OPT 
which it is difficult to decide; the former goes hack to 
1590, the latter comes down to about 1610. Itisnotof 
much confequence to fettle the priority in a matter which 
is purely accidental; yet one would not with to forget or 
miftake the names of men whom even chance had ren¬ 
dered fo great benefactors to fcience.” Suppl. to Ency. 
Brit. vol. ii. 
The pretentions of the Italians, who have willied to 
afcribe the invention of telefcopes to Galileo, cannot be 
fupported ; for Galileo himfelf fays, that, being at Venice 
when the firft rumour of this difcovery was fpread there, 
he waited'for letters from Paris, to allure himfelf of the 
wonders which fame reported; and that, after having 
received a confirmation of them, he fought for the con- 
itru£tion of the inftrument in the laws of refradtion, and 
thus dilcovered it. Being in pofTeffion of the principle, 
he by degrees formed a telefcope, which magnified the 
diameter of an objedt about thirty times; and with this 
he difcovered the Patel 1 ites of Jupiter, the lpots on the 
Sun, &c. Tlius he merely divined the mechanifm of the 
telefcope, from the defcription that was fent him of its 
effedts ; and this part of the difcovery does him fufficient 
honour, without endeavouring to exaggerate it. Bo(j'ut. 
This was the refracting telefcope ; but the refecting 
telefcope is the invention of the great fir Ifaac Newton. 
• After the invention^ of the telefcope, that of the mi- 
crofcupe was ealy ; for a telefcope may be converted into 
a microfcope, by removing the objedt-glafs to a greater 
diftance from the eye-glafs. This inftrument, by its elu¬ 
cidation of minute objedts, difcovers an immenlity on the 
one fide of man, fcarcely lefs wonderful than that which 
the telefcope difcovers on the other. The extenfion and 
divifibility of matter are thus rendered to the natural phi- 
lofopheralmoft as unlimited as the extenfion and the divi¬ 
fibility of fpace are to the geometer. We know neither 
the precife time when this curious inftrument was in¬ 
vented, nor the name of the inventor ; though it is com¬ 
monly fuppofed, that it was Cornelius Drebbel, and that 
it firft appeared about 1618, or 1620. There have been 
longdifputes on this fubjedt, into which we ftiail notenter. 
Mr. Playfairgives the invention to Galileo. Some writers 
have greatly depreciated the merit of Drebbel; but the 
truth is, he received an excellent education at Alcmaer, 
his native place, and was well verfed in all the phylical 
knowledge of his time. We need not, in this Introdudtion, 
fpeak of the different kinds of microlcopes and telefcopes, 
as we mull travel the ground over again, when we come to 
defcribe the properties and phenomena of each. 
The theory, however, of the telefcope, now become the 
main objedt in optical fcience, required that the law of 
refradtion fliould, if poffible, be accurately afcertained. 
This had not yet been efiedted ; and Kepler, whofe Diop¬ 
trics was the moft perfedt treatile on refradtion which had 
yet appeared, had been unable to determine the general 
principle which connedts the angles of incidence and re¬ 
fradtion. In the cafe of glafs, he had found by experi¬ 
ment, that thofe angles, when fmall, are nearly in the 
ratio of three to two ; and on this hypothefis he had found 
the focus of a double convex lens, when the curvature 
of both fides is equal, to be the centre of curvature of the 
fide turned toward the objedt: a propofition which is 
known to coincide with experiment. From the fame ap¬ 
proximation, lie derived other conclufions, w’hich w'ere 
found ufeful in pradtice, in the cafes where the angles juft 
mentioned were very fmall. 
The difcovery of the true law of refradtion was the 
work of Snellius. In order to exprefs this law, he fup¬ 
pofed a perpendicular to the refradling furface, at the 
point where the refradtion is made, and alfo another line 
parallel to this perpendicular at any given diftanc£from 
it. The refradted ray, as it proceeds, will meet this pa¬ 
rallel, and the incident ray is fuppoled to be produced, 
till it do fo likewife. Now, the general truth which Snel¬ 
lius found to hold, whatever was the pofition of the inci¬ 
dent ray, is, that the fegments of the refradted ray and of 
Vol. XVII. No. 1*96. 
