Dew drops, Newton's theory of
the brilliancy of, 249. pheno-
mena of them, '257-
Diacauftics defined, 544.
Diamonds, brilliant, reflect light,
251.rofe diamondsrefract it, ib.
Diffufion, circles of, 346.
Dioptra, ancient magnifiers, 202.
Dioptrics defined, 544.
Diftance, apparent, of objects,
hypothefis refpecting the, 473
-504.
Diverging beams, 514-517.
Dollond, Mr, his discoveries, 31
-33, 35-40, 47, 53, 176. im-
proves the telescope, 194-
196, 260, 588.
Dominis, Bp. De, discovers the

caufe of the rainbow, 426. his
hypothefis, ib. 428.
Doublets of diamonds, 251.
Drebell, C. faid to have invented
the microscope, 201.
Druids, their optical gems, 6.

E.

Earth's fhadow, illuminated by
the refraction of the atmo-
fphere, 518-523•
Edwards, J. his obfervation of
images reflected by the air,
102. he improves telescopes,
192.
Elaftic Fluid, hypothesis of the
undulations of an, 217. con-
futed, ib. 247, 260, 261.
Electric Fluid, 'identity of it,
with light, 4.
Emergent ray, focus of, 299.
Equatorial telescope, 198.
Euclid, a treatise of optics af-
cribed to, 57.

Euler, M. his discoveries, 32, 33,
47, 49, 52, 39. propofed im-
provements, 212, 214. his hy-
pothefis of light, 117, 247,
proved erroneous, yet obfti-
nately adhered to, 260. he
makes a capital improvement
in mathematics, 541.

310.

Eyes, feldom both equally good,
478.

F.

Fallacies, optical, caufes of, 491
-503-

Fermat, M. his hypothesis of re-
fraction, 17.

Fits of eafy reflection and tranf-
miffion, hypothefis' of, 367.
untenable, ib.

Fletcher, M. his hypothefis of
the colours of the rainbow,
425

Fluids, invifible, not capable of
explaining natural phenome-

na, 154.

Gold Leaf, optical phenomenon
refpecting, 523.

Gregorian telescope, 594-596.
Gregory, James, invents the re-
flecting telescope, 183, 184.
Grey, Mr, his aerial speculums,
118. his cheap microscopes,
208.

Grimaldi, M. his difcoveries,
27, 119, 123, 259. his experi
ments on rays of light, 123-
126.

Guerick, Otto, his explanation
of an optical difficulty, 86. his
hypothefis of the blue colour
of the fky, 508.
Gutta ferena, the iris contracts
in, 326.
H.
Hadley, Dr, completes the tele-
scope, 188. invents the reflect-
ing quadrant, 189.
Hairs, experiments with, 23, 124.
phenomena of their fhadows,
129, 138, 139, 142, 146.
Haller, Dr, his argument, that
the retina is the feat of vifion,
325.

Halley, Dr, affifts in construct-
ing a prifm, 22.
Haloes, Aristotle's opinion of, 66,

472.

Hartley, Dr, his theory of fingle
vifion, 341.

Hartfocker, M. improves micro-
fcopes, 203.

Hauksbee, Mr, his prifm, 22.

experiments with it, 23, 24.
Heat of bodies, by light, cause of
the, 112-114.
Heliometer, M. Bouguer's, 527.
Herschel, Dr, improves the tele-
fcope, 193.
Heterogeneal Light, 415.
Hevelius, his debate with Dr,
Hooke, 331.

Hey, Mr, fingular cafe reported

by, 326.

Hire, M. De La, his discoveries,
19, 20, 128, 181. his theory of
vifion, 319, 323, 325, 327. of
the apparent place, distance,
and number of objects, 475,
477. of colours, apparent
fpots, &c. 478-480.
Homogeneal Light, 415, 419-
423.
Hooke, Dr, his discoveries, 18,
20, 21, 26, 119. and experi-
ments, 120-122. his theory
of vifion, 331.
Horizon, an object fituated in
the, appears above its true
place, 352. extent of the vifi-
ble, on a plane furface, 505.
Horizontal Moon, hypothefis re-
specting the, 10, 12.

