telescope completed by him, and far exceeding in all respects any yet attempted, has an object fpeculum of forty feet focal length.

The reflecting telescope, which is most in use y at prefent, is compofed of two concave mirrors of different radii. The larger concave AR (fig. 113) forms the focal image 1 m, which serves as an object for the fmall mirror K c: a fecond image im is formed by the mirror, the rays paffing through the amplifying lens L, which is placed in a hole or perforation in the center of the great mirror AR. This image is erect, and is viewed much enlarged through the eye-glafs or lens P.

In good reflecting telescopes the object speculum z is not of a spherical form.

Reflecting microscopes are fometimes made; A the method of conftructing which, as alfo of other inftruments, may be deduced from what has been faid concerning reflecting telescopes.

END OF THE FIRST VOLUME.

IN D E X

то THE

FIRST VOLUME

ABERRATION, 254.

Acceleration, 27.

Achromatic telescope, 351.
Aerial perspective, 346.

A

Air, its colour, 297. want of transparency, 345.
Algol, 192. 196.

Alps, their colour, 297.

Analogy, 2, 4.

Analysis and fynthesis, 100,

Animalculæ, 12,

Aperture, 350.

Aphelium, 122.

Apogee, 122.

Appearances, mutual, of the planets, 113-115,
Apfides, 121, their motion, 232.

Archimedes, 319.

Attraction, 7, 8, 25. wherein it differs from impulfe, 33:
of cohesion, 44. & feq. is very properly affumed as a
general caufe of phenomena, 50. apparently fubfifting be-
tween floating bodies, 48-50. See Gravity, Cobefion.
Atwood, Mr. his inftrument for meafuring the spaces de-
fcribed by bodies moving either uniformly or otherwife, 43.
Axiom, 2.

Axis and wheel, 60.

B

Balance, 59.

Black, 61.

Bradley, Dr. 249. 254.

Buffon, his burning machine, 319.
Burning mirrors, 319. glaffes, 325,

Camera

C

Camera obfcura, 346.
Caufe and effect, 4. 6. 50. 51.
Caufe, firft, is intelligent, 103.
Center of gravity, 74-79.

Centripetal force, 94 & feq. 197 & feq.
Circumnavigation, 110.

Clocks, 88 & feq.

Clouds, why of fuch various colours, 297. a deception arif-
ing from their position, 346.

Cobefion, 44 & feq.

Cold, 16.

Colour, wherein it consists, 271, 272. 292 & feq.

Comets, 159-162. 167. their motion explained, 208.
Compofition and refolution of motion or force, 23, 24. 62, 63.
Copernicus, 103.

Creator, 50. III.

Earth, proofs of its fpherical figure, 108-110. of its an-
nual and diurnal motion, 110-112. 113–118. 236.
flattened towards the poles, 235-237.

Eccentricity, 121.

Eclipfe, 139, 145–158.

Ecliptic, 123. 181.

Effect and caufe, 4. 6.

Elafticity, 41.

Ellipfis, its defcription, 97. 122.

Elongation, 105. 113-118.

Equator, 181. measure of a degree at, 236.

Equilibrium,