with which Archimedes 
fet fire to the Roman galleys have been fubjefts of much 
difcuffion, and the faff was long difbelieved, on the 
ground of its being phyfically impoflible. The experiments 
of Kircher and Buffon ftiowed that this impoffibility was 
entirely imaginary, and that the effect afcribed to the fpe- 
cula of the Greek geometer might be produced without 
much difficulty. There remains now no doubt of their 
reality. A paffage from Ariftophanes (Clouds, aft 2.) 
gives reafon to believe that, in his time, lenfes of glafs 
were ufed for burning, by collefting the rays of the fun ; 
but, in a matter that concerns the hiftory of fcience, the 
authority of a comic poet and a fatirift would not deferve 
much attention, if it were not confirmed by more fober 
teftimony. Pliny, fpeaking of rock-cryftal, fays that a 
globe or ball of that fubftance was fometimes ufed by the 
phyficians for coliefling the rays of the fun, in order to 
perform the operation of cautery. In another paffage, 
he mentions the power of a glafs globe filled with water, 
to produce a ftrong heat when expofed to the rays of the 
fun; and expreffes his furprifethat the wateritfelf fhould " 
all the while remain quite cold. See Burning Glasses, 
vol. iii. p. 534. 
On account of the reftifineal propagation of light, the 
phenomena of optics are eafily expreffed in the form of 
mathematical propolitions, and feem, as it were, fponta- 
neoufly to offer themfelves to the ftudy of geometricians. 
Euclid, perceiving this affinity, began to apply the fcience 
which lie had already cultivated with fo much fuccefs, to 
explain the laws of vifion, before a fimilar attempt had 
been made with refpeft to any other branch of terreftrial 
phyfics, and at leaft fifty years before the refearches of 
Archimedes had placed mechanics among the number of 
the mathematical fciences. In the treatife afcribed to 
Euclid, there are, however, only two phyfical principles 
which have completely ftood the teft of fubfequent im¬ 
provement. The firft of thefe is the propofition juft re¬ 
ferred to, that a point in any objeft is feen in the direftion 
of a ftraightline drawn from the^eye to that point; and 
the fecond is, that, when a point in an objeft is feen by 
refleftion from a polifhed furface, the lines drawn from 
the eye and from the objeft to the point whence the re- 
fieftion is made, are equally inclined to the reflecting 
furface. Thefe propolitions are affumed as true ; they 
were, no doubt, known before the time of Euclid, and it 
is fuppofed that the difcovery of them was the work of 
the Platonic fchool. The firft of them is the foundation 
of Optics proper, or the theory of vifion by direft light; 
the fecond is the foundation of Catoptrics, or the theory of 
vifion by reflefted light. Dioptrics, or vifion by refrac¬ 
ted light, had not yet become an objeft of attention. 
Two other principles which Euclid adopted as poftu- 
lates in his demonftrations, have not met with the fame 
confirmation from experiment; and are, indeed, true only 
in certain cafes, and not univerfally, as he fuppofed. The 
firft of thefe is, that we judge of the magnitude of the 
objeft altogether by the magnitude of the optical angle, 
or the angle which it fubtends at the eye. It is true, that 
6 Y this 