Hortenfius,

438
Hortenfius, Prof. explains the
law of refractions, 16.
Huygens, M. his improvements
of telescopes, 176, 179–181,
187.

I. J.

Janfen, Zach. invents the tele-
scope, 166, 167; and observes
Jupiter's moons, 168. difco-
vers the microscope, 200.
Ice, cause of the lightness of, 20.
refracts lefs than water, ib.
Jewels, brilliancy of, 249.
Illumination of the earth's fha-
dow, by the refraction of the
atmosphere, 518-523.
Images, ærial, 70-73, 102. form-

ed by concave mirrors, 560-
562. experiment producing
two images of objects, 101.
Impact, hypothefis of, refuted,

249, 257.
Impulse, motion by, not perfect-
ly understood, 154, 155.
Incident ray defined, 228.
Inflammable liquid fubftances,
fruitless attempts to fire, 115,
116.

Inflection of light, difcoveries
concerning the, 119-157.
Inverfion, fingular inftance of,
118. of images on the retina,
311.

Iris of the eye described, 308.
Irradiations of the fun's light,
through the clouds, 513-517.
Jupiter's fatellites firft obferved,
168, 172.

Jurin, Dr, his opinion of the leaft
angle of vifion, 331-333. his
experiments, 339. his theory
of apparent diftances of ob
jects, 487.

K.

Kepler, John, his discoveries, 16,

74, 173, 175.

Kircher, M. his discoveries, 15,
16, 77. his optical exhibition
of the afcention, 72. and of
letters, &c. 73.
Klingenftierna, M. his difcove-
ries, 34..
Knives, experiments with, 131
-138.

Kraft, G. W. defends Dr Bar-
row's theory, 484.

L.

Lakes, heat of water in, de-

creafes downwards, 114.
Lambert, M. conftructs tables
of refraction, 57.
Laprey, John, makes telescopes,
167.

Lead, effects of, on the compofi-

tion of glass, 49-51.
Leeuwenhoeck, his microscopis
cal difcoveries, 204, 205, ac-

575.

330.

21.

Lynceus, a title of Galileo, 172.

M.

Mæftlin, M. his discoveries, 9.
Magic Lantern discovered, 15.
Magnifiers, powerful, 575, 576.
Magnitude, apparent, of objects,
Magnifying glaffes. See Glaffes,
hypothefes respecting the, 473

-504.

Maraldi, M. his experiments,
Mairan, M. his hypothefis, 148.

140-147.

Marble, black, reflects light, 37.
Marriotte, M. his hypothefis of
vilion, 315, 316. objections to
it, anfwers, and experiments,
317-327.

Martin, Benj. improves micro-
fcopes, 212, his four-glass one,
566.
Maurolycus, his difcoveries, 14,

158, 424.

Mayer, M. his experiments on

vifion, 334.

Mazeas, Abbe, his hypothesis of
the blue and green fhadows,
510.

INDEX.

Measures of light, 524-534-
Media, various appearances of
objects through different, 345
-358.

Medicean planets, 171.
Megalafcope invented, 202.
Melville, Mr, his remarks on the
heating of bodies by light, 112

114. on bodies not perfect.
ly in contact, 117. he explains
an optical phenomenon, 504:
and discovers the cause of the
Mercury. See Quick-filver.
blue and green shadows, 510.
Merfennus, his fuppofed difco.
very, 183.