ICS. 515 
the incident ray, intercepted by thefe parallels, had al¬ 
ways the fame ratio to one another. If either of the »t.?- 
dia were changed, that through which the incident ray, 
or that through which the refradted ray, palled, this ratio 
would be changed; but, while the media remained the 
fame, the ratio continued unalterable. It is feldom that 
a general truth is feen at firft under the moft limple af- 
pedt : this law admits of being more limply exprefted ; 
for, in the triangle formed by the two fegments of the 
rays, and by the parallel which they interfedt, the laid 
fegments have the fame ratio with the fines of the oppo- 
lite angles ; that is, with the lines of the angles of inci¬ 
dence and refradtion. The law, therefore, comes to this ; 
that, in the refradtion of light by the fame medium, the 
fine of the angle of incidence has to the fine of the angle 
of refradtion always the fame ratio. This laft fimplifica- 
tion did not occur to Snellius ; it is the work of Des Cartes* 
and was firft given in his Dioptrics, in 1637, where no 
mention is made of Snellius, and the law of refradtion ap¬ 
pears as the difcovery of the author. This naturally gave 
rife to heavy charges againft the candour and integrity of 
the French philolbpher. The work of Snellius had never 
been publilhed, and the author himfelf was dead ; but the 
propofition juft referred to had been communicated to his 
friends,and had been taught by his countryman, Profelfor 
Iiortenfius, in his lectures. There is no doubt, there¬ 
fore, that the difcovery was firft made by Snellius; but, 
whether Des Cartes derived it from him, or was him¬ 
felf the fecond dilcoverer, remains undecided. The quel- 
tion is one of thofe, where a man’s conduct in a particu¬ 
lar fituation can only be rightly interpreted from his ge¬ 
neral charadter and behaviour. If Des Cartes had been 
uniformly fair and candid in his intercourfe with others, 
one would have rejedted with difdain a fufpicion of the 
kind juft mentioned. But the truth is, that he appears 
throughout a jealous and fufpicious man, always inclined 
to deprefs and conceal the merit of others. He carefully 
omits to fpeak of the difcoveries of Kepler, fo nearly con- 
nedted with his own ; and, in treating of the rainbow, lie 
has made no mention of Antonio de Dominis. It is im- 
pofiible that all this fliould not produce an unfavourable 
impreflion ; and hence it is, that even the warmeft admirers 
of Des Cartes do not pretend that his condudt toward 
Snellius can be completely juftified. Like Ariftotle, he 
feems to have formed the defign of cutting oft’ the me¬ 
mory of all his predeceffors ; but the invention of printing 
had made this a far more hopelels undertaking than it 
was in the days of the Greek philolbpher. 
It has been abundantly Ihown, that Des Cartes, al¬ 
though very eminent in the Icience of optics, does not 
ftand Angle in the period at which he lived. Befides 
Snellius, who led the way to the dilcovery of the great 
law of refradtion, he had other illuftrious contemporaries; 
particularly the famous Scheiner, a Jefuit, who died in 
the lame year with Des Cartes. He carried into execu¬ 
tion the fchemes of Kepler, for conltructing telefcopes 
upon plans different from that of the original or Galilean 
one ; and fhares with Galileo the merit ot difcovering the 
fpotsof the fun. His treatife, entitled “ Oculus,” is very 
valuable, and abounds with ingenious and ilnportant il- 
luftrations of the nature of vifion. Gaflendi, alfo a con¬ 
temporary of Des Cartes, wrote largely on the flubject of 
light, and the natural phenomena which depend upon it; 
but he adhered clofely to the fyftem of Epicurus, in oppo- 
fition to that of Ariftotle, maintaining the materiality of 
light; and afferting that bodies are vilible by means of 
particles continually detaching themfelves from their fur- 
faces. Gaflendi, however, made no particular difcovery 
in optics ; and he is to be ranked rather among thelchool- 
men than the philofophers. Du Hamel, the firlt lecretary 
to the Royal Academy of Sciences at Paris, in his “ Af- 
tronomia Phyfica,” publilhed in 1681, examined the opi¬ 
nions of Epicurus, as defended by Gaflendi, and the 
Cartefian hypothefis ; and, after a laboured refutation of 
them both, pleads ltrongly in favour of the Ariltotelian 
6 Z doctrine 
