(reckoning according to the order of the signs) and a secondary to the ecliptic passing through the object. Its latitude is its distance from the ecliptic, measured on a secondary of the ecliptic passing through the object.

16. The solstitial colure is a secondary to the equator passing through the solstices, and is therefore also a secondary to the ecliptic.

The equinoctial colure is a secondary to the equator, passing through the equinoctial points.

CHAPTER II.

FIXED STARS-TELESCOPES

APPEARANCE OF STARS IN TELESCOPES.

vens.

17. LET us now return to the consideration of the fixed stars. We observe about 3000 stars visible to the naked eye, very irregularly scattered over the concave surface of the heaThere are seldom above 2000 visible at once, even on the most starry night. They are distinguished from the planets not only by preserving the same relative position to each other, but also by a tremulous motion or twinkling in their light, apparently arising from the effect of the atmosphere on the rays of light passing through it.

For the conveniency of arranging and referring to the different stars, the method of constellations was invented by the ancients. They imagined a number of personages of their mythology, also animals, &c. drawn on the concave surface, and including particular groups of stars; these they called constellations, and denominated the stars from the constellation in which they were, and from their situation in that constellation. This method, though certainly useful, is not adequate to the purposes of astronomy in its present state, but for many obvious reasons it has been retained. The stars do not form the figure of the constellation, except in a few assemblages which have a remote resemblance; such are the Great Bear, the Hyades composing the Bull's head, &c. Some of the brighter fixed stars, and those more remarkable by their position, had proper names assigned to them, as Arcturus, Sirius, Alioth, Algol, &c.

18. Bayer, who published a celestial atlas in the year 1603, much facilitated the arrangement of the fixed stars. He marked those in each constellation by the letters of the Greek alphabet, according to their then degrees of brightness." The stars are also divided according to their apparent brightness into magnitudes. The brightest are of the first magnitude, and so on to the sixth, the least magnitude visible to the naked eye. There are eleven stars of the first magnitude in the portion of the concave surface visible to us, viz., Aldebaran, Capella, Rigel, Orionis, Sirius, Regulus, Spica Virginis, Arcturus, Antares, a Lyræ, and Fomalhaut. In the remaining portion of the concave surface there are six, viz., Achernar, Canopus, ẞ Argûs, a Crucis, a and ẞ Centauri. There are about 50 of the second magnitude, and about 120 of the third magnitude, visible

to us.

a

Some assemblages of the stars are more remarkable than others; such are the Pleiades, Hyades, Cassiopea's chair, and the great Bear. The eye unassisted by a telescope, remarks also a very considerable irregular luminous belt called the milky way. Likewise other small luminous spots, called from their appearance nebulæ, viz. Præsepe, nebulæ in Perseus, in the girdle of Andromeda, &c. By the assistance of telescopes, we find that the number of the fixed stars is greater than can be ascertained; the number of those visible to the naked eye being incomparably smaller than of those which are only visible by the help of telescopes.

19. The theory of telescopes properly belongs to the science of optics, and therefore a very short account of their effects, and of the improvements that have, from time to time, been made in them, is all that is necessary here.

a The constellation called the Great Bear is an exception, in it the principal stars are marked in the order of their right ascensions.

C

The use of telescopes is to magnify objects, or to present their images under a larger angle than the objects themselves subtend; and likewise to render objects visible that would otherwise be invisible. Telescopes for common astronomical purposes magnify from 40 to 200 times, and for particular purposes from 200 to 1000 and upwards; i. e. objects appear so much nearer than when seen by the naked eye, and their parts become more visible and distinct. We are enabled by a telescope which magnifies 100 times to behold the moon the same as we should if placed 100 times nearer than at present. A telescope magnifying a thousand times, will exhibit the moon as we should behold it could we approach within 240 miles of it. Thus, although we cannot actually approach the moon at pleasure, we can form an image of the moon, and approach this image at pleasure, and so make the image subtend a greater angle than the moon itself. We can magnify the image by help of a simple microscope, as we can magnify any minute object. This is the principle of the common telescope. The object glass forms an image of the moon, and we magnify this image by help of the eye glass, which may be considered as a mi

croscope.

20. Telescopes were accidentally invented at Middleburgh, in Holland, about the year 1609. There is no foundation for supposing them known earlier, although the single lens had been in use for spectacle glasses since the beginning of the 14th century. Galileo, hearing of their effects, soon discovered their construction, and applied them to astronomical purposes, from whence a new æra may be dated in astronomy. After some trials, Galileo made a telescope which magnified upward of 30 times, and with this instrument, so inferior in power to modern telescopes, he made most important discoveries. In little more than a year he had observed the nebula of Orion, the telescopic stars in the Pleiades and in Præsepe, had discovered the satellites

of Jupiter, very accurately decribed the face of the moon, and computed the height of some lunar mountains, observed an extraordinary appearance in Saturn, occasioned by the ring, which his telescope could not clearly shew, and had observed phases in Venus similar to the phases of the moon.

Notwithstanding the importance of the telescope, it was but slowly improved. Telescopes admitting of a high magnifying power were of a very inconvenient length. A high magnifying power could not be obtained by a short telescope, without rendering the image indistinct by colour. The ardour and industry of the astronomers of the latter end of the 17th century overcame this difficulty, by using telescopes without tubes. They affixed the object glass to the top of a pole, directing it by means of a long string, so as to throw the image into its proper place. Huyghens particularly distinguished himself by important discoveries with this inconvenient kind of telescope, which has been called the aerial telescope. The discoveries of Sir Isaac Newton, respecting light, induced astronomers to desist from endeavouring to improve refracting telescopes, and to aim at perfecting reflecting ones. Soon after the discovery of the telescope, it was suggested that the image of the object might be formed by reflection instead of refraction; but as no particular advantage could be shewn to arise from this alteration, it does not seem to have been attended to, till James Gregory proposed the construction of a reflecting telescope which goes by his name. He intended by this construction to obviate the errors of the object glasses of the common telescope, arising from their being necessarily ground of a spherical form. The discoveries of Newton on light shewed these errors to be comparatively of trifling consequence. Newton himself, as soon as his experiments on light had shewn him the true obstacle to the improvement of refracting telescopes, invented and executed a reflecting telescope, which goes by his name. His construction