Mery, M. his experiment, 320.
Metius, J. his telescope, 165.
Michell, M. his theory of the
twinkling of the stars, 60, 61,
63. his remarks on their change
of colour, 64. his hypothesis
of vifion, 322, 324. his eftimate
of the light of the fun and
moon, 534.
Microscopes invented, 200—203,
210. improved, 204-213.
temporary, 208. reflecting,
209. double reflecting, 210.
folar, &c. 211. their magnify.
ing powers and other pheno-
mena,563-573. table of them,
574. their merits_compared
with those of telescopes, 599
Mines, dark in clear weather,
-611.
but the reverse in cloudy wea-
ther, 118.
Minium, effects of, in the com-

Mirrors, the principal described,
pofition of glass, 49-51.
555-phenomena of plane ones,
556, 557. of concave and con
Mift, reafon of the large appear-
vex ones, 558-562.
ance of objects feen through,
476.
Molyneux, M. his optical diffi-
culty, 355.

Monticelli, Marq. of, inftitutes

an aftronomical academy, 1730
Moon, Vitellio's hypothefis of
Montucla, M. his remarks, 208.
the horizontal, 12. estimate of
her light, 110, 530. compared
with that of the fun, 531-
534. phenomena of the, illuf-
trated, 523. affords no heat,
331.

Moon-eyed perfons, 334.
Motion not produced by im.
pulfe, 154-156. law of, 173.
hypothefis refpecting the ap
parent motion of objects 473
-504.
Mountains, different reports of
the heights of, 26. uncertain-

ty

Multiplying glass described, 554.
Murdoch, Mr, objects to Dol-
lond's difcovery, 53.
Mufcovy glafs, colours exhibited
by thin plates of, 84.
Mufchenbroek, M. his hypothe-
fis of the twinkling of the ftars,
62.

N.
Needles, experiments with, 145,
146.

Neillie, Sir Paul, improves te-

lescopes, 179.
Nettleton, Dr, his remarks on
the refractive power of the
atmosphere, 58 and on the
ufual calculations of the
heights of mountains, ib.
Newton, Sir Ifaac, his difcove-
ries, 18-30, 103, 429. and
experiments, 129-139. he
improves the telescope, 185-
187. his reafon for fingle vilion
with two eyes, 335. his theory
of the colours of the rainbow,
430-471.

Nollet, Abbe, his experiments
with burning mirrors, 115,
116.

Nuncius Sidereus published, 171.

0.

Parabolic fpeculum invented,

190, 191.

Parallel rays, focus of, 293.
Parhelia, Ariftotle's opinion of,
66, 472.

Pecquet, M. his answer to M.
Marriotte, 317.
Perrault, M. his opinion of the
feat of vifion, 319.
Place, apparent, of objects, hy-
pothefis refpecting, 473-504.
Plane glafs, defined, 288. Laws
of refraction in plane furfaces,
266-271.objects feen through
a plane medium, 353.
Planets, more luminous at their
Sedges than in the middle of
their diks, 530.
Plano-Convex, flat, defined, 289.
Plafter, quantity of light reflected
from, 106, 108, 110.
Plates, experiments with, 82,
143-145.

Plutarch, his opinion of the
moon's visibility, during an
eclipfe, 518.

Porta, J. B. his difcoveries, 15,
72. his theory of the rainbow,
425.

Portable Obfervatory, 398. T
Porterfield, Dr, his theory of
vifion, 335 of the apparent
diftance of objects, 485, 488-
492. of their vifible motion,
499, 500.

Preffure, phenomena of, 154,

Priestley, Dr, his hypothefis of

vifion, 322, 326.

Primary rainbow. See Rainbow.
Pringle, Sir J. quoted, 191.
Prifms defcribed, 22, 190. expe-
riments with, 23, 24, 28, 29,
35, 41, 103.
Ptolomy, Cl. wrote a treatife on
Optics, 7. and aftronomical
refractions, ib. his hypothefis
of vifion, 8. his account of the
horizontal moon, 10.
Pupil of the eye, described, 308.
Pur-blindness, cause of, 330.
Q.

Quadrant, reflecting, invented,
189.

Quantity of light reflected from
different bodies, 99, 100, 106,
108.

Quick-fiver, quantity of light re-
flected from, 98, 101, 102.

