FIG. 9.—SMALL TELESCOPE ON PILLAR AND CLAW STAND.
For small instruments, such as we are dealing with, the mounting generally adopted is that known as the Altazimuth, from its giving two motions, one in altitude and one in azimuth, or, to use more familiar terms, one vertical and the other horizontal. There are various types of the Altazimuth. If the instrument be of not more than 3 feet focal length, the ordinary stand known as the 'pillar and claw' (Fig. 9) will meet all the requirements of this form of motion. Should the focal length be greater than 3 feet, it is advisable to have the instrument mounted on a tripod stand, such as is shown in Fig. 10. In the simpler forms of both these mountings the two motions requisite to follow an object must be given by hand, and it is practically impossible to do this without conveying a certain amount of tremor to the telescope, which disturbs clearness of vision until it subsides, by which time the object to be viewed is generally getting ready to go out of the field again. To obviate this inconvenience as far as possible, the star or planet when found should be placed just outside the field of view, and allowed to enter it by the diurnal motion of the earth. The tremors will thus have time to subside before the object reaches the centre of the field, and this process must be repeated as long as the observation continues. In making this adjustment attention must be paid to the direction of the object's motion through the field, which, of course, varies according to its position in the sky. If it be remembered that a star's motion through the telescopic field is the exact reverse of its true direction across the sky, little difficulty will be found, and use will soon render the matter so familiar that the adjustment will be made almost automatically.
FIG. 10.—TELESCOPE ON TRIPOD, WITH FINDER AND SLOW MOTIONS.
A much more convenient way of imparting the requisite motions is by the employment of tangent screws connected with Hooke's joint-handles, which are brought conveniently near to the hands of the observer as he sits at the eye-end. These screws clamp into circles or portions of circles, which have teeth cut on them to fit the pitch of the screw, and by means of them a slow and steady motion may be imparted to the telescope. When it is required to move the instrument more rapidly, or over a large expanse of sky, the clamps which connect the screws with the circles are slackened, and the motion is given by hand. Fig. 10 shows an instrument provided with these adjuncts, which, though not absolutely necessary, and adding somewhat to the cost of the mounting, are certainly a great addition to the ease and comfort of observation.
FIG. 11.—EQUATORIAL MOUNTING FOR SMALL TELESCOPE.
The Altazimuth mounting, from its simplicity and comparative cheapness, has all along been, and will probably continue to be, the form most used by amateurs. It is, however, decidedly inferior in every respect to the equatorial form of mount. In this form (Fig. 11) the telescope is carried by means of two axes, one of which—the Polar axis—is so adjusted as to be parallel to the pole of the earth's rotation, its degree of inclination being therefore dependent upon the latitude of the place for which it is designed. At the equator it will be horizontal, will lie at an angle of 45 degrees half-way between the equator and either pole, and will be vertical at the poles. At its upper end it carries a cross-head with bearings through which there passes another axis at right angles to the first (the declination axis). Both these axes are free to rotate in their respective bearings, and thus the telescope is capable of two motions, one of which—that of the declination axis—enables the instrument to be set to the elevation of the object to be observed, while the other—that of the polar axis—enables the observer to follow the object, when found, from its rising to its setting by means of a single movement, the telescope sweeping out circles on the sky corresponding to those which the stars themselves describe in their journey across the heavens. This single movement may be given by means of a tangent screw such as has already been described, and the use of a telescope thus equipped is certainly much easier and more convenient than that of an Altazimuth, where two motions have constantly to be imparted. To gain the full advantage of the equatorial form of mounting, the polar axis must be placed exactly in the North and South line, and unless the mounting can be adjusted properly and left in adjustment, it is robbed of much of its superiority. For large fixed instruments it is, of course, almost universally used; and in observatories the motion in Right Ascension, as it is called, which follows the star across the sky, is communicated to the driving-wheel of the polar axis by means of a clock which turns the rod carrying the tangent screw (Plate [II.]). These are matters which in most circumstances are outside the sphere of the amateur; it may be interesting for him, however, to see examples of the way in which large instruments are mounted. The frontispiece, accordingly, shows the largest and most perfect instrument at present in existence, while Plate II., with Figs. [4] and [12], give further examples of fine modern work. The student can scarcely fail to be struck by the extreme solidity of the modern mountings, and by the way in which all the mechanical parts of the instrument are so contrived as to give the greatest convenience and ease in working. Comparing, for instance, Plate II., a 6-inch refractor by Messrs. Cooke, of York, available either for visual or photographic work, with the Dorpat refractor (Fig. [3]), it is seen that the modern maker uses for a 6-inch telescope a stand much more solid and steady than was deemed sufficient eighty years ago for an instrument of 96⁄10 inches. Attention is particularly directed to the way in which nowadays all the motions are brought to the eye-end so as to be most convenient for the observer, and frequently, as in this case, accomplished by electric power, while the declination circle is read by means of a small telescope so that the large instrument can be directed upon any object with the minimum of trouble. The driving clock, well shown on the right of the supporting pillar, is automatically controlled by electric current from the sidereal clock of the observatory.