at the time, is reached the former being taken as equal to the number of hundredths of inches which the diameter of the object-glass contains. Thus, a 3-inch object-glass, if really good, should bear a power of 375 on double stars where light is no object; the planets, the moon, &c., will be best observed with a much lower power.

477. In the case of stars, owing to their immense distance, no increase in their size follows the application of higher magnifiers. With planets this is different, each increase of power increases the size of the image, and therefore decreases its brilliancy, as the light is spread over a larger area. Hence the magnifying power of a good telescope is always much higher for stars than for planets, although at the best it is always limited by the state of the air at the time of observation.

478. It is always more or less dangerous to look at the Sun directly with a telescope of any aperture above two inches, as the dark glasses, without which the observer would be at once blinded, are apt to melt and crack.

A diagonal reflector, however, which reflects an extremely small percentage of light to the eye, and by reason of its prismatic form refracts the rest away from the telescope, affords a very handy method of solar observation.

Care should be taken that the object-glass is properly adjusted. This may be done by observing the image of a large star out of focus. If the light be not equally distributed over the image, or the circles of light which are always seen in a good telescope are not perfectly circular, the telescope should be sent back to the optician for adjustment.

479. The testing of a good glass refers to two different qualities which it should possess. Its quality, as to material and the fineness of its polish, should be such that the maximum of light shall be transmitted. Its quality as to the curves should be such that the rays passing through every part of its area shall converge absolutely to the same point, with a chromatic aberration

sufficient to surround objects with a faint dark blue light.

480. To give an idea of the great accuracy with which a fine object-glass refracts the light transmitted, we will take for example an object-glass of 8 inches aperture and 10 feet focal length, which, if a fine one, will separate the components of y2 Andromeda, whose angular distance is about half a second—that is, it will depict at its focus two minute discs of light fairly separated, the distance of whose centres, as above stated, is half a second. To come at the value of this half-second, as measured on a scale of inches and parts of an inch, we must consider the centre of the object-glass to be the centre of a circle whose radius is the focal length of the object-glass. The focal value of a degree of such a circle is 2'0944, or nearly 2 inches; of a minute, 0349 of an inch; of a second, 0005818, or 3000 of an inch nearly; of half a second, 0002909 inch, which is little more than the fourth part of the one-thousandth of an inch. Light from a fixed star passing through four refracting surfaces, and half an inch or more in thickness of glass, and filling 50 square inches of surface, and travelling 120 inches down the tube, is so accurately concentrated at the focal point as all to pass through the smallest hole that could be made with the most delicate needle-point through a piece of fine paper. This requires a degree of accuracy in the figuring and polishing of the material of the lenses almost inconceivable.

481. We have so far confined our attention to the principles of the ordinary astronomical telescope, and we have dealt with it in its simplest form. There are other kinds; the construction of some of which depends upon reflection; that is to say, the light is reflected by a concave mirror instead of being refracted by a lens ; but we need not dwell upon them. Let us next inquire what the very largest telescope really can do. The largest refractoras the refracting telescopes are called-in the world has recently been completed by Messrs. Alvan Clark

and Sons, American opticians of great eminence, for the Pulkowa Observatory, near St. Petersburg. The objectglass is 30 inches in diameter. Now, the pupil of our eye is one-fifth of an inch in diameter : this object-glass, therefore, will grasp 22,500 times more light than the eye can: if used when the air is pure, it should easily bear a power of 3,000 on the Moon; in other words, the Moon will appear as it would were it 3,000 times nearer to us, or at a distance of 78 miles, instead of, roughly, 234,000; measuring from the surfaces of the Earth and Moon, and not from their centres.

The largest reflector in the world has been constructed by the late Earl of Rosse; its mirror, or speculum, is six feet in diameter, and its illuminating power is such that it enables us to see, 66 as clearly as the heavens shine

to us on a cloudless evening, the details of a starry universe, stretching into space five hundred times further than those depths at which we are accustomed to gaze almost in oppressive silence."*

482. An astronomer wants telescopes for two kinds of work he wants to watch the heavenly bodies, and study their physical constitution; and he wants to note their actual places and relative positions; so that he mounts or arranges his telescope in several different ways.

483. For the first requirement it is only essential that the instrument should be so arranged that it can command every portion of the sky. This may be accomplished in various ways: the best method of accomplishing it is shown in Plate XIII., which represents an 8-inch telescope, equatorially mounted—or, shortly, an equatorial—that is, an instrument so mounted that a heavenly body may be followed from rising to setting by one continuous motion of the telescope, which motion may be communicated by clockwork.

484. In this arrangement a strong iron pillar supports a head-piece, in which is fixed the polar axis of the instru

* Nichol.