R.

Rainbow, Vitellio's theory of it,
12. Ariftotle's opinion of it,
66. phenomena of the, 424.
hypothefes respecting it, ib.
425. firft difcovery of the
caufe, 426. further explained,
427-469.

Ramfden, M. his invention, 198.
Rationale of the telescope, dif-
covered, 173.

Ray of light defined, 222. See
Light and Vifion.
Reflecting Telescope, 183, 572-
598.

Reflection of light, difcoveries
of the, 65-118. phenomena
of the, 359. hypothefes re-
specting the cause, 360–367.
Laws of it, 368-395. appear-
ances of bodies seen by it from
plane furfaces, 396, 397; and
from spherical ditto, 398-
408.*

Refracted Ray, defined, 228.
Refracting Telescope, 183, 578
-582.

Refraction of Light, discoveries
/ of the, 5—64 ; causes and laws

of it, 225-271. by spherical
furfaces, 272-286. experi-
ments and phenomena, 409–
423.
Refrangibility, rays of light have

different degrees, of, 29, 30.
Reid, Dr, his discoveries, 162.
his theory of fingle vifion with
two eyes, 338.
Reive, Mr, improves telescopes,
279.

Repulfive Force. See Forces.
Retina of the eye, defcribed, 308.
generally fuppofed to be the
feat of vifion, ib. 314. new
hypothefis, experiments, and
arguments, 315-327.
Rheita, F. his discoveries, 177.
Robins, M. refutes Dr Smith's
hypothefis, 482.

Rohault, M. quoted, 128.
Rowning, Mr, his account of the
chief theories of reflection,
361.
Royal Academy, experiments
made by the, 22, 25, 319.
Royal Society, experiments made
by the, 18-20, 22-24.

¡S.
Saturn's ring firft obferved, 172.
his fatellites discovered,< 178.
his diameter, 253. an expe-
riment proposed and difco-
very expected, respecting his
ring, ib.

Scheiner, M. his experiments and
difcoveries, 159, 172.
Sclerotica, described, 308.
Secondary Rainbow. See Rain-
bow.

Seneca, his opinion of light, and
its colours, 5.
Shadows, bordered with colours,
130. blue and green, observed,
509. hypothefes refpecting
them, 510-512.

Short,

of it, s09512.
Smith, Caleb, attempts to im-

prove the telescope, 197.
Smith, Dr, invents a double re-
flecting microscope, 20. his
· ́experiment on vision, 331. obi
jects to Bp. Berkeley's theory,
481. his method of judging of
diftances of objects, 486. his
opinion of the colour of the
fky, 508. his theory of ir-
radiations of light through
clouds, 513517. of the illu-
mination of the earth's fhadow,
1518523 his estimate of the
light of the moon, 533.1
Snellius, Profeffor.. Willebrord,
discovers the law of refraction

16.

Snow, proved to have no native
light, 75%

Soap and water, colours exhi-
bited by bubbles of, 83.
Solar microfeope, 211. its mag-
nifying power, 576. 1
Specific velocity defined, 245.1
Spectacles, first hint of 163. the
invention of, afcribed to dif-
ferent perfons, 164–1664,9
Spina, Alexander, his invention,
164.
Spirit of Wine, refractive power
of 31. not easily fired, 116.
Spots on the fun, firft obferved,
172, vifibility of spots and lines
compared, 331, 332, 334.
Stars, hypothefes refpecting the
twinkling of the, 9, 13. ac-
counted for, 26, 60-63. fome
of them change colour, 64. vi-
fible in day-light at the bottom
of a pit, 86. velocity of their
light equal to that of the fun,

255.

Sun, light of the. See Light
Reafon of his apparently great-
er fize when near the horizon
8..more luminous at the centre
of his disk, than in its extremi-
ties, 530. estimate of his light,
ib.-534.
Surfaces, fpherical, refraction by,
272-284.+

T.

༈༔ ན་རྩྭར་ས

Tchirnhaufs, M. his diftinction
of the curves of rays, 544.
Telescope, invented, 165. the
discovery afcribed to different
31, 32, 37-40; 44-49; 551
perfons, ib. 169. improved,
~56. 170—176; 185—194; 598.
chief effects of it, 174. the ter-
reftrial, 175; Galilean, 176:
binocular, 177, ‍342; \refract
ing, 183, 578-582; reflecting,
183, 592-598; aerial, 1891
equatorial, 198. Mr Dollond's
$88-591; Newtonian, $92,
593; Gregorian, 594; merits
of telescopes and microscopes
compared, 599-611

₹

Thin Plates, experiments with,

82.

-74 A

Tlapazatli, experiments...with,

776

Torre, T. Di,› his magnifiers,
213-
Torricellian Vacuum, experi-
ment with a, 21.
Tour, M. Du, his hypothefis
and experiments, 149–152,
340.
Tournieres, M. De, affifts
Clairaut in his experiments,
43.

Transparent Bodies always re-
flect fome light, 359.
Twinkling. See Stars,
U, V.

Variations of the light of the fun,

moon, and planets, 530.
Velocity of light, 222. fpecific,
245. that of the fun and ftars,
the fame, 255.
Venice, Senate of, rewards Gali-
leo, 170.
Venice Glafs, refractive power

of, 37

Venus, fometimes observed to

twinkle, 61. changes her
phases like the moon, 172.

Vertex defined, 273.
Verulam, Lord. See Bacon.
Visible Objects, how to judge
if they be in motion or at rest,
502
Vilion, difcoveries concerning,
158-161. theory of, 307, 311.
manner of 312-314. hypo-
thefes as to the feat of, 315-
319. experiments and argu-
ments refpecting it, 320-317.
diftinctions of vifion, 328-
330. experiments and theories
refpecting it, 331-344. de-
ceptions, 342, 343. various
appearances through different
media, 345-358. causes of in-
diftinct and fallacious vifion,
Visual base, 337.
475 504.
Vitellio, his discoveries, 9, 12.
his account of images feen in
the air, 73.

ОРТ

ОРТ

mat

cy, and inward murmur that it was not greater, he died unlamented. Clarendon.-He had been a perfon, not only of great opulency, but authority. Atterbury.

man people, opposed to POPULARES. They were warm supporters of the dignity of the chief magiftrate, and promoters of the grandeur of the ftate, who cared not if the inferior members fuffered, provided the commanding powers were advanced : whereas the populares boldly ftood up for the rights of the people, pleaded for larger privileges, and laboured to bring matters nearer to a level. In short, they resembled the court and country parties of Britain. Tully fays, that the optimates were the best citizens, who wished to deferve the approbation of the better fort; and that the populares courted the favour of the populace, not fo much confidering what was right, as what would please the people, and gratify their own thirft of vain glory and empty applause.

OPTIMISM. 7. f. [optimus, beft, Lat.] the doctrime of the OPTIMISTS, that "Whatever is, is right", and that "All that happens is for the best." See Voltaire's Candidus.

OPTIMIST, n. f. One who believes in OPTI

MISM.

* OPTIMITY. #. f. [from optimus.] The ftate of being beft.

OPTIO, an officer in the Roman army, an affiftant or lieutenant to every centurion; fo called because he was the choice or option of the centurion in later times: at first, however, he had been chofen by the tribune, or chief commander of the legion. These optiones are also fometimes called SUCCENTURIONES and TERGIDUCTORES; the laft name was given them because their poft was in the rear of the company. Some authors mention fub optiones, or fublieutenants. Optiones were not peculiar to the camp, but were also used in a variety of other offices of life.

(1.) * OPTION. n. f. [optio, Lat.] Choice; election.-He decrees to punith their contumacy finally, by affigning them their own options. Hammond-Tranfplantation must proceed from the option of the people, elfe it founds like an exile. Bacon.-Which of these two rewards we will receive, he hath left to our option. Smallridge.

(2.) OPTION, in ecclefiaftical polity. When a new fuffragan bishop is confecrated, the archbishop of the province, by a cuftomary prerogative, claims the collation of the firft vacant benefice, or dignity, in that fee, according as he fhall choofe; which choice is called the archbishop's option. But if the bishop dies, or is tranflated, before the incumbent of the promotion chofen by the archbishop die or be removed, it is generally fuppofed that the option is void; as the granter, fingly and by himself, could not convey any right or title beyond the term of his continuance in that fee. And if the archbishop dies before the voidance happen, the right of filling up the vacancy fhall go to his executors or adminiftrators.

OPTIONAL, adj. in one's option; free to be chofen, or refused.

OPTO-KILTEKAS, a town of Chinese Tartary. Lon. 110. 40. E. Ferro. Lat. 44. 8. N.

* OPULENCE. n. f. [opulence, Fr. opulentia, * OPULENCY. Latin.] Wealth; riches; affluence. It must be a discovery of the infinite flatteries that follow youth and opulency. Shak. Timon. After eight years spent in outward opulenVOL. XVI. PART II.

There in full opulence a banker dwelt, Who all the joys and pangs of riches felt. Swift. * OPULENT. adj. [opulent, Fr. opulentus, Lat.] Rich; wealthy; affluent.-He made him his ally, and provoked a mighty and opulent king by an of fenfive war in his quarrel. Bacon.-To begin with the fuppofed policy of gratifying only the rich and opulent. South.

* OPULENTLY. adv. {from opulent.] Richly ; with splendour.

OPUNTIA, the prickly pear tree, a fpecies of cactus; fee CACTUS. The fruit of the opuntia is remarkable for colouring the juices of living animals, though it appears not to be poisonous or even hurtful to the body. In a letter from Charlestown in S. Carolina, inferted in the 5oth vol. of the Philof. Tranf. the author writes thus:-"As you defired, I have tried the effects of the prickly pear in clearing the urine. I went down to one of the iflands, and gathered fome of the fruit, and gave 4 pears to a child of 3 years of age, and fix to one of 5. The next morning I examined the urine of both, and it appeared of a very lively red colour, as if tart wine had been mixed with water. I gave likewife fix pears to a negro wench, who was fuckling an infant, and strictly forbade her to put the child to her breaft for 6 or 8 hours; and then taking fome of her milk in a tea-cup, and setting it by for fome hours, the cream had a reddish luftre, though it was very faint." He adds, that the prickly pear grows in great abundance about Carolina; and that the cochineal infects are found upon it, though no attempt has yet been made to cure them for ufe as the Spaniards do.

OQUENDO, a town of Spain in the province of Alava; 5 miles SE. of Orduna.

(I, 1.) * OR. conjunct. [other, Saxon.], 1. A dif junctive particle, marking distribution, and some times oppofition.-Inquire what the ancients thought concerning this world, whether it was to perish or no; whether to be destroyed or to stand eternally? Burnet.

He my mufe's homage fhould receive, If I could write, or Holles could forgive. Garth. By intenfe ftudy, or application to bufinefs that requires little action, the digeftion of foods will foon proceed more flowly. Blackmore. Every thing that can be divided by the mind into two or more ideas, is called complex. Watts' Logic. 2. It correfponds to either; he muft either fall or fly.-At Venice you may go to any houfe either by land or water. Addifon. 3. It fometimes, but rather inelegantly, ftands for either.

Cowley

For thy vaft bounties are so numberless, That them or to conceal, or else to tell, Is equally impoffible. 4. Or is fometimes redundant, but is then more properly omitted. How great foever the fins of any unreformed perfon are, Chrift died for him becaufe he died for all; only he muft reform or for fake his fins, or elfe he fhall never receive benefit of his death. Hammond. 5. [Or, or are, Sax.] before; or ever, is before ever. Obfolete. Or Kkk

we